Inorganic Chemistry

Chemistry of Lanthanoids

Dr. Reena Jain

Dept. of Chemistry Hindu College Delhi -110052

(26.10.2006)

CONTENTS Introduction Position of lanthanoids in the periodic table Terrestrial abundance and distribution Extraction of lanthanoids from minerals Electronic structure Atomic and ionic radii : Lanthanoid contraction Oxidation states Separation of lanthanoid elements Magnetic properties Colour and spectra Chemistry of “0” oxidation state – The metal Chemistry of lanthanoid ions in +2 oxidation state Chemistry of lanthanoid ions in +3 oxidation state Chemistry of lanthanoid ions in +4 oxidation state Complex formation Uses of lanthanoids and their compounds Some comparisons and contrasts

Introduction Group 3 of the periodic table contains scandium , yttrium and lanthanum . Strictly speaking , actinium should also be included , but in practice , it is studied separately . There are fourteen elements that follow lanthanum and these are called lanthanides . The lanthanides comprises of the largest occurring group in the periodic table . These lanthanides are placed below the main body of the periodic table in the manner of a footnote . The full - width version of the periodic table shows the position of the lanthanides more clearly (Table 1) .

Table 1: Modern Periodic Table

These lanthanides are associated with a major confusion regarding their terminology. Whether the term “Lanthanides” refers to the fifteen elements from La to Lu or the fourteen elements from Ce to has been debated for long . “Rare Earth Elements” and “ Rare Earth Metals” are trivial names sometimes applied to a collection of these elements in the periodic table . Earth is an obsolete term for “oxide” . At the time of their discovery , the oxides of these elements were believed to be scarce in abundance , as minerals . This terminology is no longer appropriate , as these elements are no longer rare , except promethium , with a t1/2 of 2.6 years . To avoid any confusion , now the term “lanthanoid” rather than “lanthanide” , is used to represent these elements as the suffix “-ide” is generally used to indicate anions . However , even now there is a confusion regarding the position of La , i.e. , whether the group is made up of fifteen elements , La to Lu , or fourteen elements , Ce to Lu . Lanthanides are chemically similar to each other , to scandium as well as yttrium . Currently, the general symbol, Ln, is used for the fourteen elements ( Ce - Lu ) and group III elements Sc , Y and La. The story of the lanthanoids begins in 1787 when a young Swedish artillery officer, Lieutenant Carl Axel Arrhenius , who was a keen amateur geologist , was exploring a quarry at a small town called Ytterby , near Stockholm . He found a new , very dense black mineral which he named ytterbite . Its chemical analysis carried out by Johan Gadolin , a Finnish chemist in 1794 showed that the new mineral contained oxides of iron , beryllium, silicon and a new , previously unidentified 'earth' which he named 'yttria' . Yttria was later shown to be a mixture of the oxides of six rare earth elements . The history of discovery and naming of the lanthanides are summarized in Table 2 .

Table 2: Discovery & origin of names of lanthanoids, including yttrium, thorium & scandium Year Element Origin of

name Discoverer Nationality Comments

1794 Yttrium Yttterby mine, Sweden

Johan Gadolin Finn

1803 Cerium After the asteroid Ceres (which in turn named after a Greek deity )

Baron Jons Jakob Berzelius and William Hisinger

Swedish Also discovered independently in same year by Martin Heinrich Klaproth (German) . The pure element was not isolated until 1875 .

1828 Thorium After Thor , the Scandinavian god of war

Baron Jons Jakob Berzelius

Swedish

1839 Lanthanum From Greek lathano = to lie hidden (because it lay concealed in the earth )

Carl Gustav Mosander

Swedish

1843 Erbium Derived from Yttterby mine, Sweden

Carl Gustav Mosander

Swedish

1878 Terbium Derived from Yttterby mine, Sweden

Carl Gustav Mosander

Swedish

1878 Ytterbium Derived from Yttterby mine, Sweden

Jean Charles de Marignac

French In 1907 and 1908 , Georges Urbain (French) and Carl Auer Von Welsbach (Austrian) independently separated Marignac’s ytterbium into two elements , which are now called ytterbium and lutetium

1879 Samarium After the mineral samarskite , in turn after the minerals discoverer , a Russian mining official V. E Samarsky

Paul E . Lecoq de Boisbaudran

Swedish

1879 Scandium After Scandinavia

Lars Fredrik Nilson

Swedish

1879 Holmium After the Latin word for Stockholm, Holmia

Per Teodor Cleve

Swedish Also discovered independently by Jacques Louis Soret and Marc Delafontaine (Swiss)

1879 Thulium From the Latin Thule, an ancient name for Scandinavia

Per Teodor Cleve

Swedish

1880 Gadolinium In the honour of Johan Gadolin , a Finnish chemist

Jean Charles de Marignac

Swiss of French origin

Paul E . Lecoq de Boisbaudran independently isolated the element from Mosander’s yttria in 1886

1885 Praseodymium From Greek prasios = green , in reference to the colour of the salts and didymos = twin , because the earth didymia was separated into two salts ; Pr and Nd

Carl Auer Von Welsbach

Austrian

1885 Neodymium From Greek neo = new and didymos = twin , because the earth didymia was separated into two salts; Pr and Nd

Carl Auer Von Welsbach

Austrian Not isolated in relatively pure form until 1925

1886 Dysprosium From Greek dys = bad and prositos = aproachable, , dysprositos means hard to get because of the difficulty involved in its detection and isolation

Paul E . Lecoq de Boisbaudran

French

1901 Europium After Europe Eugene Demarcay

French

1907 Lutetium After Lutetia , Latin name for the place where Paris was founded

Independently by Georges Urban and Carl Auer Von Welsbach

French and Austrian

1947 Promethium After Prometheus , in greek mythology , who brought fire to mankind in reference to harnessing of the energy of the nuclear fission and warning against its dangers

Charles DuBois Coryell Lawrence E . Glendenin and Jacob A . Marinsky

American

On the basis of their separablility , the lanthanoids were conveniently divided into the “cerium group minerals” or “light earths” (including light lanthanoid elements , from La to Euro) and the “yttrium group minerals” or “heavy earths” (including heavy lanthanoid elements from Gd to Lu , along with Y) . Yttrium is lighter than other “yttrium group minerals” , but is still grouped with them , as it has a comparable ionic radius and occurs in nature associated with the ores of heavier lanthanoids. This unit shall deal with the general chemistry of lanthanoids, including the implementation of the conceptual approach . The unit also provides the background essential to understand the problems of their recovery and separation , along with their applications. Position of Lanthanoids in the Periodic Table The lanthanoids have atomic numbers between those of barium (Z = 56) and hafnium (Z = 72) , and hence must be placed between these two elements . Ba is an alkaline earth metal belonging to group 2 , below Sr . Hf is present is group 4 , below Zr , thus leaving only one place between them , which lies exactly below Υ ( Z = 39 , group 3) . Since all the lanthanoids resemble each other in many aspects , therefore it become necessary to accommodate all of them together at one place . This problem is solved by placing the first element i.e. La below Υ and the remaining elements separately in the lower part of the periodic table (Table 1). Terrestrial Abundance and Distribution The lanthanoid elements are not particularly rare. Apart from the unstable 147 Pm (half life 2.6 years) of which traces occurs in uranium ores , all the lanthanoids are actually more abundant than iodine . Cerium is the twenty - sixth most abundant of all elements , being half as abundant as Cl and more abundant than lead . Even Tm , the rarest after Pm , is more abundant than iodine , and Lu is more abundant than gold . The abundance of these elements and the number of naturally occurring isotopes vary regularly , in accordance with Harkin′s rule (Table 3) .

Table 3: Abundance of the lanthanoides in the earth’s crust by mass and number of natural isotopes

Atomic number

Element Abundance (ppm) in earth’s crust

Relative abundance

Naturally occurring isotopes

58 Ce 66 26 4 59 Pr 9.1 37 1 60 Nd 40 27 7 61 Pm 0 0 62 Sm 7.0 40 7 63 Eu 2.1 49 2 64 Gd 6.1 41 7 65 Tb 1.2 56 1 66 Dy 4.5 42 7 67 Ho 1.4 55 1 68 Er 3.5 43 6 69 Tm 0.5 61 1 70 Yb 3.1 44 7 71 Lu 0.8 59 2

According to this rule , the elements with even atomic numbers are more abundant and have more stable isotopes , than those with odd atomic numbers . The graphical representation of their abundance is given in fig 1 .

Fig. 1: Abundance of Lanthanoids in Earth’s crust The non - existence of promethium in nature may be explained by Mattauch′s rule . This states that if each of the two elements with consecutive atomic numbers have an isotope of the same atomic mass , one of the isotopes will be unstable . Since Nd (Z = 60) has stable isotopes with mass numbers 142 , 143 , 144 , 145 , 146 , 148 , 150 and 152 and Sm (Z = 62 ) has the isotopes with mass numbers 144 , 147 , 148 , 149 , 150 , 152 and 154 , there are not many stable mass numbers available for promethium (Z = 61) . If Pm is to have a stable isotope , it must have a mass number outside the range 142 – 150 . The isotopes of Pm which have been identified so far are radioactive .

Many minerals are known to contain lanthanoids (Table 4) . The symbols Ce and Υ in the table represent the elements in cerium group and yttrium group , respectively . Out of the list of the mentioned minerals , only two, namely, monazite and bastnasite are of commercial importance . Monazite is sparsely distributed in various rocks but , due to its high density and inertness , it is concentrated by weathering into sand or beaches , usually in presence of other similar minerals such as cassiterite (SnO2) . Their rich deposits occur in Travancore , South Africa , Brazil , Malaysia , the U.S.A and Australia . In fact , before 1960 , monazite was the only source of lanthanoids . However , a vast deposit of bastnaesite was explored in the Mountain Pass , California has since then become the most important single source of lanthanoids . Apart from the U. S. A. it is also found in Madagascar . Table 4: Important Minerals of Lanthanoids

Minerals Composition Location of significant deposits

(1) Cerium group minerals (i) Monazite Sand- Mixture of ortho-phosphates of Ce-earths, (Ce)PO4

50-70% Ce-earths(i.e. elements of at. no. 57 to 62 calculated as oxides) 1-4% Y-earths (i.e. elements of at. no. 63 to 71 calculated as oxides) 5-10% ThO2 1-2% SiO2 22-30% P2O5 Traces of U

Occurs in the sand beaches of Travancore(India) Brazil South Africa U.S.A.

(ii) Bastnaesite-cerium earth fluoro-carbonate,(Ce)FCO3

65-70% Ce-earths , < 1% Y-earths

Sweden, California, New Maxico

(iii) Cerite-A hydrated silicate of the composition, (Ce)3 MII H3Si3O11(M-Ca,Fe)

Traces of thorium 51-72% Ce-earths 7.6% Y-earths Traces of Th, U, Zr

Sweden Caucausus

(2)Yttrium group minerals (i) Gadolinite or Ytterbite- A ytterium-earth , iron and beryllium silicate, (Fe, Be)3 (Y2) Si2 O10

35-48% Y-earths (Calculated as oxides) 2-17% Ce-earths Upto 11.6% BeO Traces of ThO2

Sweden, Norway USA(Texas and Colorado)

(ii) Xenotime –An orthophosphate of Y-earth(analogous to Monozite), (Y), PO4

54-65% Y-earths ∼ 0.1% Ce-earths Upto 3% ThO2, upto 3.5%U3O3 2-3% ZrO2

Norway Brazil

(iii) Euxenite- Mixture of titanates, niobates and tentalates of Y-earths, (Y) (Nb, Ta) TiO6. XH2O

13-35% Y-earths (Calculated as Oxides) 2-8% Ce-earths (Calculated as Oxides) 20-23% TiO2, 25-35% (Nb, Ta)2O5

Australia, Idaho(U.S.A.)

At present , China is estimated to have the world’s largest deposits of lanthanides (43%) and is now the largest producer of these elements . India contributes only 3% towards the production of lanthanoids. Promethium is not available from rare earth ores. It occurs only in traces in uranium ores where it is formed by spontaneous fission of 238U . It was first isolated as 147Pm by exchange methods from products of nuclear fission reaction . Extraction of Lanthanoids from Minerals The distribution of the lanthanoids in the two commercially important minerals , monazite and bastnaesite , is quite similar . Both these minerals contain metals such as Ce , La , Nd and Pr . However , monazite typically contains 5 – 10% ThO2 and 3% yttrium earths , which are almost absent in bastnaesite . The complex composition makes the chemical treatment of monazite very extensive and lengthy . Moreover , though thorium is only weakly radioactive , it is contaminated with daughter elements such as 228Ra which are more active and hence require careful handling during the processing of monazite . Processing includes cracking the minerals , recovering the lanthanoids (along with thorium) , removing thorium if present and separating the lanthanoids . The concentration of the mineral usually begins with gravity separation on Wilfley table. Since these minerals are heavy , their sand gets caught up on the riffles and the gangue material is washed off and dried . The magnetic impurities are removed by magnetic separation. The concentrated mineral is then subjected to chemical treatment , which is technically known as “opening up” or “cracking” . These treatments depend on the ore being used and the extent to which the metals are to be separated from each other . The chemical treatment of monazite is done either by NaOH or concentrated H2SO4 solution . The principle underlying the cracking is the difference in solubilities of Ln2 (SO4)3 . Na2 SO4 . x H2O for the light and the heavy lanthanoids and also the low solubility of the hydrous oxide of thorium. (i) Cracking of monazite by conc. H2SO4 – The finely powdered and concentrated ore is treated with 93% H2SO4 at 2000 C for several hours . The reaction is exothermic and the resulting viscous paste is leached with cold water . Th , La and the lanthanoids dissolve as sulphates , leaving behind the insoluble residues which mainly contain radioactive 228Ra . The solution of sulphates of Th , La and Ln , on partial neutralization with NH4OH , precipitates out ThO2 . The remaining solution is then treated with Na2SO4 , to salt out La and the light lanthanoids as sulphate , leaving the heavy lanthanoids in solution . This scheme is summarized in fig.2 (a). The solution containing the sulphates of heavy lanthanoids can then be used to separate the individual components by various methods , such as valency change, ion exchange and solvent extraction , discussed later in the unit . (ii) Cracking of monazite by NaOH - The finely powdered and concentrated ore is treated with 65% NaOH solution at 1400C , followed by extraction with water . A slurry of impure hydrous oxides is obtained , which is treated with boiling aq. HCl until the pH is 3.5 . Crude ThO2 separates out leaving behind a solution of impure lanthanoid

chlorides. The solution is then treated with a solution of BaCl2 and Ln2 (SO4)3 in stoichiometric amounts. BaSO4 precipitates out, along with radioactive 228Ra as RaSO4 . This scheme is summarized in fig.2 (b). The remaining solution containing Ln and lanthanoid chlorides can be used to separate the individual components by special techniques.

Symposium on Symposium on Symposium on “ Emerging Areas In Chemistry” Dec. 1 – 3 , 2005 Hindu College University of Delhi Symposium on Symposium on

Fig. 2: Cracking of Monazite / xenotime by (a)conc. H2 SO 4; (b) NaOH

Out of the two cracking methods, the conc. H2SO4 treatment is more economical than the NaOH process. However, the latter process gives higher yields and a cleaner separation . (iii) Cracking of bastnaesite - The bastnaesite mineral contains very small amounts of Th and heavy lanthanoids and requires a comparatively simpler treatment . The concentrated ore is treated with conc. H2SO4 at 2000 C , when CO2 , HF and SiF4 are evolved . The dried product is then treated with water to obtain a solution containing lanthanum and the light lanthanoids , which are then separated out via special techniques discussed later in the unit . A schematic representation of the procedure followed is given in fig 3.

Fig.3: Cracking of Bastnaesite

Electronic Structure The electronic structure of the members of group 3 , in the modern periodic table , indicates that the elements usually listed in this family are the first members of the four d – type transition series .

Sc , Z = 21 1s2 2s2 2p6 3s2 3p6 3d1 4s2

Or , [Ar] 3d1 4s2

Y , Z = 39 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d1 5s2

Or , [Kr] 4d1 5s2

La , Z = 57 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 5s2 5p6 5d1 6s2

Or , [Xe] 5d1 6s2

Ac , Z = 89 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 4f14 5s2 5p6 5d106s2 6p6 6d1 7s2

Or , [Rn] 6d1 7s2

In elements succeeding scandium and yttrium , the electrons are added to the 3d and 4d levels respectively , giving the first and the second transition series . However , after lanthanum , the energy of the 4f level falls below that of the 5d and thus the electrons are added to the inner , well - shielded 4f orbitals before entering into the 5d subshell . Hence, the lanthanoid series is defined by the progressive filling of the 4f

orbitals , namely , z3 , xz2, yz2 , xyz , z ( x2 - y2 ) , x ( x2 - 3y2 ) and y ( 3x2 - y2 ) . The general set f orbitals are illustrated in fig. 4. Since there are seven such orbitals , each with a capacity of two electrons , a total of fourteen elements of this f - type series may result before the 5d orbitals starts filling up again . This accounts for the elements cerium through lutetium (Z = 58 to 71).

Fig.4: Shape of seven f orbitals

The electronic configuration is established on the basis of the emission spectra of the element under consideration . If the spectrum is simple , containing only a few lines , its interpretation becomes easier and the correct ground state configuration can be established for the atom in question . However , the emission spectra for many lanthanoids is highly complex , making the establishment of an absolutely correct configuration extremely difficult . The difficulty arises due to the fact that the 5d and 4f orbitals have comparable energy , so that the distinction between the two is not easy . The configuration of the lanthanoids is summarized in table 5. Table 5: Ground state electronic configuration of Lanthanoids

Element Atomic number (z)

Electronic configuration Idealized observed

La 57 [Xe]5d16s2 [Xe]5d16s2

Ce 58 [Xe]4f15d16s2 [Xe]4f15d16s2

Pr 59 [Xe]4f25d16s2 [Xe]4f3 6s2

Nd 60 [Xe]4f35d16s2 [Xe]4f4 6s2

Pm 61 [Xe]4f45d16s2 [Xe]4f5 6s2

Sm 62 [Xe]4f55d16s2 [Xe]4f6 6s2

Eu 63 [Xe]4f65d16s2 [Xe]4f7 6s2

Gd 64 [Xe]4f75d16s2 [Xe]4f75d16s2

Tb 65 [Xe]4f85d16s2 [Xe]4f9 6s2 or [Xe]4f85d16s2

Dy 66 [Xe]4f95d16s2 [Xe]4f10 6s2

Ho 67 [Xe]4f105d16s2 [Xe]4f11 6s2

Er 68 [Xe]4f115d16s2 [Xe]4f12 6s2

Tm 69 [Xe]4f125d16s2 [Xe]4f13 6s2

Yb 70 [Xe]4f135d16s2 [Xe]4f14 6s2

Lu 71 [Xe]4f145d16s2 [Xe]4f145d16s2

It clearly indicates that the 4f orbitals are not occupied regularly , mainly in case of Ce , Gd and Lu . Hence , the general electronic configuration for lanthanoids is [Xe] 4f1-

145d06s2 , with the exception of Ce , Gd and Lu . In cerium , even the sudden contraction and reduction in energy of the 4f orbitals immediately after La , is not yet sufficient to avoid occupancy of the 5d orbital . Gd has a 5d1 arrangement , leaving a half filled 4f , which leads to increased stability . Lu has a 5d1 arrangement , as the f shell is already full . On the basis of the similarity in the outer electronic configuration , scandium , yttrium and actinium should be placed along with the lanthanoids . However , due to the physical limitations of the modern periodic table , the lanthanoids and actinoids are placed separately from the main body of the periodic table . Neither scandium nor yttrium as well as lanthanum can be called properly as lanthanoides , since the 4f orbital does not have any electron . Property wise , yttrium and lanthanum are better discussed with the lanthanoids than with any other elements . On the other hand , scandium is markebly different , even though it was first isolated from yttria sources . Atomic and Ionic Radii : Lanthanoid Contraction In the periodic table , the atomic as well as ionic radii normally increase on descending down a group , due to the inclusion of extra filled shells of electrons . However , on moving from left to right across a period , the atomic and ionic radii decrease . This is due to the fact that the extra orbital electrons are not able to shield the extra nuclear charge completely and hence the increase in the effective nuclear charge is responsible for the decrease in size . The atomic and ionic radii of lanthanoids are given in Table 6.

Table 6 : Size relationship

Symbol Atomic number

Atomic radius, A0

Crystal or ionic radius, A0

Ln2+ Ln3+ Ln4+

Sc 21 1.641 0.68 Y 39 1.801 0.88 La 57 1.877 1.061 Ce 58 1.82 1.034 0.92 Pr 59 1.828 1.013 0.90 Nd 60 1.821 0.995 Pm 61 - (0.979) Sm 62 1.802 1.11 0.964 Eu 63 2.042 1.09 0.950 Gd 64 1.802 0.938 Tb 65 1.782 0.923 0.84 Dy 66 1.773 0.908 Ho 67 1.766 0.894 Er 68 1.757 0.881 Tm 69 1.746 0.94 0.869 Yb 70 1.940 0.93 0.858 Lu 71 1.734 0.848

The table clearly shows that on moving from Sc to Y to La , there is a steady increase in the atomic as well as the ionic radii . This is due to the fact that addition of electrons to higher energy levels overcome the increasing contractive effects resulting from the enhanced attraction produced by larger nuclear charge. However, as we move along the lanthanoid series , there is a decrease in atomic as well as ionic radii . A similar but more limited trend characterizes the non - tripositive ions. The contraction in size from one element to another is fairly small , so that the additive effect over the lanthanoid elements from Ce to Lu is just 0.2 A0 . This limited contraction in the atomic and ionic radii of the lanthanides is known as Lanthanoid Contraction. The shielding effect of the electrons decreases in the order s > p > d > f . In lanthanoids the additional electron enters 4f sub - shell and not the valence shell , namely , sixth shell. The “Lanthanoid Contraction” , therefore , occurs because , although each increase in nuclear charge is balanced by a simultaneous increase in electronic charge , the directional characteristics of the 4f orbitals cause the 4fn electrons to shield themselves and other electrons from the nuclear charge only imperfectly . Thus each unit increase in nuclear charge produces a net increase in attraction for the whole extranuclear electron charge cloud and each ion shrinks slightly in comparison with its predecessor . On the other hand , although a similar overall reduction is seen in the atomic radii , the trend for Eu and Yb is spectacularly irregular . Mathematically , lanthanoid contraction could be understood in terms of effective electron potential , Veff , which is expressed as :

Veff = rZeff−

+ 22)1(

rll +

Or, Veff = columbic potential + centrifugal potential The angular momentum for an f orbital (l = 3) is large and hence the centrifugal potential , which tends to keep the electrons away from the nucleus , is also large . Increase in the atomic number increases the columbic attraction to a large extent for a smaller value of n , due to a proportionately greater change in Zeff . This can be viewed empirically , to differing penetration effects , 4f orbitals (and the atoms in general) steadily contract across the lanthanoid series . Fig. 5 shows that the graph has two peaks , one at Eu and the other at Yb , which possess exceptionally high values for their atomic radii. This is regarded to be due to the difference in their metallic bonding . Most of these metals are composed of a lattice of Ln3+ ions with a 4fn configuration and three electrons in the 5d / 6s conduction band . Metallic Eu and Yb , however , are composed predominantly of larger Ln2+ ions with 4fn+1 configurations and only two electrons in the conduction band , thus leaving behind half - filled and completely filled 4f orbitals , respectively . The rough parallel between these metals and barium supports this contention. Correspondingly , the slightly reduced atomic radius of cerium is due to the presence of ions in an oxidation state somewhat above +3 . Similar discontinuities are found in other properties of the metals , particularly at Eu and Yb . The decrease in the crystal or ionic radii is indicated in Fig 6 . The general decrease in crystal radius from Ln2+ to Ln3+ to Ln4+ reflects the effect of increasing cationic charge.

Fig.5: Atomic radii of lanthanoides, barium and Hafnium

Fig.6: Crystal radii of Ln2+ , Ln3+ and Ln4+ ions It is to be noted that a contraction resulting from the filling of the 4f electron shell is of course not exceptional . Similar contraction occurs in each row of the periodic table. In the d – block for instance , the ionic radii decrease by 20.5 pm from Sc3+ to Cu3+ ,

and by 15 pm from Y3+ to Ag3+ . However, the importance of the lanthanide contraction arises from its consequences:- 1] Basic character of oxides , Ln2O3 and hydroxides , Ln(OH)3 : The atomic and ionic radii affect those properties of the metal and their respective cations , which mainly reflect the attraction or lack of attraction for the electrons . This includes basicity . In broad terms , basicity is a measure of the ease with which a species loses electrons and it decreases with decrease in the atomic or ionic radii . In terms of ionic radii , basicity may be expected to decrease in the order : La3+ > Ce3+ > Pr3+ > Nd3+ > Pm3+ > Sm3+ > Eu3+ > Gd3+ >Tb3+ > Dy3+ > Ho3+ > Y3+ >Er3+ >Tm3+ > Yb3+ >Cu3+ > Sc3+ Amongst the tripositive ions , due to the decrease in size from La3+ to Lu3+, the covalent character in Ln(III) hydroxides increases . This is in accordance with Fajan’s rule . Hence , the basicity of the hydroxides decreases from La to Lu . Thus La(OH)3 is the most basic while Lu(OH)3 is the least basic hydroxide . The hydroxide of more highly charged Ce4+ is even more basic than any tripositive ion . Basicity differences are reflected in the hydrolysis of ions – more basic ion hydrolyses less readily ; solubilities of salts ; thermal decomposition of oxy - salts – more basic oxy - salts decompose with difficulty ; and the formation of complex species . 2] Occurrence of yttrium with heavy lanthanoids :The magnitude of lanthanoid contraction is such that the ionic radii of Y3+ (0.88 A0) is reached in the holmium - erbium region ( 0.894 - 0.881A0 ) . This similarity in ionic radii , along with the equality in ionic charge (i.e. +3) accounts for the invariable natural occurrence of yttrium with these heavier lanthanoids . The marked similarity in the crystal structure , chemical properties and solubility between yttrium compounds and those of the heavier lanthanoids make it difficult to separate yttrium from heavy lanthanoids . In fact , the behaviour of yttrium is considered to be so characteristic of these elements that these elements are referred to as yttrium earths . It is to be noted that the marked differences observed between the chemistry of scandium and that of the lanthanoids are consistent with the fact that Sc3+ is much smaller than even the Lu3+ ion . 3] Melting and boiling point : The hardness and melting and boiling points of the elements increase from Ce to Lu . This is because of the increase in attraction between the atoms , with decrease in size . 4] Separation of lanthanoids : The chemical properties of an ion are mainly dependent on its size and charge . For the various Ln3+ ions , since the charge remains the same and the decrease in size is just marginal , their chemical properties are very similar , making their separation difficult . But , it is this small variation in properties that permits the separation of the lanthanoids by fractional means . 5] Effect on the post - lanthanoid elements : The elements which follow lanthanoids in the third transition series of the periodic table , are called post - lanthanoid elements . Due to the lanthanoid contraction , the post - lanthanoid elements are considerably smaller than expected .

Normally , the covalent radii increase with increase in the atomic number , on moving down the group . This , in fact , holds good when we compare the covalent radii values of the elements of the first , second and the third transition series (Table 7 ) . Table 7 : Covalent radii of the transition elements (Αο)

Sc 1.61

Ti 1.32

V 1.22

Cr 1.17

Mn 1.17

Fe 1.17

Co 1.16

Ni 1.15

Y 1.80

Zr 1.45

Nb 1.34

Mo 1.29

Tc -

Ru 1.24

Rh 1.25

Pd 1.28

La 1.87

Hf 1.44

Ta 1.34

W 1.30

Re 1.28

Os 1.26

Ir 1.26

Pt 1.29

On comparing the covalent radii values of the elements of the second transition series with those of the third series , it is observed that the normal increase in the covalent radii values from Y to La disappears after lanthanoids . Thus pairs of elements such as Zr / Hf , Nb / Ta and Mo / W are almost identical in size . The close similarity of properties in such a pair makes their chemical separation difficult . The sizes of the third row of the transition elements are very similar to those of the second row of transition elements . Thus , the second and the third rows of transition elements resemble each other more closely than do the first and second rows . These similarities between members of the second and third transition series continue through the platinum metals to at least silver and gold . As a consequence of the contraction in the atomic radii of the elements of the third transition series , the packing of atoms in their metallic crystals become so much compact that their densities become very high . Thus , while the densities of the elements of second transition series are only slightly higher than those of the elements of first series , the densities of the elements of third transition series are almost double than those of the elements of the second transition series . Hence , the increase in atomic mass from 91.2 to 178.4 g/mol and almost similar radii leads to an increase in density from 6.51 to 13.35 g/cm3 , on moving from Zr to Hf . Indeed , due to similar chemical behaviour , Hf (1923) was discovered 134 years later than Zr(1789) . Oxidation States The ionization enthalpy and standard electrode potential values for the lanthanoids are listed in table 8 . The sum of the first three ionization enthalpy values is low. Thus , the oxidation state ( + 3 ) is ionic and the stability of Ln3+ dominates the chemistry of these elements . In just the same way as for other elements , the higher oxidation states occur in the fluorides and oxides and the lower oxidation states occur in the other halides , mainly bromides and iodides . Oxidation numbers (+2 ) and (+4) do occur , particularly when they lead to a noble gas configuration (f0 in Ce4+) , or a half - filled f shell (f7 in Eu2+ and Tb4+) , or a completely filled f shell (f14 in Yb2+) . In addition , (+2 ) and (+4) states also exist for elements that are close to these configurations . Thus , Sm2+ and Tm2+ occur with f6 and f13 configurations , while Pr4+ and Nd4+ have f1 and f2 configurations , respectively (table 9 ) . However , of the non - tripositive state , only tetrapositive cerium , praseodymium , and terbium and dipositive samarium , europium and ytterbium have sufficient chemical stability .

Table 8 : Some properties of the lanthanoid elements

Element Ionization enthalpy / kJmol-1

E0(M4+/M3+)/V

E0(M3+/M2+)/V

E0(M3+/M) /V

∆atmH /kJmol-

1

∆hydH /kJmol-

1

1st 2nd

3rd

Ce 541 1047 1940 1.61 - -2.483 419 -3370 Pr 522 1018 2090 ∼2.860 - -2.462 356 -3413 Nd 530 1034 2128 - - -2.431 328 -3442 Pm 536 1052 2140 - - -2.423 301 -3478 Sm 542 1068 2285 - -1.000 -2.414 207 -3515 Eu 547 1085 2425 - -0.360 -2.407 178 -3547 Gd 595 1172 1999 - - -2.397 398 -3571 Tb 569 1112 2122 2.7 - -2.391 389 -3605 Dy 567 1126 2230 - - -2.353 291 -3637 Ho 574 1139 2221 - - -2.319 301 -3667 Er 581 1151 2207 - - -2.296 317 -3691 Tm 589 1163 2305 - - -2.278 232 -3717 Yb 603 1175 2408 - -1.205 -2.267 152 -3739 Lu 513 1341 2054 - - -2.255 - -3760

Table 9 : Distinguishing electronic configuration for observed oxidation states

Symbol

Configuration

+2

+3

+4

La [Xe]4f0(La3+) Ce [Xe]4f2(CeCl2) [Xe]4f1(Ce3+) [Xe]4f0(Ce4+) Pr [Xe]4f2(Pr3+) [Xe]4f1(PrO2 ,

Na2PrF6) Nd [Xe]4f4(NdI2) [Xe]4f3(Nd3+) [Xe]4f2(Cs3NdF7) Pm [Xe]4f4(Pm3+) Sm [Xe]4f6(Sm2+) [Xe]4f5(Sm3+) Eu [Xe]4f7(Eu2+) [Xe]4f6(Eu3+) Gd [Xe]4f7(Gd3+) Tb [Xe]4f8(Tb3+) [Xe]4f7(TbO2 ,

TbF4) Dy [Xe]4f9(Dy3+) [Xe]4f8(Cs3DyF7) Ho [Xe]4f10(Ho3+) Er [Xe]4f11(Er3+) Tm [Xe]4f13(TmI2) [Xe]4f12(Tm3+) Yb [Xe]4f14 (Yb2+) [Xe]4f13(Yb3+) Lu [Xe]4f14(Lu3+)

The direct correlation between oxidation state and electronic configuration is the exception rather than the rule . Since , 6s2 configuration is the most characteristic configuration for the lanthanoids, a uniform dipositive state might be expected . However, the dominance of the +3 oxidation state results from the stabilizing effects of increasing positive charge on different orbitals . When electrons are removed from a lanthanoid atom , the orbitals are stabilized in the order 4f > 5d > 6s . This is the same order in which the orbitals penetrate through the inert core of electrons towards the nucleus . By the time an ionic charge of +3 is reached , the 4f have become core - like , with any remaining electrons occupying the 4f , leaving 6s and 5d orbitals empty . Removal of further electrons from the core then becomes energetically unfavourable. This explanation can further be expanded by examining the relative stabilities of other oxidation states from a thermodynamic view point with the help of fig. 7 , 8 , and 9 .

Fig.7: Plot of I1 , I2 , I3 , I4 and ∆Hatom for lanthanoids

Fig. 8(a): Plot of [ I1 + I2 + I3 + ∆atomH] Fig. 8(b): Plot of (b) [ ∆hydH(Ln3+)]

Fig. 8(c): Plot of (c) [I1 + I2 + I3 +∆atomH + ∆hydH(Ln3+)] Fig. 9 : Plot of [ I1 + I2 + I3 + I4 + ∆atomH]

Stability of +2 oxidation state - a so called “ anomalous” state The stability of the +2 oxidation state can be studied by examining ∆rG for a process such as : Ln2+ (aq) + H + (aq) Ln3+ (aq) + ½ H2 (g) A thermodynamic cycle for this reaction can be constructed as:- Ln2+ (aq) + H+ (aq) ∆rH Ln3+(aq) + 1/2 H2(g) +∆hydH(Ln2+) -∆hydH(Ln3+) Ln2+(g) + H+(aq) Ln3+(g) + 1/2 H2(g) I3 + (∆hydH(H+)) According to the Hess’s law of constant heat summation : ∆rH = ∆hydH(Ln2+) + I3 + ∆hydH(H+) + (-∆hyd H(Ln3+)) also , ∆rG = ∆rH - T∆rS

Hence , ∆rG = [ ∆hydH(Ln2+) + I3 + ∆hydH(H+) + (-∆hyd H(Ln3+) )] - T∆rS

∆hydH(Ln2+) , ∆hydH(H+) and (-∆hyd H(Ln3+)) represent the enthalpy of hydration for Ln2+ , H+ and Ln3+ , respectively . I3 is the third ionization enthalpy for the lanthanoid ,. ∆rH is the enthalpy of reaction , T is the temperature , ∆rG is the Gibb’s free energy and ∆rS is the entropy change for the reaction . Here , ∆hydH(H+) remains constant and can be ignored , as variation in ∆rG across the Ln series is sought . Due to the lack of data , it is assumed that ∆rS is constant across the series – as for the 3d metals . Hence, ∆rG = I3 + [ ∆hyd H(Ln2+) + (-∆hyd H(Ln3+) )] + constant Since the ionic radii moves smoothly along the series, ∆hydH(Ln2+) and ∆hydH(Ln3+) are also expected to vary regularly . ∆hydH(Ln3+) is larger in magnitude than ∆hydH(Ln2+) due to the higher charge and smaller radius of the ion with higher oxidation state . The term in square brackets is thus negative and favours the +3 oxidation state. As the ions gets smaller across the series , this term smoothly increases so that the +3 state becomes more favoured . On the contrary , I3 increases across the series , disfavouring the existence of +3 oxidation state . However , the dominance of the +3 oxidation state arises from the balance between the increasing enthalpy of hydration and the increasing ionization enthalpy . The occurrence of the +2 state for some of the lanthanoids arises from the non - smooth changes in the third ionization enthalpy values .

The overall trend across the series, in general, is for the increase in I3 values. This is due to the increase in nuclear charge across the f – block. This outweighs the increasing repulsion between the electrons. However , the ‘break’ between f7(Eu2+ ) and f8(Gd2+) cannot be explained easily . This break is actually due to the extra stability associated with the f7 (half – filled) configuration. Similarly the higher than expected I3 for Yb2+ with f14 (fully - filled) configuration can also be explained on the same basis . This can be explained even on the basis of the Hund’s rule , according to which the electrons prefer to adopt arrangements in degenerate orbitals so as to have maximum number of parallel spins . Destroying pairs of such parallel spins requires extra energy. In Gd2+(f8) , there are seven electrons with parallel spin and one with anti - parallel spin . A pair of electrons with anti - parallel spins repels more than ‘expected ’ and so is destabilized , leading to a low I3 , despite the increase in nuclear charge . A similar decrease in I3 is observed on moving from Yb2+ to Lu2+. To understand small irregularities in the third ionization energy values, a detailed inspection of the electronic configuration is required. For example , for Pr2+ (4f3 ) and Pm2+(4f5 ) , Hund’s ule predicts the lowest energy by way of arranging the electrons as : r

ml 3 2 1 0 -1 -2 -3

All the three electrons have ml values of the same sign and can thus be taken to be orbiting in the same direction . There is slightly less repulsion between electrons which are orbiting in the same direction as they come across one another less frequently . The electrons are thus repelling one another slightly less than expected and hence ionization enthalpy is higher than expected. Now consider Pm2+(4f5) , for which the lowest energy arrangement is : ml 3 2 1 0 -1 -2 -3

The electron that is removed upon ionization is that with ml = -1 . As this is orbiting in the opposite direction to the other electrons, it is repelled slightly more than ‘expected’ and its ionization becomes easier. Thus, the ionization enthalpy of Pm2 + is less than expected. This is known as quarter - shell effect. An identical situation occurs between f10 and f12 in the second half of the series leading to the three quarter - shell effect. The I3 values of Nd and Dy are thus larger than expected, leading to the occurrence of their compounds in the +2 oxidation state. Amongst the various lanthanoids in dipositive state, samarium, europium, and ytterbium have sufficient chemical stability to be of importance to exist in aqueous solution and to form series of compounds. Standard electrode potential values indicate that in aqueous solution, these ions are all strong reducing agents, with reducing strength decreasing as : Sm2+ > > Yb2+ > > Eu2+

Stability of +3 oxidation state – the “characteristic” state

The observed oxidation states of the lanthanoids noted either in solution or in insoluble compounds (Table 9) , clearly indicates that whatever is the electronic configuration of the lanthanoids in the ground state , they all form the tri - positive ions . This can be examined more closely by considering the following thermodynamic cycle : Ln (s) ∆H0 Ln3+ (aq) ∆atmH - ∆hyd H(Ln3+) Ln (g) I1 + I2 + I3 Ln3+ (g)

According to the Hess’ s law of constant heat summation : ∆H0 = ∆atmH + I 1 + I2 + I3 + (-∆hyd H(Ln3+))

where ∆atomH is the enthalpy of atomization of the lanthanide ; I1, I2 and I3 are the first, second and third ionization enthalpies respectively and ∆hydH(Ln3+) is the enthalpy of hydration for Ln3+(g) . The standard electrode potential, E0

(Ln3+ (aq) / Ln (s) ) can

be calculated using a thermodynamic relation ∆rG0 = ∆H0 - T∆rS0 = - n FE0

The value of standard electrode potential of a lanthanoid thus depends on the balance between the energy supplied in the form of enthalpy of atomization and the sum of the first three ionization enthalpies [ ∆atm H + I1 + I2 + I3 ] , and the energy released in the form of enthalpy of hydration for Ln3+(g) . These enthalpy terms are graphed in Fig 8. The production of Ln3+ (g) shows a smooth trend, based on size effects. Shell structure effects superimposed with clear maxima at half shell (f 7 ) and full shell (f 14) are also reflected . The enthalpy of hydration of Ln3+(g) shows only a smooth ionic size based trend and no shell structure effects . Balance of trends in ionization enthalpy along with enthalpy of atomization and hydration reflects the dominance of the shell effects over the size effects. It can also be shown that the tri-positive state is a more preferred state in comparison to +2 and +4 state. In other words, in aqueous solution, oxidation represented by the equation:

Ln3+(aq) + 41

O2 (g) + H+ (aq) Ln4+ (aq) + 21

H2O (l)

or reduction represented by the equation :

Ln3+ (aq) + 21

H2 (g) Ln2+ (aq) + H+ (aq)

are not very favourable . The conversion from one oxidation state to another in aqueous solution is controlled by the magnitudes of the energy required to remove an electron from the gaseous ion in its lower oxidation state (i.e. , ionization

enthalpy) and of the energy released when gaseous ions combine with water to form the hydrated species (enthalpy of hydration) . Calculations show that in solution , all tetra - positive species (except possibly Ce4+) and all di - positive species (except Eu2+) must revert to the tri – positive state . This leads to an important conclusion that the tri - positive state owes its general stability to a somewhat fortuitous combination of ionization and hydration enthalpies rather than to any particular electronic configuration . The ease of formation of the tri - positive state in solution is indicated by the standard electrode potential data , summarized in table 8 . It is apparent that the elemental lanthanoids are very powerful reducing agents and their oxidation to the tri – positive state occurs readily and vigorously. Similarly , oxidation and reduction among solid compounds can be related to the magnitude of the ionization enthalpy and the energy released when the gaseous ions combine to produce crystalline solids (i.e., the enthalpy of lattice formation) . Again , a fortuitous combination of these energy terms renders the tri - positive state the most common , but the energy conditions are more favourable to the existence of non tri - positive species (table 9) in the solid state than in solution . Stability of +4 oxidation state - Another “Anomalous” state :- The occurrence of the +4 oxidation state for Ce , Pr and Tb can be treated in a similar manner by examining ∆rG for a process such as :-

LnX3 (s) + 21

X 2(g) LnX4 (s)

where X is a halide . A thermodynamic cycle for this reaction can be constructed as :

LnX3 (s) + 21

X2(g) ∆rH LnX4(s)

∆diss H U (LnX3) X (g) -U (LnX4) -∆egH Ln3+(g) + 3X-(g) + X- (g) I4 Ln4+(g) + 4X- (g) where , U is the enthalpy of lattice formation , ∆diss H is the enthalpy of dissociation of X2 and ∆eg H is the electron gain enthalpy for X(g) . According to Hess’s Law of constant heat summation : ∆rH = U(LnX3) + ∆dissH (X2) + (- ∆egH (X)) + I4 + (-U (LnX4))

Or , ∆rG = [ I4 + [ U (LnX3) + ( - U(LnX4))] + ∆diss H(X2) + (- ∆egH(X))] - T∆rS Or, ∆rG = I4 + [ U (LnX3) – U(LnX4) ] + constant

In an ionic model , the enthalpy of lattice formation is directly proportional to the charges of the ions (Z+ and Z) and inversely proportional to the sum of the ionic radii ( r+ and r-) i.e.

U ∝ )()(

−+

−+

+ rrZZ

Since the charge Z+ on lanthanoid is higher in LnX4 and the radii of Ln4+ is smaller than that of Ln3+ , the enthalpy of lattice formation is greater for LnX4 . Hence , the terms in the square brackets become negative , to favour the +4 oxidation state . As the radii of the lanthanoide ion in +3 and +4 oxidation states are expected to decrease smoothly across the series , this term also gets more negative across the series . However , the high values of I4 favours the existence of lanthanoide ions in +3 oxidation state , but still the +4 oxidation state is stable where the fourth ionization enthalpy values are low ( fig. 7 ) . Ce(+4) and Pr (+4) can be formed as the nuclear charge is lowest at the start of the series . Tb(+4) can be formed as I4 corresponds to the process of removal of an electron from Tb(+3) with f8 configuration to form Tb(+4) , with a more stable f7 configuration . The plot of (∆atmH + I1 + I2 + I3 + I4) also favours +4 oxidation for Ce , Pr and Tb , to some extent (fig . 9) . Separation of the Lanthanoid Elements As discussed earlier, the lanthanoids are all typical trivalent , almost identical in size and chemical properties . Hence, the separation of one lanthanoid from another is an exceedingly difficult task . The methods of separation exploit slight differences in their basic properties , stability and solubility . Most of the methods involved in separation are fractional in character , where in each step there is a concentration of one species at the expense of the other , but the separation results only if that step is repeated many times . The efficiency of a fractional procedure is given by the magnitude of its separation factor (∝). For two lanthanoids , Ln and Ln′ , being changed from an initial concentration condition (1) to a final one (2) , the separation factor , ∝ may be defined as

∝ = )1()2('

)1(')2(

)1(')1(

)2(')2(

..

//

LnLn

LnLn

LnLn

LnLn

CCCC

CCCC

=

No separation results for ∝ = 1 , but greater the departure of ∝ from unity , the more efficient the process is . Some classical techniques for the separation of lanthanoides are discussed below : Fractional crystallization: This technique utilises the slight differences in the solubility of various lanthanoid salts. Various anions found to form soluble salts with tri – positive lanthanoid ions have been investigated . However , only those anions

which yield easily crystallizable isomorphous salts , possessing measurable solubility differences as the Ln3+ ion changes , can be effective . Such systems are very limited . The basicity decreases along the series , resulting in decrease in solubility of their hydroxides , oxides , carbonates and oxalates . Thus, the salts at the Lu end of the series crystallize out first . Not only simple salts such as Ln (BrO3)3. 9H2O and Ln (NO3)3. 5H2O , but even double salts like Ln(NO3)3 . 3Mg(NO3)2. 24H2O and K2CO3 . Ln 2 (CO3)2 .12H2O are used . This technique is very tedious and needs to be repeated many times to obtain good separation. The mixed solution of lanthanoid salts in water is evaporated to such an extent that about half of the dissolved salts crystallize out on cooling. The crystals obtained are separated , dissolved and the solution is again evaporated , to such an extent that on cooling it would deposit about one – half of the dissolved salts as crystals . Even the mother liquor is further evaporated, so that about one – half of the dissolved salt is obtained as crystals. Each series of operation gives one more fraction and such series of operation are repeated a number of times, until the desired degree of separation is achieved . The systematic and tedious fractional crystallization of any given mixture of lanthanoids results in the separation of less soluble components (such as the salts La3+ , obtained as crystals) from the more soluble components (such as the salts of Sm3+ obtained in the mother liquor) . The intermediate components such as the salts of Pr3+ and Nd3+ , are distributed between the final crop of crystals and the mother liquor , in the order of solubility . Fractional crystallization is normally most effective at the lanthanum end of the series where the solubilities differ the most . The most effective compounds used are :-

(1) Double ammonium nitrates , Ln (NO3)3. 2NH4NO3 . 4H2O for the removal of lanthanum and the separation of praseodymium from neodymium.

(2) The double manganese nitrates , 2Ln (NO3)3. 3Mn(NO3)2. 24H2O for the separation of members of cerium group (but not the yttrium group) .

(3) The bromates , Ln (BrO3)3. 9H2O , and the ethyl sulphates , Ln (C2H5SO4)3 . 9H2O , for the separation of the members of the yttrium group . Separation in the samarium - europium - gadolinium region and within the yttrium group , using this technique is inefficient and often involves hundreds or even thousands of crystallizations . Even a non - lanthanoid such as Bi (III) can be used as a separating ion . If bismuth (III) nitrate is added to a mixture of cerium group nitrates , containing magnesium ion , its double magnesium nitrate crystallizes between the isomorphous ones of Sm3+ - Bi3+ and Bi3+ - Eu3+ mixtures . Bismuth can be removed readily with hydrogen sulphide . This tedious method of separation is later on, however, replaced by a more effective technique i.e. solvent extraction technique, which can be used on a large scale. Fractional Precipitation Method : The separation by this method depends fundamentally upon the differences in the basic character of hydroxides of lanthanoid elements . However , these differences can be operative only when there exists true equilibration between the solution and the solid phase . The condition is

hardly fulfilled in practice and for addition of a precipitating anion to a solution of mixed lanthanoids , all the species are precipitated out due to localized excess of reagent . The precipitates are then separated out and are dissolved . The redissolution may then be too slow or too far from completion to permit true equilibration before more reagent is added . This problem is avoided either by diluting the gaseous precipitating agent such as ammonia with inert nitrogen gas or by generating the precipitating ion slowly and homogeneously throughout the solution. Examples of such precipitating agents include generation of hydroxyl ion by the cathodic decomposition of water , carbonate and hydroxyl ions by adding urea and heating , and oxalate ion by thermally hydrolyzing dimethyl oxalate . Another method of controlling the precipitating agent is by addition of another reagents that may give a complex species . For example , addition of solid oxides (eg. MgO, Ln2O3) or carbonates (eg. PbCO3) provides a slowly increasing and controlled source of hydroxyl ions . Sometimes , even a weak complexing agent like EDTA is added before the addition of a precipitant . These reagents tie up all of the lanthanoid ions and hence the least readily complexed lanthanoid ion is the most available one for precipitation . Let us now discuss this technique in detail by using hydroxyl ions as precipitants . The basic character of the hydroxides of lanthanoid elements decreases from La to Lu. Hence , the hydroxides of “cerium group minerals” are more basic than those of “yttrium group minerals”. On adding a precipitating agent , like hydroxyl ions into a solution containing a mixture of lanthanoid nitrates , the hydroxides of the “heavy earths” are preferentially precipitated out , with those of “light earths” being left behind , in the solution . Within a given sub - group, the individual lanthanoids are separated by further fractional precipitation. However, the complete separation of the adjacent members in a given sub – group by this method is not possible, since the basicity differences in the hydroxides of lanthanoid elements are small. Fractional Thermal Decomposition Of Oxy Salts: The temperature at which an oxy salt (eg. acetate , sulphate , nitrate) is converted into an oxide or a basic salt of lower solubility , decreases with decreasing basicity of the tri – positive cation . Yttrium is concentrated into the more basic fractions and hence is effectively separated from the heavier lanthanoids . When a mixture of Ln (NO3)3 is fused, it undergoes thermal decomposition, i.e. a temperature will be reached when the least basic nitrate changes to the oxide . The mixture is leached with water , the nitrates are dissolved and can be filtered off , leaving behind the insoluble oxides . The oxides are then removed, dissolved in HNO3 and the process is repeated. Apart from these classical methods of separation which are generally employed for small scale separation, some modern techniques have also been devised. Some of the modern methods now used for separating individual lanthanides are discussed below: Solvent Extaction : The separation of lanthanoids by this method was introduced to separate and identify various lanthanoids produced by the fission of uranium.

This method is important and used for large scale separation. This method is based on the difference in solubility of lanthanoid salts in water and in an immiscible organic solvent. For example, the heavier lanthanoid ions are more soluble in an organic solvent, than the lighter ions. Their solubilities in water and ionic solvents, however, are reversed. The technique involves the extraction of aqueous solution of lanthanoid salts in a continuous counter - current process into a non - polar organic liquid. The most widely used extracting solvent is tri - n - butyl phosphate , (m BuO)3 PO , ( TBP ) , in an inert medium like kerosene , to extract the lanthanides from nitric acid solution . TBP form complexes with Ln3+ (aq) ions in presence of NO3

- ion and hence, the complex moves into the organic phase. Ln3+ (aq) + 3NO3

- (aq) + 3TBP(org) Ln (NO3)3 (TBP)3 (org) The distribution ratio (λ) represents the distribution of lanthanoid ions between the two phases . Hence , λ = Total concentration of solute in one solvent Total concentration of solute in other solvent = [Ln(NO3)3(TBP)3(org)] [Ln3+(aq)] For two tri – positive lanthanoide ions , Ln/ 3+ and Ln3+, the separation factor , ∝ is given as ,

∝ = λλ '

= [Ln/ (NO3)3(TBP)3(org)][Ln3+(aq)] [Ln (NO3)3(TBP)3(org)][Ln/ 3+(aq)] The distribution equilibrium may be described as :

K = [Ln (NO3)3(TBP)3(org)] [Ln3+(aq)][NO3

-(aq)]3[TBP(org)]3

Hence , a relation between K and λ can be established as : λ = K [NO3

- (aq)]3 [ TBP (org)]3

A plot of log10 λ vs. [NO3- (aq)] at constant concentration of TBP should give

a straight line with a slope 3 . This is , however , true for cases with the concentration of nitric acid above 10 M . It is also to be noted that the extent of extraction increases with decreasing atomic radii . A mixture of La (NO3)3 and Gd (NO3)3 can be easily extracted using TBP. The separation factor for La(NO3) and Gd(NO3)3 is 1.06 , for 15.8 M nitric acid – 100% TBP system . The difference no doubt is very small, but by using this technique a very large number of partitions can be performed automatically.

Another organic solvent that can be used as extractant is bis(2-ethyl hexyl) phosphoric acid i.e. DEHPA ( fig 10(a)). Lanthanoids of high purity (∼ 95%) have been obtained using this solvent. This reagent is a more efficient extractant , as well as more resistant to hydrolytic decomposition (in presence of aqueous nitric acid ) in comparison to TBP. Esters of this type better represented as HDGP exists as dimers in the organic phase. Ln3+ (aq) + 3(HDGP)2 (org) Ln[(HDGP)2]3 (org) + 3H+ (aq)

The distribution ratio may be derived to be λ = K [(HDGP)2 (org)] [H+ (aq)]3

An average separation factor of 2.5 for adjacent lanthanoids has been reported . Such esters form chelates , having the units of the type drawn in fig 10(b). CH3(CH2)3 CH(Et) CH2O OH P CH3(CH2)3 CH(Et) CH2O O (a) CH3(CH2)3 CH(Et) CH2O O H O OCH2 P P CH3(CH2)3 CH(Et) CH2O O O OCH2 Ln/3

(b) Fig 10 : Structure of (a) DEHPA ; (b) a unit of [Ln(HDGP)2]3 Solvents such as mono alkyl phosphoric acids , RP(O) (OH)2 ; pyro phosphate esters , (RO)2P(O)-O-P(O)(OR)2 ; 1 , 3 - diketones and 8 – quinolinols have also been used for extraction . However , these solvents are not as successful as TBP and DEHPA . Solvent extraction , no doubt is an efficient technique for separation of lanthanoids . This technique is mainly helpful in separation and purification of Ln3+ ions from Ce4+ and Th4+ ions , using TBP . La3+ is somewhat less readily extracted than the other tripositive ions and hence can be separated out easily . This technique is also useful for separation and purification of scandium .

Complex Formation : - In the presence of complexing agent like ethylenediammine tetraacetic acid ie : EDTA , the lanthanoids can be separated by precipitation , as oxalates . When a mixture of lanthanoids is treated with EDTA , all the ions form complexes . However , they differ in their stabilities . The heavy lanthanoids form stronger complexes than the lighter lanthanoids . The oxalates of the lanthanoids are insoluble but addition of oxalate ions to this solution does not give a precipitate , since the La3+ ions are not freely available and are complexed with EDTA . If some acid is also added to the solution , the less stable EDTA complexes decompose first and are immediately precipitated as the oxalates . Hence , the lighter lanthanoids such as Ce3+ , Pr3+ , Nd3+ are precipitated out first , which are filtered off . The separation is not complete , so the oxalates are redissolved and the process is repeated many times . Change of oxidation states by selective oxidation or reduction : +3 oxidation state is most characteristic of all the elements of lanthanoid series . Some of the lanthanoids also show +2 and +4 oxidation states in addition to +3 state . But the properties of Ln4+ and Ln2+ are so different from those of Ln3+ that their separation becomes fairly easy . Cerium can be separated from a mixture of lanthanoids , as it is the only lanthanoid which has a stable (+4) oxidation state in aqueous solution . Oxidation of a solution containing a mixture of the lanthanoid ions in +3 oxidation state with NaOCl , or KMnO4 or chlorine , in alkaline medium , leads to the oxidation of Ce3+ to Ce4+ .only . Since Ce4+ ion is smaller , less basic and less soluble than any Ln3+ or Ce3+ ion , Ce4+ can be easily precipitated as Ce(OH)4 by the addition of a small amount of an alkali to a solution of mixture of lanthanoids . Eu can be separated from a solution containing various Ln3+ ions by reduction of Eu3+ to Eu2+ . Zn - amalgam is the main reducing agent and once Eu3+ is reduced to Eu2+ , the latter can be precipitated as insoluble EuSO4 . It is to be noted that the sulphates of all the trivalent lanthanoids are soluble in water . Lanthanoids such as europium , samarium and ytterbium can be removed effectively from mixtures by their reduction to amalgams . This can be carried out either by extracting a buffered acidic solution with liquid sodium amalgam or by electrolyzing an alkaline citrate solution with a lithium amalgam cathode . This method of separation is convenient , rapid and can be operated continuously on a large scale . Ion exchange: This is the most effective method now available for the separation of lanthanoids. The method utilizes synthetic cation exchange resins , which are generally organic compounds containing sulphonic (- SO3H) or carboxylic acid (- COOH) groups . Thus , if an aqueous solution containing a mixture of Ln3+ ions is allowed to pass down a column packed with a cation exchange resin (represented as HR(s)) , the Ln3+ ions replaces the equivalent number of H+ ions of the resin and

hence themselves get fixed on the resin . This exchange of Ln 3+ ions in solution with protons on a solid ion exchanger is an equilibrium process , represented as : Ln3+(aq) + 3HR (s) LnR3 (s) + 3H+ (aq) The tenacity with which a cation is held by an ion exchanger decreases with decrease in its crystal radii and charge as : Th4+ > La3+ > Ce3+ > Y 3+ > Lu3+ > Ba2+ >Ca2+ > K+ > NH4

+ > H+

Hence , amongst the tripositive lanthanoide ions , La3+ is attached to the resin in the column with maximum and Lu3+ with minimum firmness . In order to remove the Ln3+

ions , fixed on the resin as LnR3 , the column is eluted with an eluting agent . Generally , sodium chloride is used as an eluting agent . But for the separation of lanthanoids , it is not considered very effective , as it results in a low degree of separation . However , when eluted with a suitable complexing agent , such as a buffer solution of ammonium citrate - citric acid and amine polycarboxylates , separation of lanthanoid ions into individual bands occurs on the column and the species ultimately leave the column in reverse order of their atomic numbers (Fig 11) . Hence , the fractionating ability of an ion exchanger is enhanced by a complexing agent . This can be explained on the basis of the value of separation factor ∝ .

Fig 11 : Graphical representation of the elution order for the lanthanoids The distribution of a lanthanoid between a solid exchanger (HR or simply r) and an aqueous phase in terms of equilibrium : Ln3+(aq) + 3HR(s) Ln(R)3(s) + 3H+ (aq) is described as : λ = [Ln3+(r)] / [ Ln3+(aq)] where λ is the distribution coefficient . For convenience, neglecting the ionic charges, we get

λ = )]([

)]([aqLnrLn

Similarly , for a second lanthanide (Ln / ) λ0 =

)]('[)]('[

aqLnrLn

Thus , separation factor , ∝ = λλ0

= )]()].[('[)]()].[('[

rLnaqLnaqLnrLn

Under these conditions , the value of ∝ remains almost equal to unity , making the separation of Ln and Ln/ difficult . However , if a chelating agent (say , AA) is introduced , the additional equilibria are also established in the solution . Ln(aq) + n(AA)(aq) Ln (AA)n(aq)

and , Ln/ + n(AA) Ln/ (AA)n The formation constant , K , for the formation of Ln(AA)n can be expressed as ;

= nAALnK )( n

n

aqAAaqLnaqAALn

)])()][(([)]()([

or , [Ln(AA)n (aq)] = nAALn aqAAaqLnK

n)])()].[((.[)(

Hence , in presence of the chelating agent , the total concentration of the lanthanoid ion in aqueous phase is the sum of the uncomplexed and complexed ion . The distribution coefficient , λ , (for Ln), thus get modified to λ/ .

λ/ = )]}()[ln()]({[

)]([aqAAaqLn

rLn

n+

or , λ / = })])()][(([)]({[

)]([

)(n

AALn aqAAaqLnKaqLnrLn

n+

= ))])([(1)](([

)]([

)(n

AALn aqAAKaqLnrLn

n+

or, ))])([(1(

')(

nAALn aqAAK

n+

=λλ

If the formation constant is sufficiently large , the above equation reduces to :

nAALn aqAAK

n)])([(

')(

λλ =

Similarly , the behaviour of other lanthanoid ion , Ln / would be described in terms of K/ Ln’(AA)n and λ0

/ , in presence of a complexing agent . Hence , the separation factor , ∝/ , in presence of a complexing agent is represented as :

∝/ = n

n

AALn

AALn

KK

)('

)(00

'''

λλ

λλ

=

Thus , if a complexing agent gives complexes of different stability with different lanthanoid ions , the value of separation factor will deviate largely from unity , making the separation of lanthanoid ions easy . This holds good only when the lanthanoid ion reacts with the complexing agent in 1:1 mole ratio , resulting in the formation of a single complex species . It is also assumed that the chelated species are of sufficient stability, so that almost all the lanthanoid ions in the solution exist as complex species . This is true with chelates such as amine polycarboxylates , but with citrates , metal chelates of variable compositions are formed , depending upon the pH . A buffer solution of ammonium citrate – citric acid is thus used so that the pH is sufficiently high (5.0 – 8.0) and the various equilibria of the type Ln(AA)m-

n + H+ H Ln (AA)n(m-) + 1

can be avoided and only the species [Ln(cit)2]3- is significant . Using this complexing agent , the separation is rapid , effective , complete and also provides hydrogen ion as a suitable retaining ion . By this trechnique , large quantities of lanthanoids of purity better than 99. 99% can be separated . However, it is not important commercially due to its low capacity. Other complexing agents such as amine polycarboxylates and ethylenediamine tetraaceteic acid (EDTA) have also been found to be effective eluants for separating the lanthanoide ions . Infact , the average separation factor for adjacent lanthanoids is observed to be 2.4 , using EDTA . Another more easily adapted technique is “displacement chromatography”. In this technique, two separate columns of cation exchange resin are employed . The first column is loaded with a mixture of various tripositive lanthanoid ions and the second column , known as development column , is loaded with a salt of retaining ion such as Cu(II) or Zn(II) or Fe(III) . The two columns are coupled together . An aqueous solution of complexing agents such as triammonium salt of EDTA4- is generally used as eluant . When this eluant is passed through the first column , the resin bounded lanthanoid ions (LnR3) are displaced by the equivalent number of ammonium ions .

LnR3 (s) + (NH4)3(EDTA – H) (aq) (NH4)3 R3 (s) + Ln(EDTA – H) (aq)

With the addition of more and more solution of the complexing agent and with the removal of products so formed , the reaction progresses in the forward direction . The solution of Ln(EDTA – H) and (NH4)3 (EDTA – H) reaches the development column , where Cu(II) is displaced and Ln3+ is redeposited in a compact band on the top of the column .

3CuR2(s) + 2Ln (EDTA – H)(aq) 2 LnR3(s) + Cu3(EDTA – H)2(aq)

Cu(II) , inspite of its lower charge is able to form a more stable complex with (EDTA – H)3- , due to its smaller size than Ln(III) . The importance of using Cu(II) should be understood. Instead of Cu(II) if the resin is loaded with H+ or with citrate which provide hydrogen ion as a retaining ion, EDTA.H4 will precipitated out and would clog the resin . Once the Ln3+ ions have been deposited on the resin , they are displaced again by the NH4

+ in the eluant . The affinity of Ln3+ ions for the resin decreases almost negligibly with increasing atomic mass and thus the elution of the development column with NH4

+ alone is not able to discriminate effectively between different lanthanoids . However, the value of formation constant for Ln(EDTA-H) complex increases by approximately 25% from Ce3+ to Lu3+ . Thus , in the presence of complexing agent like EDTA4- , the tendency to leave the resin and go into the solution is significantly greater for heavier than for the lighter lanthanoides , leading to the concentration of heavier cations in the solution . The Ln3+ ions pass down the development column in the band , being repeatedly deposited and redissolved , concentrating the heavier members in the solution phase . Thus , when all copper has come off the column , the lanthanoids emerge in succession , starting from the heaviest . They may then be precipitated from the eluant as insoluble oxalates and ignited to oxides , if required . Once the different lanthanoid elements have been separated completely or partially , it is not easy to obtain pure metals because of their high melting points and ease of oxidation . The unseparated mixture of La and the lanthanoids is used mainly as misch metal . Misch metal contains 50% Ce , 40% La , 7% Fe and 3% other lanthanoids , and is used to strengthen steel . Though , the metals are of little importance of their own , two important methods namely electrolysis of fused salts and metallothermic treatment of the anhydrous salts , are available for producing the metals . Magnetic Properties The major magnetic properties of any chemical substance result from the fact that each moving electron is itself a micro – magnet . Since , an electron has both spin and orbital motion , it may contribute to magnetic behaviour in two ways . The magnetic properties of a substance thus represent the combined contribution of all the electrons present . When a substance is placed in a magnetic field , it is observed to align itself either in opposition to the field (diamagnetic behaviour) or parallel to the field (paramagnetic behaviour) . Diamagnetism results when there are no unpaired electrons and pairing of all electrons nullifies their individual contribution . The ions Y3+ , La3+ and Lu3+ are diamagnetic . Paramagnetism results when unpaired electrons are present to prevent such compensation . All of the other lanthanoid tripositive ions are paramagnetic . Let us now consider the contribution of two or more electrons in the outer shell to the total angular momentum of the atom . There are two different ways in which we might sum the orbital and spin momentum of several electrons : (i) First sum the orbital contributions and then the spin contributions separately , finally add the total orbital and total spin contributions to reach the grand total . Symbolically: Σml = L , ΣmS = S , L+S = J .

S is the resultant spin quantum number , L is the resultant orbital momentum quantum number and J is the total angular momentum quantum number . (ii) Sum the orbital and spin momenta of each electron separately , finally summing the individual totals to form the grand total : ml +ms = j ; Σj = J The first method , known as Russell – Saunders coupling , gives reasonably accurate results for small and medium sized atoms , such as lanthanoids . The second method called j - j coupling applies better to large atoms. Russell – Saunders coupling shall be discussed in detail and will correlate it with the calculation of magnetic moment . The magnetic moment of transition i.e. d – block elements may be calculated from the expression :- µ (S+L) = )1()1(4 +++ LLSS

where , µ (S+L) is the magnetic moment in Bohr magnetons calculated using both the spin and orbital momentum contributions . For the elements in the 3d - series , the orbital contribution is usually quenched out by interactions of the metal ion with the electric fields of the ligands . Thus their magnetic moments can be calculated using only the spin – only formula . µS = )2()1(4 +=+ nnSS where µS is the spin only magnetic moment in Bohr magnetons and n is the number of unpaired electrons . This simple spin only formula works with La3+(f0) , Gd3+(f7) and Lu3+ (f14) . La3+ and Lu3+ have no unpaired electrons i.e. , n = 0 and hence µS= 0)20(0 =+ . Gd3+ has seven unpaired electrons , n = 7 and µS = 63)27(7 =+ = 7.9 BM . The other lanthanoid ions , however , do not obey this simple relationship . The 4f electrons are deeply placed and shielded from external fields by the overlying 5s2

and 5p6 shells. Thus , the magnetic effect of the motion of the electron in its orbital is not quenched out . The magnetic moments , therefore , are calculated taking into account both the magnetic moment from the unpaired electron spins and that from the orbital motion . This is true not only for lanthanoids , but also for the elements belonging to the 4d and 5d series . In principle , the electronic configurations of the lanthanoids are described by using the RUSSELL – SAUNDERS coupling scheme as follows – (i) Term with highest S and highest L lies lowest . Hence , for Nd3+ (4f 3 configuration) , the lowest energy arrangement is : ml 3 2 1 0 -1 -2 -3

Thus , L = Σml = 3 + 2 + 1 = 6

S = ΣmS = 3 x 1/2 = 3/2 The value of L is then expressed for each ion in the form of a ground term with the symbolism extended so that S , P , D , F , G , H , I-------, correspond to L = 0 , 1 , 2 , 3 , 4 , 5 , 6------ , in that order . The term symbol is written as 2S + 1L . For f 3 configuration , it is 4I . At this point , however , a crucial difference emerges between the lanthanoid and 3d element . Since the d electrons of the transition metal ions are exposed directly to the influence of the neighbouring groups , the effect of the crystal field on the free - ion ground terms has to be considered before that of spin - orbit coupling . But the 4f electrons of the lanthanoid ions are deeply buried in the inner electron core and are very effectively shielded from their chemical environment . The spin contribution S and orbital contribution L couple together to give a new quantum number J called the total angular momentum quantum number . This coupling (mostly of the order of 2000 cm-1) , is , therefore , much larger than the crystal field (∼ 100 cm-1) and its effect must be considered first . J can take the following values : J = L+S , L+S-1-------- L-S

or , the ground state of the ion is that with J = L – S (when the shell is less than half full) and J = L + S (when the shell is more than half full)

The complete term symbol is represented as 2S+1LJ and thus , the ground state term symbol for Nd3+ is 4I9/2 . Similarly , the ground state term symbol for Tb3+ with f8 configuration can be calculated as follows :- ml 3 2 1 0 -1 -2 -3

L = 3 , S = 6 X 1/2 = 3 , J = 6 , 5 , 4 , 3 , 2 , 1 , 0 Since it is more than half filled , for ground state the value of J = L + S = 6 . Hence , for f8 configuration , the ground state term symbol is 7F6 . The magnetic moment µ is calculated by using Lande’s formula as : µ = g )1( +JJ

where , g = )1(2

)1()1(23

++−+

+JJ

LLSS

Due to a large spin – orbit coupling the lanthanoid ions , with a few exceptions , have ground state with a single well - defined value of the total angular momentum quantum number J , with the next lowest J state at energies many times above and hence virtually unpopulated . Fig 12 shows the calculated magnetic moments for the lanthanoid ions using both the simple spin only formula and the coupled spin – orbital momentum formula . For most of the lanthanoide ions , there is excellent agreement between the calculated and the experimental values. However, the agreement for Eu3+ and Sm3+ is poor (table 10 )

because of the fact that for Sm3+ and Eu3+ , the spin – orbit coupling is only about 300 - 400 cm-1 . This means that the difference in energy between the ground state and the next state is small . Hence , the energy of thermal motion is sufficient to promote some electrons to the first excited state (and in the case of Eu3+ even to the second and the third excited state) . Due to this , the magnetic properties are not solely determined by the ground state configuration . Since these excited states have higher J values than the ground state , the actual magnetic moments are higher than those calculated by considering the ground states only . However , magnetic moments of Eu3+ measured at low temperatures give a value close to zero , as expected . The electronic configuration suggested for the non – tripositive species are in agreement with magnetic data . Thus , Ce(+4) and Yb(+2) are diamagnetic , corresponding to 4f 0 and 4f 14 configurations , Sm(+2) and Eu(+3) are magnetically similar (both 4f6) and Eu(+2) shows similar to Gd(+3) (both 4f7) .

Fig. 12 : Paramagnetic moments of Ln3+ lanthanoid ions at 300 K . Table 10 : Magnetic moments of Ln3+ ions . Number of 4f

electrons Magnetic Moment Calculated (BM) Observed (BM)

La3+ [Xe]4f0 0 0 Ce3+ [Xe]4f1 2.54 2.3 - 2.5 Pr3+ [Xe]4f2 3.58 3.4 - 3.6 Nd3+ [Xe]4f3 3.62 3.5 - 3.6 Pm3+ [Xe]4f4 2.68 2.7 Sm3+ [Xe]4f5 0.84 1.5 - 1.6 Eu3+ [Xe]4f6 0 3.4 - 3.6 Gd3+ [Xe]4f7 7.94 7.8 - 8.0 Tb3+ [Xe]4f8 9.72 9.4 - 9.6 Dy3+ [Xe]4f9 10.63 10.4 - 10.5 Ho3+ [Xe]4f10 10.60 10.3 - 10.5 Er3+ [Xe]4f11 9.57 9.4 - 9.6 Tm3+ [Xe]4f12 7.63 7.1 - 7.4 Yb3+ [Xe]4f13 4.50 4.4 - 4.9 Lu3+ [Xe]4f14 0 0

Several of the paramagnetic lanthanoid ions , especially Pr3+ , Eu3+ and Yb3+ are useful as NMR shift reagents . When an organic molecule with a complex NMR spectrum is coordinated to one of these ions , the large magnetic moment of the ion causes displacements and a spreading out of the spectrum . This often helps in assigning and interpreting the peaks .

Colour and Spectra Electronic absorption spectra are produced when electromagnetic radiation promotes the ions from their ground state to the excited state . Many trivalent lanthanoid ions are strikingly coloured , both in the solid state and in aqueous solution . Interestingly , the colours from the ions from La3+ through Gd3+ repeat themselves , atleast qualitatively from Lu3+ back through Gd3+ (table 11 ) . It is tempting to conclude , that a given colour is related to a given number of unpaired electrons and the elements with nf electrons often have a colour similar to those having (14 - n) f electrons . However , the divergence in colour between the tri – positive species and isoelectronic non - tripositive ions (table 12 ) suggests that the situation , in reality , is somewhat more complex . Table 11 : Colour of Ln3+ ions Electronic

configuration Ground state Number of

unpaired electrons Colour

La3+ [Xe]4f0 1S0 0 Colourless Ce3+ [Xe]4f1 2F5/2 1 Colourless Pr3+ [Xe]4f2 3H4 2 Green Nd3+ [Xe]4f3 4I9/2 3 Lilac Pm3+ [Xe]4f4 5I4 4 Pink Sm3+ [Xe]4f5 6H5/2 5 Yellow Eu3+ [Xe]4f6 7F0 6 Pale pink Gd3+ [Xe]4f7 8S7/6 7 Colourless Tb3+ [Xe]4f8 7F6 6 Pale pink Dy3+ [Xe]4f9 5H15/2 5 Yellow Ho3+ [Xe]4f10 5I8 4 Pale yellow Er3+ [Xe]4f11 4I15/2 3 Pink Tm3+ [Xe]4f12 3H6 2 Pale green Yb3+ [Xe]4f13 2F7/2 1 Colourless Lu3+ [Xe]4f14 1S0 0 Colourless Table 12 : Colour of Ln2+ , Ln4+ and their isoelectronic Ln3+ counterparts Non-tripositive ions

Colour Electronic configuration

Colour Tripositive ion

Ce4+ Orange-red [Xe]4f0 Colourless La3+

Sm2+ Blood-red [Xe]4f6 Pale pink Eu3+

Eu2+ Straw yellow [Xe]4f7 Colourless Gd3+

Yb2+ Yellow [Xe]4f14 Colourless Lu3+

The ions Ce3+ , Gd3+ , Yb3+ , all of which contain unpaired electrons are colourless along with the lanthanoid ions having no unpaired 4f electrons (La3+ with 4f0 and Lu3+ with 4f14 configuration) . Colour arises because light of a particular wavelength is absorbed in the visible region . Tripositive ions except Y3+ , La3+ and Lu3+ absorb in the wavelength range 2000 – 10000 A0 . The colourless species absorb either in ultraviolet (Ce3+ , Gd3+) or the infrared region (Yb3+) . The dipositive ions absorb strongly in the ultraviolet region . The only tetrapositive ion stable in aqueous solution , the Ce4+ ion , absorbs in the blue and ultra - violet regions . In the lanthanoids , spin – orbit coupling is more important than crystal field splitting . This is in contrast to the transition metals, where the crystal field splitting is of major importance and the crystal fields even lift some of the orbital degeneracy of the terms of dn ions . Nevertheless , crystal fields can not be completely ignored for lanthanoide ions . The intensity of a number of bands show a distinct dependence on the actual ligands which are coordinated . Also , the crystal fields lift up some of the orbital degeneracy of the states of fn ions ( by the order of 100 cm-1 ). Thus , when the electronic transitions , called f - f transitions , occur from one J state of an fn configuration to another J state of this configuration , the absorption bands are extremely sharp . The colour remains unaffected by alternation of the ligands present or addition of colourless complexing groups . They are apparently characteristic of the cation themselves . Strictly , the f - f transitions are Laporte forbidden (since the change in the subsidiary quantum number is zero i.e. ∆ml = 0) . Moreover , the vibration in the metal - ligand bond is very small leading to only a slight relaxation in the Laporte rule . Hence , the transitions are not very intense , giving only pale colours to the complexes . This is in marked contrast to the transition elements where d - d spectra gives absorption bands whose position depends on the number and nature of the ligand and the width of the peak is greatly broadened because of the vibration of the ligands . Though d - d transitions are also Laporte forbidden , but these are partially relaxed by a mechanism which depends on the effect of the crystal field in distorting the symmetry of the metal ion . Hence , the transition metal complexes have more intense colour than the lanthanoids . Ce3+ (4f1) and Tb3+(4f8) are exceptional in providing bands of appreciably higher intensity in the ultra - violet region . The absorption is very intense for two reasons . Firstly , the transitions involved are of the type 4fn 4f n-1 5d1 i.e. 4f to 5d and hence ∆ml =1 , making them orbitally allowed , as against forbidden f – f transitions . Furthermore , promotion of electrons in these ions is easier than for other ions . The electronic configuration for Ce3+ is 4f1 and Tb3+ is 4f8 . Loss of one electron gives the extra stability of an empty or half – full shell . The absorption bands of the Ce3+ and Tb3+ ions are broad and are altered by complexing groups . The spectra of Ln2+ ions , stabilized in CaF2 type crystal can be studied . It is expected that their spectra would resemble those of the +3 ions of the next element in the series . However , due to lower ionic charge of the Ln2+ ion , their 4f orbitals have not been stabilized relative to the 5d to the same extent as those of the Ln3+ ions . Therefore , the spectra of Ln2+ consist of broad , orbitally allowed , 4f 5d bands

with much weaker and sharper f - f bands . These dipositive ions absorb strongly in the ultra – violet region . Charge transfer spectra are also possible along with the f - f and f - d transitions . The charge transfer spectra arise due to the transfer of an electron from the ligand to the metal . Such spectra are more intense than those obtained due to f – f transitions . Charge transfer is more probable if the metal is in a high oxidation state or the ligand has reducing properties . The orange - red colour of Ce4+ solutions arises from charge transfer rather than f - f spectra . Similarly , the blood - red colour of Sm3+ is also due to charge transfer . The unique absorptions associated with the f electrons make some of the lanthanoids particularly useful in light filters . Glass containing praseodymium and neodymium ions (known as didymium glass) absorb yellow sodium light so strongly that it is used mainly in glass blower’s goggles . The sharp and characteristic absorption spectra of Ln3+

are used extensively for both the qualitative and quantitative analysis of the lanthanoids in mixtures . Calibration of wavelength in optical devices is done by taking the advantage of the sharpness of these absorption bands . Fluorescence or luminescence of certain lanthanoid ions , notably Tb , Ho and Eu is another important feature . Luminescence is the emission of light by a material as a consequence of its absorbing energy . It is known as PHOTO LUMINESCENCE , if photons are used for excitation . A photo luminescent material generally requires a host (crystal structure) doped with an activator along with a sensitizer . Luminescence with a short time lapse (∼ 10-8 s ) between excitation and emission is known as FLUORESCENCE . However , if luminescence continues for long even after the removal of excitation source and the decay time is very long , it is known as PHOSPHORESCENCE . These phenomenon are commercially very important . For example , red phosphor in TV tubes is obtained from Eu3+ doped into Y2O3 . Other luminescence colours are : green ( from Ce3+ doped in CaS or Eu3+ doped in SrGa2Su ) , blue ( from Eu2+ doped in (SrMg)2) .P2O7 ) . Mixing red , green and blue is effective in giving white luminescence , used in fluorescent lamp coatings to convert UV / blue discharge to white light . The luminescence of Eu3+ is used to probe its environment , giving information on ligand charges , binding constants , ligand exchange rates , site symmetry etc. Further , Ln3+ can be used as a probe for Ca2+ sites in bioinorganic chemistry . Since , the ionic radii and coordination number of Ln3+ are close to Ca2+, Ln3+ may replace Ca2+ in its binding sites in proteins .

Chemistry of “0” Oxidation State – The Metal Yttrium and the elemental lanthanoids are all metals , as might be expected from their electronic configurations , large atomic radii and their position in the periodic table . However , ease of oxidation of the metals suggests that reduction of ions to the metallic state is difficult . In aqueous solution , neither electrolytic nor chemical reduction is effective . Electrolysis of molten halides and metallothermic treatment of anhydrous salts constitute successful reduction systems. (i) Electrolysis of molten halides : A mixture of LnCl3 with either NaCl or CaCl2 is fused and electrolysed in a graphite lined steel cell which serves as the cathode and the

graphite rod as anode . This method is mainly used for misch metal , for Ce , Sm , Eu and Yb . (ii) Metallothermic reduction : La and the lighter metals are obtained by reducing anhydrous chlorides with Ca at 1000 – 1100оC in an argon filled vessel . 1000 - 1100оC 2 LnCl3 + 3 Ca 2 Ln + 3 CaCl2

( Ln = La , Ce , Pr , Nd , Gd )

The heavier lanthanoids have higher melting points and hence require a temperature of 1400оC . However , at this temperature CaCl2 starts boiling . Thus , LnF3 are used in place of LnCl3 .

2 LnF3 + 3 Ca 2 Ln + 3 CaF2

( Ln = Tb , Dy , Ho , Er ,Tm , Y ) A mixture of LnF3 and Ca is heated in a tantalum crucible to a temperature 50о - 100 оC above the melting point of Ln under an inert atmosphere . The charge is then cooled and the slag and metal is broken apart . The impurities of Ca are removed by melting under vaccum . Promethium is obtained by reducing PmF3 with Li

PmF3 + 3 Li Pm + 3 LiF

Trichlorides of Eu , Sm and Yb are reduced only to the dihalides by calcium , but the metals can be prepared by reduction of the oxides Ln2O3 with La at high temperatures . Ln2O3 + 2 La 2 Ln + La2O3(Ln = Sm , Eu , Yb )

The metals are best purified by distillation in tantalum apparatus with a vaccum of atleast 10-5 mm of Hg . The metals are silvery – white and are very reactive . They are relatively soft with hardness increasing with atomic number . They have high density , and melting and boiling points which increase with increase in the atomic mass . Eu and Yb have lower values of melting and boiling points , which probably reflect the differences in crystal structure . Their compounds are generally ionic . They are strong reducing agents and react readily with most non - metals to form binaries . They are malleable and ductile . These elements also exhibit the nuclear property of capturing the neutrons . Cerium is a weak neutron absorber , but Sm , Eu and Gd are more effective than boron or cadium - the substances normally used in neutron control devices . Europium is of particular interest because its isotopes undergo a series of n , γ reactions , each of which gives a europium isotope of large absorption cross sections . In electrical conductivity , they cover

the same range that cesium and mercury do . Most of the metals are paramagnetic and gadolinium is unique in being ferromagnetic , but upto 16оC only . The occurrence of reactions of the type : Ln (s) + xH2O Ln (H2O)x

3+ + 3e- (acidic medium )

and Ln (s) + 3 OH- Ln (OH)3 (s) + 3e- (basic medium )

indicate their ease of oxidation under aqueous conditions . The metals are strong reducing agents under these conditions due to the large amounts of energy released in the hydrolysis of their gaseous tripositive ions as compared to the smaller quantities of energy needed to form these ions . Even in absence of water the metals are readily oxidized , though slowly at room temperature and rapidly at high temperatures . The lanthanoids are much more reactive than Al (E0 = - 1.66 V) and are slightly more reactive than Mg (E0 = - 2.37 V) . Thus , they all react directly with water , slowly in the cold , rapidly on heating , to liberate hydrogen and forming insoluble hydroxides or hydrous oxides . Eu is attacked much more rapidly than other metals , forming first the soluble , yellowish Eu(OH)2.H2O and then hydrous Eu2O3 . The hydroxides are ionic and basic . Dry oxygen attacks pure metals very slowly at room temperature , but at higher temperatures the metals ignite and burn . They tarnish readily in moist air and burn to give the sesquioxides , Ln2O3 except cerium which gives CeO2 . Yb and Lu form protective oxide films , on exposure to moist air , which prevents the bulk of the metal forming the oxide unless it is heated to 10000 C . Praseodymium and terbium yield non – stoichiometric products , Pr6O11 and Tb4O7 , respectively . Chalcogenides of the type LnZ and Ln2Z3 ( Z = S , Se , Te ) are formed by the direct combination with the respective lanthanoid ( except Pm ) . Their characterization is made more difficult by the prevalence of non – stoichiometry . In general , they are stable in dry air but are hydrolysed if moisture is present . The metals react exothermically with hydrogen , though heating to 300 - 4000C is often required to initiate the reaction . The resulting phases MH2 and MH3 , which are usually in a defect state , have remarkable thermal stability , in some cases even upto 9000C . The metals also react readily with C , N , Si , P , halogens and other non – metals at elevated temperatures . Reactions of the metals with typical reagents are summarized in Table 13 . Like alkali and alkaline earth metals , metallic Eu and Yb dissolve in liquid ammonia at -780 C to give a dark blue solution . The solutions are strongly reducing , decompose slowly and are believed to contain the ammoniated electron , e(NH3)-

y . The solution changes to golden on concentration .

Table 13 : Typical chemical reactions of the elemental lanthanoids Reagent(s) Product(s) Conditions H2 LnH2, LnH3 Rapid above 3000C X2(X= F, Cl, Br, I) LnX3 Slow at room temperature,

burn above 2000C S Ln2S3 At boiling point of S N2 LnN Above 10000C C LnC2, Ln2C3 (also LnC, Ln2C,

Ln3C, Ln4C) At high temperature

B LnB4, LnB6 At high temperature O2 Ln2O3 Slow at room temperature,

burn above 150-1800C Dil. Acids(HCl, H2SO4, HClO4)

Ln3+ + H2 Rapid at room temperature

H2O Ln2O3 or Ln(OH)3 + H2 Slow at room temperature, more rapid at high temperature

H2O + O2 Ln2O3 or Ln(OH)3 Rapid with Eu, slow with others

Metal oxides metal At high temperatures (except CaO, MgO, Ln2O3 in general)

Chemistry of Lanthanoid Ions in +2 Oxidation State All lanthanoids can give Ln2+ ions under unusual conditions . For example , reduction of tri –chlorides of Nd , Dy or Tm by sodium naphthalenide or reduction of tri - bromides and tri - iodides of Sm , Dy , Tm and Yb by alkali metals (M/ ) give M/ LnX3 . Though the reduction potentials for these ions (in THF) are known [ Tm2+ (-2.3 V) , Dy2+(-2.5 V) , Nd2+(-2.6 V )] , only Sm2+ , Eu2+ and Yb2+ have significant normal aqueous chemistry in ( + 2 ) state . The most stable divalent lanthanoid is Eu2+, which is highly stable in water . Both Sm2+ and Yb2+ are rapidly oxidized by hydronium ions : 2 Ln2+ + 2 H3O+ 2 Ln3+ + 2 H2O + H2

(g)

but the Eu2+ ion is oxidized only very slowly . All the three ions are oxidized rapidly in the presence of oxygen . 4 Ln2+ + 4 H3O+ + O2 4 Ln3+ + 6 H2O

The general instability of the dipositive species with respect to oxidation , in aqueous medium has already been discussed . However , in less strongly solvating media , such as alcohol , some stabilization with respect to oxidation may be expected and is best achieved by including these ions in solid compounds . More extensive stabilization is noted when the ions are trapped in an SrO , Ln2+ in silicate glasses , LnF2 in LnF3 . The divalent lanthanoids are generally obtained by reduction of anhydrous fluorides or chlorides or fused tri – halides or oxides with the corresponding metal . Eu2+SO4 can be prepared by electrolyzing Eu2

3+(SO4)3 . Amalgamated Zn can reduce Eu(+3) to Eu(+2) , but Sm(+3) and Yb(+3) can not be reduced by this method . Electrolytic reduction of aqueous solutions of Eu(+3) and Yb(+3) yields the respective dipositive ion at the

mercury cathode . But Sm(+2) ion is too strongly reducing to be so prepared . Sm , Eu and Yb amalgams , on treatment with acids , apparently give the dipositive ions as intermediate oxidation products . The best way of obtaining these dipositive ions is by thermal decomposition of the trihalides at high temperature : high temperature 2LnX3(s) 2LnX2(s) + X2(g) The ease of reduction increases in the series Sm3+ - Yb3+ - Eu3+ and Cl- - Br- - I- . SmI2 and YbI2 can be prepared by reacting the metal with 1, 2 – diiodomethane in anhydrous THF . Ln + ICH2CH2I LnI2 + CH2=CH2 There are striking similarities in the crystal structure and solubility between these divalent ions and the alkaline earth metals , especially Sr2+ and Ba2+ . All dipositive fluorides are isomorphous , having the fluorite type structure . Anhydrous SmCl2 is isostructural with SrCl2 and BaCl2 . EuBr2 and SmBr2 are isostructural with SrBr2 . It is for this reason that the Sr2+ and Ba2+ ions are commonly used to carry the dipositive lanthanoids in separations . In low oxidation state , iodides are more numerous than fluorides . In solid state , even the diiodides of Dy , Tm and Yb are known to form layer structures like CaI2 . Most of the dihalides are air and moisture sensitive . Eu (+2) resembles Ca (+2) in several ways. Both form insoluble sulphates and carbonates and are soluble in liquid NH3 . EuH2 is ionic and similar to CaH2 . The major difference between the two is that Ca compounds are diamagnetic whereas EuX2 have a magnetic moment of 7.9BM , corresponding to seven unpaired electrons . The halides of Ln(II) are finding increasingly more uses . SmI2 is considered as a highly versatile reagent in organic chemistry . It can be used in the reduction of organic functional groups ( i.e. R - X can be converted to R - H) . It can also be used in the formation of carbon - carbon bond forming reactions , such as in Barbier or Reformatsky or Pinacol coupling type of reactions . Apart from the insulating dihalides , conducting diiodides of Ce , Pr and Gd are also known . They are known for their metallic lusture and high conductivities . They are best formulated as Ln3+ , 2I- , e - , the electron being delocalized in conduction band . The observed isomorphism between the compounds TmI2 and YbI2 suggests that the Tm2+ exists . Apart from this , other reduced phases such as Smx+

5Br13 (x = 2.20) , Smx+11Br24

(x = 2.18 ) and Smx+6Br13 (x = 2.16) have also been identified . They have fluorite - like

structure in which the anions are rearranged to accommodate the additional anions , giving an irregular 7 - and 8 - coordinated lanthanoid ion . Gd 5 Cl11 and Ho 5 Cl11 have also been characterized . Further reduction, below the + 2 oxidation state in LnCl3 / Ln melt gives rise to Ln2Cl3 and finally “graphite - like” LnCl phase . Dioxides of the type NdO , SmO ( both golden yellow ) , EuO (dark red) and YbO (greyish white) are known . They are obtained by reducing Ln2O3 with the metal at high temperature , except for Eu , which requires high pressure . EuO and YbO are insulators or semiconductors and have NaCl like structure . EuO is found to be

ferromagnetic at low temperatures . However , NdO and SmO consist of Ln3+ , O2- and an e- in the conduction band . Monochalcogenides , LnZ (Z = S , Se , Te) , have been prepared for all the lanthanoids except Pm , mostly by direct combination . They , like oxides , have rock salt - like structure , and are mostly black . Except SmZ , EuZ , YbZ , TmSe and TmTe , all of them exhibit conductivity , illustrating the fact that they consist of Ln3+ and Z2- ions , with an electron in the conduction band . EuZ and YbZ are semi –conductors or insulators with Ln2+ ion . SmZ seems to be intermediate between Sm2+ and Sm3+ involving the equilibrium Sm2+ + Z2- Sm3+ + Z2- + e-

The hydrated water - soluble Sm(+2) and Yb(+2) salts are oxidized by their water , but the salts of Eu(+2) are comparatively stable . Water insoluble compounds (e.g. the sulphates , carbonates , fluorides) resist oxidation even in the presence of water . Chemistry of Lanthanoid Ions in +3 Oxidation State It is apparent from previous discussion that the properties of this state very largely determine the chemistry of lanthanoids . The tri - positive species are found in crystalline compounds containing essentially all known anionic species . Compounds of Ln3+ cation with the anions such as OH- , CO3

2- , SO42- , C2O4

2- and NO3- decompose on heating ,

giving basic salts and finally the respective oxides . Hydrated salts that contain thermally stable anions such as F- , Cl- , Br- and PO4

3- also give similar products on heating because of hydrolysis . However, their anhydrous compounds melt without decomposition . Their high melting points and electrical conductivities in the fused state reflect the high degree of ionic bonding . The hydroxides , Ln(OH)3 are definite compounds and may be obtained in crystalline form by ageing Ln2O3 in strong alkali at high temperature and pressure . They can also be precipitated out by adding alkali or ammonium hydroxide to a solouble lanthanum salt such as chloride , nitrate etc . They have hexagonal structures with nine - coordinated tricapped trigonal prismatic geometry . These hydroxides are nearly insoluble in water but are sufficiently basic to dissolve readily in acids . They are less basic than Ca(OH)2 but more basic than Al(OH)3 . However , their basicity decreases as the ionic radius decreases from Ce to Lu . Thus Ce(OH)3 is most basic and Lu(OH)3 is least basic . The decrease in basicity is evident by the fact that the hydroxides of latter elements dissolve in hot concentrated NaOH forming complexes . Yb(OH)3 + 3NaOH 3Na+ + [Yb(OH)6]3-

Lu(OH)3 + 3NaOH 3Na+ + [Lu(OH)6]3-

Paralleling this change , the [Ln(H2O)x]3+ ions are subjected to an increasing tendency to hydrolyse and hydrolysis can be prevented by the use of increasing acidic solution . All lanthanoids , except Eu and Yb form hydrides of the type LnH2 simply by heating the lanthanoid directly with H2 at a high temperature of 3000 - 3500C . These hydrides are black , metallic and conduct electricity . They have the fluorite type structure. These hydrides are better formulated as Ln3+ , 2H- and an electron which is delocalized in a

metallic conduction band . More hydrogen can be accommodated in the interstices of the lattice and a limiting stoichiometry of salt like hydride , LnH3 can be achieved if high pressures are employed . The composition of LnH3 is Ln3+ and 3H- , with no delocalized electrons and hence no metallic conduction . In addition , Yb forms non –stoichiometric compound approximately to YbH2 . 5. The hydrides are remarkably stable to heat often upto 9000C . They are decomposed by water and react with oxygen .

CeH2 + 2H2O CeO2 + 2H2 Due to the low boiling point , low density and explosion risk involved , it is difficult to store and transport hydrogen fuel . The lanthanoids can form intermetallic alloys of the type LnNi5 , which can reversibly absorb large quantities of hydrogen . The intermetallic hydrides of lanthanoids which are capable of storing and producing large amount of ultra pure hydrogen , can be used as a source of fuel for motor vehicles , in fuel cells , as well as hydrogenating agent in organic chemistry . An intermetallic alloy of the type LaNi5 crystallises in CaCu5 type structure , having 9 interstitial sites for H . ∆H = - 31 kJmol-1 LaNi5 + 3 H2 LaNi5H6 ∆H = 31 kJmol-1

The absorption and desorption of hydrogen occurs topotactically , without making any drastic change in structure but expanding the lattice . The oxides , Ln2O3 are obtained by heating metal in oxygen or by the thermal decomposition of the Ln(OH)3 or oxy salts such as Ln2(CO3)3 and Ln(NO3)3 . These are well characterized . However , Ce , Pr and Tb form stable oxides as CeO2 , Pr6O11 and Tb4O7 , from which sesquioxides can be obtained by controlled reduction with H2 . These oxides are generally pale in colour , are strongly basic and have properties resembling those of the oxides of alkaline earth metals . They readily absorb water and CO2 from air to form hydroxides and carbonates respectively . Ln2O3 adopts three structure types conventionally classified as:

A - type : This type consists of LnO7 units , approximate to capped octahedral geometry (Fig. 13). This structure is favoured by the lightest lanthanoids .

B - type : This type also consists of LnO7 units but in this case two are capped trigonal prisms and one is a distorted octahedron ( Fig. 14). This type is favoured by the middle lanthanoids .

C- type: This type is related to fluorite structure and is favoured by middle and heavy lanthanoids. They have a complex structure with LnO6 units which are not octahedral ( Fig 15 ).

A – type structure

B - type structure C- type Structure

Fig. 13-15 : Geometry of Ln2O3

Trivalent chalcogenides of the type Ln2Z3 ( Z = S, Se , Te ) with stoichiometry similar to oxides can be obtained by a variety of methods including direct combination . Direct action of H2S on the chloride or oxide results in the formation of respective sulphides . On heating with excess of chalcogen in a sealed tube at ∼6000C , products with composition up to or nearing LnZ2 are obtained . They seem to be polychalcogenides. Except Promethium , all lanthanoids form trihalides of the type LnX3 . The trifluorides are of particular importance because of their insolubility. They can be precipitated as LnF3. ½ H2O by the action of HF on aqueous Ln(NO3)3 . However , the fluorides of heavier lanthanides are sparingly soluble in HF due to the formation of soluble complex , [LnF(H2O)n]2+ . Aqueous solutions of other trihalides are obtained by dissolving the oxides or carbonates in aqueous HX . Hydrated salts with 6 or 8 water molecules can then be crystallized though with difficulty due to their high solubilities . The chlorides are non – volatile , deliquescent solids , soluble in H2O and alcohol and crystallize with six or seven molecules of water of crystallization . Bromides and iodides are rather similar to the chlorides . Iodides of the first few lanthanoids are orthorhombic while those of the remaining lanthanoids are hexagonal . Heating of hydrated halides leads to the formation of oxohalides instead of the dehydration of the salt heat LnCl3 . 6 H2O LnOCl + 5 H2O + 2 HCl The bromides and iodides are even more susceptible to hydrolysis and dehydration . However, heating CeX3. (H2O)n results in the formation of CeO2 . The anhydrous trihalides can be obtained by heating the metal and halogen , or by heating theb oxide with the appropriate ammonium halide . 3000C Ln2O3 + 6 NH4Cl 2 LnCl3 + 6 NH3 + 3 H2O They can also be prepared as methanolates, LnCl3. 4 CH3OH by treating the hydrates with 2,2 - dimetheoxypropane or as etherates , LnCl3(ether)n by reaction of the oxides or

carbonates with HCl produced in - situ from SOCl2 and H2O in the presence of glyme (MeOCH2CH2OMe) under mild conditions . These anhydrous trihalides are ionic , high melting crystalline deliquescent substances . The coordination number of Ln3+ changes with the radii of the ions . It varies from 9 for LnF3 of the large lanthanoids to 6 for the LnI3 of the smaller lanthanoids . The trihalides react with glass giving oxohalides (LnOX) at high temperatures . high temperature 2 LnX3 + SiO2 2LnOX + SiX4

Lanthanoid salts of most oxy acids such as sulphates , nitrates , perchlorates and bromates are also known . These salts are readily obtained by dissolving the oxide in acid and crystallizing . Their aqueous solutions furnish hydrated cations , [Ln(H2O)n]3+ which tend to undergo slight hydrolysis in aqueous solution [Ln(H2O)n ]3+ + H2O [Ln(H2O)n - 1(OH)]2+ + H3O+

The tendency of the [Ln(H2O)n]3+ ion to hydrolyze increase with decreasing ionic radii , on moving from La3+ to Lu3+ . Lanthanoid salts with common anions frequently contain species like Ln(H2O)9

3+ , which have a tricapped trigonal prismatic (ttp) structure . These hydrated lanthanoid ions usually have high primary hydration numbers varying from 8 to 9 . The increased polarization of the primary hydration sphere by a smaller cation enhances further the hydrogen bonding to water molecules leading to the formation of a secondary hydration sphere . Polymeric species such as [ Ln2(OH)3]3+ and [Ln3(OH)3]6+ may also be present. Lanthanoid compounds form a large number of double salts like double nitrates such as 2 Ln(NO3)3. 3 MII(NO3)2. 24 H2O; (MII = Mg, Zn, Ni, Mn ) and Ln (NO3)3 . NH4NO3 .4 H2O and double sulphates such as Ln2(SO4)3 . 3 M2

ISO4 .12 H2O ; (MI = alkali metal) . On the basis of solubility in Na2SO4 , the double sulphates are categorized into two groups : cerium group (Ln = La to Eu) which are only sparingly soluble and yttrium group (Ln = Gd to Lu) which are highly soluble . Since the double salts crystallize well , these are used to separate the rare - earths from one another . All lanthanoids are quantitatively precipitated as oxalates from Ln3+ solution containing nitric acid and oxalate , (ox2-) ion . The insolubility of oxalates in acidic medium ( pH 4 or less) is very important . Precipitation of oxalates under these conditions may be used to separate the tripositive lanthanoids from all other cationic species ( excepting the tri - and tetrapositive actinoids ). The precipitate, on drying and ignition, gives Ln2O3. The nature of the oxalate precipitated depends on the reaction conditions. In nitric acid and in presence of ammonium oxalate double salts of the type NH4Ln (ox)2. y H2O( y = 1 or 3) are obtained . However , in neutral solution , ammonium oxalate gives the normal oxalate with lighter lanthanoids and mixtures with heavier lanthanoids. The phosphates are insoluble but become soluble in presence of dilute acid solution. Carbonates of the type Ln2(CO3)3 are well known.They can be prepared by passing CO2 into an aqueous solution of Ln(OH)3 or by adding Na2CO3 solution to Ln3+ salt solution . They are best prepared by hydrolysis of chloroacetates 2 Ln(Cl3CCO2)3 + (n + 3) H2O Ln2 (CO3)3.n H2O + 3 CO2 + 6 CHCl3

Most of the carbonates are basic , insoluble in water but dissolve in acids with liberation of CO2 . An interesting aspect of the chemistry of lanthanoids in + 3 oxidation state is the solubility of their salts . Water soluble Ln(+3) compounds include the chlorides , bromides , iodides, nitrates, acetates , perchlorates , bromates and a number of double nitrates . Water insoluble compounds include the fluorides , hydroxides , oxides , carbonates , chromates , phosphates and oxalates and the salts of most di - and trinegative anions . The normal sulphates vary from very soluble to difficultly soluble . Ideally , the trend in solubility for a given salt should be directly related to crystal radii . However , the experimental observations indicate that the solubility may decrease with decreasing radius (e.g , OH- ) , increase with decreasing radius (e.g , double magnesium nitrates ) , or change irregularly (e.g. , SO4

2- , BrO3- ) . No explanation has been offered for these observations.

Table 14 records the general trends in the solubilities of Ln (+3) salts in water . The lanthanoids also form well defined compounds with non - metals and metalloids . They react directly with B at elevated temperatures to give LnB4 and LnB6 . LnB6 are the most important type of borides , better formulated as Ln3+(B

62- ) (e-) , excepting EuB6

and YbB6 which are represented as Ln2+(B62-) and do not contain any free electron.

They are black coloured metallic conductors which are isomorphous with CaB6 and contain B6 octahedral clusters . MT4B4 type of compounds ( where M = Sc , Y , Ln , Th , U and T = Ru , Os , Co , Rh , Ir) are of current interest due to their super conductivity . CeCo4B4 is a well characterized compound . The reaction of metals with C or t reduction of Ln2O3 yields carbides of stoichiometry LnC2 and Ln4(C2)3 . Carbides of lanthanoids Sm to Lu are known . They do not contain Ln(+2) but are best described as acetylides of Ln3+ and C2

2- with extra electron in a conduction band . They are more reactive than CaC2 , show metallic conductivity and react with water giving ethyne . 2 LnC2 + 6 H2O 2Ln(OH)3 + 2C2H2 + H2 C2H2 + H2 C2H4 + H2 C2H6

Similarly , the lanthanoids react with N , P , As , Sb and Bi to give LnN , LnP , LnAs , LnSb and LnBi respectively . They all crystallize in NaCl type lattice.

Table 14 : General trends in the solubility of Ln(+3) salts in water

Anions Cerium group Yttrium group Cl- , Br- , I- , NO3

- , ClO4-

, BrO3- , C2H3O2

-Soluble Soluble

F- Insoluble Insoluble OH- Insoluble Insoluble HCO3

- Slightly soluble Moderately soluble C2O4

2- Insoluble ; insoluble in C2O4

2- Insoluble ; soluble in C2O4

2-

CO32- Insoluble ; insoluble in CO3

2- Insoluble ; soluble in CO32-

Basic NO3- Moderately soluble Slightly soluble

PO43- Insoluble Insoluble

Double MI sulphate Insoluble in M2SO4 solution Soluble in M2SO4 solution

Chemistry of Lanthanoid Ions in +4 Oxidation State It is very rare to find lanthanoids with + 4 oxidation state , in solution . The only lanthanoid with + 4 oxidation state which exists in solution and has any aqueous chemistry is Ce4+ (i.e. ceric ion) . The elements Pr , Nd , Tb and Dy also form +4 states . However , these states are generally unstable in water and are found as non - stoichiometric oxides and fluorides . Moreover , these species are required to be trapped in a suitable crystal lattice for their stabilization . Tetrapositive lanthanoid species can be obtained by carrying out oxidation with oxygen or fluorine at elevated temperatures ( ∼ 450 – 5000C) . Ce(III) can be oxidized even at room temperature . Cerium (+4) in solution is obtained by treatment of Ce(+3) solutions with very powerful oxidizing agents , such as ammonium peroxodisulphate (NH4)2S2O8 in nitric acid . The listed electrode potential (table 8 ) describes the equilibrium when neither complexing by the anion nor hydrolysis occurs. Ce3+(aq) Ce4+(aq) + e- In acidic medium , Ce(+4) is slightly stronger oxidizing agent than PbO2 and slightly weaker than H2O2 . However, no potential data is available for alkaline systems. Experiments show that Ce(+4) is a much weaker oxidizing agent in alkaline medium . Solid ceric compounds include CeO2 , hydrous oxide CeO2. n H2O and CeF4. CeO2 is a white solid when pure and acquires a pale – yellow colour on standing . It is obtained by heating the cerium metal , or any of the Ce(+3) oxo – salts , in air or oxygen . Ce + O2 CeO2 2 Ce(OH)3 + ½ O2 2 CeO2 + 3 H2O Ce2(C2O4)3 + 2 O2 2 CeO2 + 6 CO2

The oxidation of Ce (+3) oxide or hydroxide gives green , blue and purple coloured products , before the final off - white to pale yellow CeO2 . The almost white coloured CeO2 further darkens on heating. The coloured products are obtained due to the presence of both tri - and tetravalent cerium in the same crystal (mixed - valence compounds of a given element are often deeply coloured ) . CeO2 is an inert material , not attacked by either strong acids or alkalis but dissolve in acids in the presence of reducing agents (H2O2/ Sn(+2)) giving Ce(+3) solutions .

Hydrous ceric oxide CeO2.n H2O is a yellow , gelatinous precipitate obtained on heating Ce(+4) solutions with bases . It redissolves without reduction in acids . In low acidic medium , such solutions readily hydrolyze giving the basic salts . If the acidity is kept sufficiently high , they can yield hydrated Ce (+4) salts . Double salts such as Ce (NO3 )4 . 2 NH4NO3 and Ce (SO4)2 . 2 (NH4)2 SO4 can be prepared easily .

Pr forms a non - stoichiometric black oxide often formulated as Pr6O11 which can be converted to PrO2 . Pr6O11 readily dissolves in acids to give aqueous Pr(+3) and

liberate oxygen , chlorine etc depending on the acid used . Ignition of oxo - salts of terbium under normal conditions results in the formation of terbium (+4) oxides , with composition ranging from TbO1.7 to TbO1.81 . Black coloured TbO can be obtained by oxidation of Tb2O3 with air , at 4500C . All these oxides have the fluorite type structure. Ce , Tb and Pr form tetrafluorides . Due to the high oxidizing power of fluorine , it is able to oxidize these lanthanoids to the (+4) oxidation state also . CeF4 is comparatively stable and crystallizes as a monohydrate . TbF4 and PrF4 are thermally unstable and are capable of oxidizing H2O . All these tetraflourides are white solids with UF4 / ZrF4 structure , in which the metal shows dodecahedral coordination . As mentioned earlier , except for Ce(IV) , only oxide and fluoride are known to stabilize tetrapositive species . The compound CeF4 was prepared and characterized long ago . Later on , TbF4 was also prepared and found to be isostructural with CeF4 . Pure PrF4 has not been prepared , however the complex compounds Na[PrF5] and M2[PrF6] (M = Na , K , Rb , Cs) have been prepared and characterized . Terbium compounds such as M2[TbF6] and dysprosium compound of the type Cs3[DyF7] have been successfully prepared and characterized . Complex Formation The lanthanoids are moderate in their complex forming ability which increases with moving to the right in the periodic table . This is in contrast with the transition metals which form complexes readily . The difference arises may be due to larger size of the lanthanoids and the presence of “very well - shielded” 4f orbitals which are not available for interaction . Consequently, each lanthanoid ion effectively attracts the ligands only by overall electrostatic forces . Hence , a decrease of complex ion formation with a specific ligand is observed in the series Ln4+ > Ln3+ > Ln2+ . The tendency of the anhydrous Ln3+ ions to form complexes increases from La3+ to Lu3+ but that of hydrated Ln3+ decreases in the same order . This is due to the fact that while the size of gaseous Ln3+ ions decreases that of the corresponding hydrated ion increases with the increase in atomic number . Fluoro complexes are formed easily by small sized Ln3+ , but chloro complexes are not readily formed in aqueous media or concentrated HCl . This property makes them distinct from their counterparts , the actinoids . Lanthanoids have no or very little tendency to form complexes with π -bonding ligands such as CO and CN- . This is again in contrast to the transition metals which form numerous metal carbonyls and the difference is again attributed to the unavailability of the 4f orbitals for bonding . Lanthanoids form less stable complexes with monodentate oxygen ligands which tend to dissociate in aqueous solution . However , in ethanolic solution , lanthanoids form triphenylarsine oxide , [Ln(NO3)2] . (OAsPh3)4 ; pyridine - N - oxide , [Ln(pyO)8] (ClO4)3 and numerous complexes with DMSO , of the type (DMSO)n Ln(NO3). Complexes with amines are formed only in non - aqueous medium because water is a stronger ligand than amine. Aryloxo complexes such as Sm(OAr)3 . (THF) and K[Ln(OAr)4], ( Ln = La , Nd , Er) are also known. Mixed siloxo / amido complexes of the type, Y[O Si Bu Ar2] [ N(Me3Si)2]3

have also been reported. The complex, [Nd6 (OPr)17 Cl] is of special interest as it is made up of 6 Nd atoms held together around Cl by means of bridging - OCHMe2 group . Chelating ligands such as oxalates , citrates , tartarate , EDTA and acetylacetone form most common and stable complexes with lanthanoids( Fig. 16 ). These complexes water soluble and frequently have high and variable coordination numbers . Water or solvent molecules are often attached to the central metal . Formation of such complexes is used in ion - exchange separation. The bonding in complexes is preferentially ionic. These chelates form highly soluble complexes in water , such that even their crystallization may be extremely difficult . The formation of such chelated species in aqueous solution involves stepwise equilibria of the type Ln3+ + AA Ln (AA)

Ln (AA) + AA Ln (AA)2

Ln (AA)2 + (n - 2)AA Ln (AA)n

The overall equilibrium may be represented as: Ln3+ + n AA Ln (AA)n

where , AA is the chelating group. and both equation and the charges of the chelated species are neglected . The equilibrium constant for such an equilibrium is represented as KLn (AA)n = [Ln (AA)n] [Ln3+] [AA]n

Such type of constants are known as formation or stability constants . For convenience, log10K values are used.

(a) (b) (c) Fig. 16: Structures of (a) Oxalate (b) citrate and (c ) tartarate ions Studies reveal that the most stable chelated species of the lanthanoids are those derived from the aminepolycarboxylic acids . The anions of these acids are capable of forming more than one chelate ring by utilizing the several oxygen and nitrogen donors that are available (table 15) . It is apparent that an increase in the number of like chelate rings on the lanthanoid ions , increases the value of stability constant . In general , for a given ligand , the stability constant increases with decrease in crystal radius . This increase is invariably in the region La3+ through Eu3+ with a break at Gd3+ as evident from fig. 17. The gadolinium break can be associated with the corresponding break in crystal radii (fig 6) but the latter alone is insufficient to account for the former , especially for heavier lanthanoids . Hence , for heavier lanthanoids it would be appropriate to consider the fact that the formation constant actually measures the displacement of an equilibrium of the type [Ln (H2O)n ]3+ + EDTA4 - [Ln (EDTA ) (H2O )m]- + (n –m ) H2O

Thus , the effect of hydration on crystal radii , the rupture of Ln – OH2 bonds , the formation of new bonds and the retention of water in the product are important factors in determining the actual stability . Table 15: Structure of some common Aminepolycarboxylic acids with possible chelate rings

Name Formula Possible

chelate rings

Nitrilotriacetic acid (H3NTA) CH2COOH N CH2COOH CH2COOH

3

N-Hydroxyethylethylenediamine-triacetic acid (H3HEDTA)

HOCH2CH2 CH2COOH NCH2CH2N HOOCCH2 CH2COOH

4

Ethylenediaminetetracetic acid (H4EDTA)

HOOCCH2 CH2COOH NCH2CH2N HOOCCH2 CH2COOH

5

1,2-Diaminocyclohexane-tetraacetic acid (H4DCTA)

CH2 CH2COOH

CH2 CHN CH2COOH CH2COOH CH2 CHN CH2 CH2COOH

5

Fig 17 : Stability constants of lanthanoid – EDTA complexes in aqueous solutions at 20оC

The β – diketonates ( fig.18) are also commonly used as chelating ligands . These chelates have the same colours as the aquated tripositive ions with only a slight modification in the absorption bands. Anionic catecholate complexes with higher coordination numbers 7 and 8, such as [Gd2(cat)6]6- and [Gd(cat)4]5- are also well characterized. Various crown ethers with differing cavity diameters and high coordination numbers are also known . For Ln = La to Gd , the most thermally stable product has Ln : crown ether ratio as 4 : 3 , with the general formula [Ln(NO3)2 L]3 [Ln(NO3)6] . The larger lanthanoids i.e. , La , Ce , Pr and Nd form 1:1 complexes of the type [Ln(NO3)3L] ; where L represents the crown ether . The commonly used crown ether is 18 - crown - 6 – ether (or 1 , 4 , 7 , 10 , 13 , 16 – hexaoxacyclo – octadecane ) (Fig. 19 ) . The coordination number of the former complex is 12 while that of the latter is 10 . The smaller lanthanoids (Tb - Lu) occupy the cavities in the crown ether and very large lanthanoids form [Ln(NO3)3 (H2O)3]L , in which the crown ether L remains uncoordinated . These complexes generally dissociate instantly in water. The 8 – quinolinol chelates are uniformly yellow but their absorption spectra consists of sharply defined , weak bands at the wavelengths of those for the aquated ions superimposed on the broad , intense bands of the 8 – quinolinol grouping . Different types of polyoxo ligands such as polyphosphates and mixed phosphoryl / carbonyl species also form complexes with Ln3+ , mainly in aqueous medium .

Fig.18: Structure of β – diketonates Fig. 19 : Structure of 18 - crown - 6 - ether R(CO)(CH-)(CO)R Some super oxide complexes have also been reported . Lanthanoids form larger number of complexes with O – donor ligands as compared to N - donor ligands . This is due to the higher eletronegativity associated with the former type of ligands which helps in the formation of a stable ionic bond . Conventional complexes of amines such as en , dien , dipy and N – bonded thiocyanato can be prepared easily in polar organic solvents . These complexes also have high coordination numbers , usually varying from 8 to 10 . Complexes such as [Ln(en)4]3+ , [Ln(dien)4(NO3)]2+ , [Ln(en)3Cl3] have also been reported . Sc (III) , Nd (III) , Eu (III) and Gd (III) form complexes with substituted amido ligands such as ( Me3Si)2 N- . Common complexes with such ligands are [(Me3Si)2N]3 Ln (THF) and (Me3Si) [(C6H5)N]3 Nd(THF). These complexes are very stable . The lanthanoid - EDTA complex shows no destabilization due to the presence of two N - donor atoms. Similarly , the N - donor analogues of crown ethers are sufficiently stable in water . Various complexes of porphyrins , porphyrinogens and phthalocynanine are known . Lanthanoids form complexes with sulphur as well as phosphorus ligands . Amongst the S - ligands , dithiocarbamates and dithiophosphinates are common . Examples include eight - coordinated [Ln(S2CNR2)4]- and [Ln(S2PMe2)4]- . Even mixed S / N - donor ligands form stable complexes of the type {Ln[N(SiMe3)2] . [`µ-SBu/ ]}2 , where Ln = Eu , Gd and Y . A very few phosphido complexes of the type , Ln [N(SiMe3)2]2 (PPh2) , (Ln = La , Eu) and Tm [P(SiMe3)2]3 (THF)2 are known .

From the above discussion , it can be concluded that lanthanoids form numerous complexes with high coordination numbers such as 7 , 8 and 9 . Lighter lanthanoids are able to form 10 and even 12 coordinated complexes with small chelating ligands such as NO3

- and SO42. For example , [Ce(NO3)5]2- forms a 10 - coordinated bi - capped

dodecahedron , while [Ce(NO3)6]3- forms a 12 – coordinated icosahedron (fig. 20) .

Fig. 20 : Structure of [Ce(NO3)6]3—

Lanthanoids form only a few complexes with coordination number 6 which is the most common coordination number in case of transition metal complexes ( Table 16 ) . Table 16 : Oxidation states , coordination numbers and stereochemistries of the compounds of lanthanoids Oxidation state

Coordination number

Complex Stereochemistry

2 6 LnZ ( Ln = Sm, Eu, Yb ; Z = S, Se, Te)

Octahedral

8 LnF2 ( Ln = Sm, Eu, Yb Cubic 3 3 [Ln{ N(SiMe3)2}3];( Ln = Nd, Eu, Yb) Pyramidal 4 [Lu(2,6-dimethylphenyl)4]- Tetrahedral [Ln{ N(SiMe3)2}3(OPPh3}] ;

(Ln = Eu, Lu) Distorted Tetrahedral

6 [LnX6]3-; (X= Cl, Br), LnCl3; ( Ln = Eu, Lu)

Octahedral

7 [Ho{PhC(O)CH=C(O)Ph}3(H2O)] Capped octahedral 8 [Ho(tropolonate)4]- Dodecahedral [Eu(acetylacetone)3(phenanthroline)] Square antprismatic LnF3; ( Ln= Sm – Lu) Bi-capped trigonal

prismatic 9 [Ln(H2O)9]3+ , [Eu(terpy)3]3+ Tri-capped trigonal

prismatic [Pr(terpu)Cl3(H2O)5].3 H2O Capped square

antiprismatic [Ln (NO3)5]2- ; (Ln = Ce, Eu) Bi-capped

dodecahedral [Gd(NO3)3(DPAE)](a) Irregular [La(acetylacetone)3(H2O)2] Square antiprism [CeIV(acetylacetone)4] Square antiprism 12 [CeIV(NO3)6]3- Icosahedral

(each NO3- is

bidentate)

4 6 [CeCl6 ] 2- Octahedral 8 LnO2; ( Ln = Ce, Pr, Tb) Cubic [Ce(acac)4], LnF4; ( Ln = Ce, Pr, Tb) Square

antiprisamatic 10 [CeIV(NO3)4(Ph3PO)2] Complex

(each NO3- is

bidentate) 12 [Ce(NO3)6]2- Icosahedral

(each NO3- is

bidentate) (a) DPAE = 1,2- di(pyridine-2- aldimino) ethane However, lanthanoids can be induced to form complexes with exceptionally low coordination numbers by using very bulky ligands such as (2 , 6 - dimethylphenyl) and [N(SiMe3)2]- . Complexes such as [Ln{N(SiMe3)2}3] are volatile , air - sensitive , undergo hydrolysis easily and have coordination number of 3 , the lowest possible coordination number reported for the lanthanoids . The complex has a zero dipole moment and pyramidal geometry in solid state which changes to planar in solution . Ph3PO forms 4 - coordinated distorted tetrahedral complex of the type [Ln{N(SiMe3)2}3 (OPPh3)] . Six coordinated trigonal prismatic complex , Pr[S2P(C6H11)2]3 is also known . The bonding in complexes with low coordination numbers can be explained easily as compared to those with high coordination numbers . Even if all the valency shell orbitals (i.e. one s , three p and five d) are used for bonding , it accounts only for a maximum coordination number of 9 . Higher coordination numbers of 10 and 12 indicate the participation of f - orbitals in bonding or bond orders of less than one . Cerium is the only lanthanoid with a significant chemistry even in + 4 oxidation state . The most important complexes formed by Ce(IV) are 12 - coordinated , water soluble double nitrate , (NH4)2 [Ce(NO3)6] and 10 - coordinated [Ce(NO3)4 (OPPh3)2] . Complexes with β - diketonates and other O - donor ligands are also known . The sulphates such as , Ce(SO4)2.n H2O (n = 0 , 4 , 8 , 12) and (NH4)2 [Ce(SO4)6] are very common . Iodates and fluoro complexes such as [CeF8]4- and [CeF6]2- are well known . [CeF6]2- is an 8 - coordinated , square antiprismatic complex . Orange coloured octahedral chloro complex , [CeCl6]2- is also stable . Organometallic compounds : The organometallic chemistry of the lanthanoids is not as extensive as that of transition metals . This is due to the inability of lanthanoids to engage in bonding with π - bonding ligands . However , it has shown appreciable growth in the recent past and has become predominantly , though not exclusively , the chemistry of yclopentadienyls , alkyls and aryls . These organometallic compounds are thermally stable but sensitive to air and moisture . Cyclopentadienyl Compounds : The cyclopentadienyl compounds are actually the salts of C5H5

- anion . A series of cyclopentadienyl (C5H5 or Cp) compounds of the type [Ln (C5H5)3] , [Ln(C5H5)2Cl] and [Ln(C5H5)Cl2] are prepared in anhydrous medium by the following reaction : LnCl3 + n Na( C5H5) THF LnCl3-n ( C5H5 ) n + n NaCl

These compounds are well characterized . The properties and structures of these compounds depend on the nature of the solvent as well as on the size of the lanthanoid ion and the steric demand of the cyclopentadienyl group . The structures of LnCp3 and Ln2Cp4Cl2 are given in fig 21 .

(a) (b)

Fig. 21 : Structures of (a) [Ln(C5H5)3] and (b) [Ln2(C5H5)4Cl2]

The tris ( C5H5) compounds are insoluble in non – polar solvents whereas the bis (C5H5) compounds dimerize . However , in polar solvents they exist as solvated monomeric species . The heavily substituted cyclopentadienyl rings such as C5Me5 (i.e. , Cp*) make the existence of [Ln(Cp*)3] difficult due to the steric demand . (Cp*)2LnIIIX and SmII(Cp*)2

are very well characterized and generally serve as the centre for catalytic polymerization of olefins . SmII(Cp*)2 undergoes dimerization with nitrogen bridging in presence of N2 . The entire sequence of reaction is explained in fig. 22.

Fig. 22 : Formation and dimerization of SmII(Cp*)2 The structure of these cyclopentadienyl compounds is very complicated . In [Sm(C5H5)3] , each C5H5

- is pentahapto towards one metal atom but some also act as bridges by presenting a ring vertex (η1) or edge (η 2) towards an adjacent SmIII atom . The blue solid , [Nd(C5H4Me)3] is tetrameric , each Nd being attached to three rings in a pentahapto manner with one ring being further attached in a monohapto manner to an adjacent Nd . The structure of red complex , [Yb(C5H4Me)2Cl] is comparatively simpler . It exists as a dimer with simple pentahapto rings and two chloride bridges . Complexes with analogous ligands , C9H-

7 (fig. 23 ) and C8H82- are also known .

Fig. 23 : Structure of C9H-

7

For example , complexes of the type [Sm(C9H7)3 ] and K[Ln(η8-C8H8)2] are common with lighter lanthanoids . Due to the bulky nature of the ligand , such complexes show very little tendency to solvate . The various Ln(III) members of the series are presumed to have “sandwich” structure. The hydrido derivatives of the Cp and Cp* lanthanoid compounds are also known . Such compounds are highly reactive and show activity as hydrogenation catalysts . Simple alkyl and aryl compounds of the type LnRn and LnArn are very difficult to prepare and are rare , mainly due to the preference of large Ln(III) ions for high coordination numbers . LnR3 and LnAr3 compounds are usually solvated . For example Ph3Er(THF)3 . They are generally prepared with lithium reagents in ether solution . LnCl3 + 3LiR LnR3 + 3LiCl LnCl3 + 4LiR Li[LnR4] + 3LiCl3 Bulky alkyl groups such as – CH2CMe3 and – CH2SiMe3 form stable complexes of the type [LnR3(THF)2] . The triphenyls are usually polymeric . The first fully characterized complex of the type was [Li(THF)4] [Lu(C6H3Me2)4] . Octahedral [LnMe6]3- have also been isolated for most of the lanthanides . With bulky ligands , lanthanides form bis – arene sandwich compounds (fig 24 ) .

Fig 24 : Lanthanoid Bis - Arene sandwich compounds The divalent lanthanides , Sm , Eu and Yb form Grignard - like compounds , which are structurally more complex than the grignards but can be effectively used as alkylating agents .

Uses of Lanthanoids and their Compounds (i) Uses of alloy (misch metal ) : Certain alloys of lanthanoid elements , known as misch metals containing 50% of Ce together with smaller quantities of other light lanthanoids are used in the production of heat resistant , stainless and instrumental steels . These are pyrophoric alloys and when scrachted gives off sparks capable of igniting flammable gases . Hence the alloys can be used as flints in cigarette lighters , miners safety lamps and automatic gas lighting devices . Another rapidly expanding application of misch metal is in batteries . Misch metal is a component of nickel metal hydride batteries that are replacing nickel cadmium batteries in powering portable electronic equipment such as lap tops , computers and mobile phones . Mg alloys containing about 3% misch metal and 1% Zr are used for making parts of jet engines because these have sufficiently high strength and creep resistance at 450 - 600оF . Misch metal is an

excellent scavenger for oxygen or sulphur in many metal systems . Both misch metal and yttrium have a marked nodulizing effect upon graphite and thus enhance the malleability of cast iron . (ii) Ceramic applications : These include cerium (IV) oxide and cerium – rich (> 40 % CeO2 ) oxide mixtures as highly efficient glass – polishing compositions ; neodymium and praseodymium oxides as colouring agents for glass and in the production of standard filters . Lanthanum oxide is used in the preparation of low – dispersion , high – refraction optical glass . Cerium (IV) oxide is used to improve the stability and discolouration resistance of glass to gamma or electron – beam radiations . Cerium (IV) and neodymium oxides are used to counteract the iron (II) – produced green in glass . 3 % praseodymium oxide in combination with zirconium (IV) oxide is used to give a yellow ceramic glaze . It is used to opacify enamels . The high melting points of the oxides , certain sulphides (e.g. , CeS ) , borides , carbides , and nitrides suggest their use as refractories , although some of these are reactive at high temperatures . Lanthanoids are also used as UV absorbers and antibrowning agents , additives to structural ceramics such as stabilised zirconia and silicon nitride Si3N4 , and in optical lenses and glasses . CeO2 is used in the manufacture of protective transparent glass block . The mixture of CeO2(47%) , La2O3 + Nd2O3 + Pr2O3 (51%) + SiO2 , CaO , Fe2O3 is called polirite . It is used as abrasive for polishing glasses . (iii) Catalytic applications : Lanthanoid compounds are good catalysts for a number of reactions such as hydrogenation , dehydrogenation , refining of crude oil and oxidation of various organic compounds . Addition of 1 to 5% lanthanoid chloride to a zeolite catalyst increases the catalysts cracking efficiency . They are also included in catalytic converters in automobiles where they stabilise the gamma - alumina support and enhance the oxidation of pollutants. The anhydrous lanthanoid chlorides are used in polyesterification processes ; and the chlorides and cerium phosphate in petroleum cracking . In the sense that heterogeneous catalysts are commonly characterized by unpaired electrons , paramagnetism , defect structures , or variability in oxidation state , catalysis is a promising practical area . (iv) Magnetic and electronic applications : Industry has been slow in taking advantage of the para - and ferromagnetic properties of the lanthanoids . The low electrical and eddy current losses of the ferrimagnetic garnets , 3 Ln2O3 . 5 Fe2O3 , make these substances useful in microwave devices and as magnetic core materials . The yttrium compound “yig” is particularly important . Certain compounds , e.g. , selenides , tellurides are of potential interest as semiconductors or thermoelectrics . The titanates and stannates, are useful ceramic capacitors , because of high dielectric constants and small temperature coefficients of capacitance . (v) Nuclear applications : These include actual and potential uses of high – cross - section metals or compounds mainly in control , shielding and flux – suppressing devices . Their oxides are used as diluents in nuclear fuels . Metals are also used for making structrural compounds of the reactor . For example , yttrium pipes are used which are not attacked by molten U even at 1000оC . Tm and Eu are used as radiation

sources . Pr is used in the production of atomic batteries . Thorium is a potential atomic fuel source . (vi) Metallurgical applications : Lanthanoids are used as reducing agents in metallurgical operations . About 30% of lanthanoids are used in metallurgy as an alloying agent to desulphurise steels , as a nodularising agent in ductile iron , as lighter flints and as alloying agents to improve the properties of superalloys and magnesium , aluminium and titanium alloys . CeS is used in the manufacture of special crucibles which can withstand temperatures up to 1800○C . (vii) Miscellaneous applications : Cerium salts are used for increasing haemoglobin content of blood and for the treatment of vomitting and sea – sickness . Lanthanoid compounds are filled into carbon electrodes to give brilliant light . La , Ce , Eu , and Sm salts are used in luminophores as activators and in the coatings of luminescent lamps . Salicylates of Pr, Nd a known as dimals are used as germicides . Lanthanoid compounds are used as insectofungicides and as trace elements in fertilizers . Ce (SO4)2 is used as an oxidizing agent in volumetric titrations . Cerium salts are used in dyeing cotton and lead accumulators . Ce (NO3)4 is used as a mordant for alizarin dyes . Radio isotopes like La140 , Ce142 , En152 , Tb160 are used in co – precipitation and chromatographic separation . The major applications of lanthanoids and yttrium , scandium and thorium are listed in table 17 . Table 17 : Major applications of the lanthanoids , yttrium , scandium and thorium

Element Comment Applications Lanthanum Component of mischmetal. Ceramic glazes, high quality

optical glass, camera lenses, microwave crystals, ceramic capacitors, glass polishing, petroleum cracking.

Cerium Most abundant amongst the lanthanides. Chief component of mischmetal.

Glass polishing, petroleum cracking catalysts, alloys - with iron for sparking flints for lighters, with aluminium, magnesium and steel for improving heat and strength properties, radiation shielding, many others.

Praseodymium Yellow ceramic pigments, tiles, ceramic capacitors. With neodymium in combination for goggles to shield glass makers against sodium glare, Permanent magnets. Cryogenic refrigerant.

Neodymium Important in magnetic alloys. Ceramic capacitors, glazes and coloured glass, lasers, high strength permanent magnets as neodymium-iron-boron alloy, petroleum cracking catalysts.

Promethium Not found in nature. Radioactive; produced only in nuclear reactors.

Radioactive promethium in batteries to power watches, guided missile instruments, etc, in harsh environments.

Samarium Important in magnetic alloys. In highly magnetic alloys for permanent magnet as Samarium-Cobalt alloy; probably will be superseded by neodymium. Glass lasers. Reactor control and neutron shielding.

Europium One of rarest, and most rare reactive of rare earths. Absorbs neutrons.

Control rods in nuclear reactors. Coloured lamps, cathode ray tubes. Red phosphor in colour television tubes.

Gadolinium Solid state lasers, constituent of computer memory chips, high temperature refractories, cryogenic refrigerants.

Terbium Associated with gadolinium. Cathode ray tubes, magnets, optical computer memories; future hard disk components; magnetostrictive alloys.

Dysprosium Absorbs neutrons. Magnetic alloy. Controls nuclear reactors. Alloyed with neodymium for permanent magnets. Catalysts.

Holmium Absorbs neutrons. Controls nuclear reactors; catalysts; refractories.

Erbium Physical properties almost identical with Holmium and Dysprosium.

In ceramics to produce a pink glaze; infra-red absorbing glasses.

Thulium Gives x-rays on irradiation in nuclear reactor.

X-ray source in portable X-ray machines.

Ytterbium Properties very similar to Lutetium - not well known.

Practical values presently unknown. Research.

Lutetium Chemical and physical properties not well known.

Deoxidiser in stainless steel production, rechargeable batteries, medical uses, red phosphors for colour television, superconductors.

Yttrium Associated with Holmium, Erbium. Cold or hot forged

Deoxidiser in stainless steel production, rechargeable batteries, medical uses, red phosphors for colour television, superconductors.

Scandium Close to Aluminium in chemical and physical properties

X-ray tubes, catalysts for polymerisation, hardened Ni-Cr superalloys, dental porcelain.

Thorium Resembles nickel, as soft and as plentiful as lead. Radioactive.

Gas mantles. Can be used as nuclear fuel in place of uranium.

Some Comparisons and Contrasts As mentioned in the beginning , both yttrium and scandium could be considered with lanthanoids because of the similarities in their physical and chemical properties . Yttrium is similar to the lanthanoids in following respects : 1] Y occurs with lanthanoids in rare earth minerals , eg. monazite ;

2] The radius of Y3+ is comparable to that of Ho3+ and hence it is difficult to separate Y3+ from Ho3+ ; 3] Y occurs effectively exclusively in + 3 oxidation state . 4] YH2 is better known to exist as Y3+(H-)2(e-) , similar to Ln3+(H-)2(e-) . Likewise , scandium is similar to the lanthanoids , as detailed below :

1] Sc occurs effectively exclusively in +3 oxidation state . Compounds like Sc2O3 , ScX3 are well known ; 2] Sc forms conducting hydride ScH2 , better known as Sc3+(H-)2(e-) ; 3] Sc forms reduced halides of the type Sc7Cl12 , which is Sc3+ with Sc6 clusters . 4] Sc forms complexes of high coordination number with chelating O – donors e.g. Na+[Sc(CF3COCHCOCF3)]4 with coordination number 8 . 5] Most of the scandium compounds are obtained as hydrated salts e.g. Sc(NO3)3.4H2O and Sc2(SO4)3.5H2O. 6] Scandium forms organometallic compounds such as Sc(C5H5)3 . However , there are sufficient reasons to consider scandium as a 13 group element , with aluminium . In fact , it can be considered to be intermediate between the lanthanoids and aluminium . 1] Sc3+ (r = 74 pm) is appreciably smaller than any of the Ln3+ . 2] Sc2 O3 is amphoteric like Al2O3 and not like Ln2O3 . 3] ScF3 dissolves in excess F- to give ScF6

3 - . Lanthanoids , on the contrary , show scarcity of halogen compounds . 4] Anhydrous ScCl3 is easily obtained by dehydration of respective hydrated halide with P2O5 . However , unlike AlCl3 , ScCl3 is not a Friedal - Crafts catalyst . In some of the properties , scandium even resembles the first row transition metals . Like 3d metals , Sc usually forms complexes with coordination number 6 and its aqua ion , [Sc(H2O)6 ]3+ is susceptible to hydrolysis . As discussed earlier , the lanthanoids show some similarities and some dissimilarities with transition elements as well as actinoides . Some contrasts between lanthanoids , pre – transition and transition metals are given in table 18. Comparison between lanthanoids and actinoids are listed in table 19 .

Table 18: Some contrasts between Lanthanoids , Pre – Transition & Transition Metals Property Pre-Transition Metals Lanthanoids Transition Metals Valence Essentially Monovalent

Show Group(n+) oxidation state

Essentially Monovalent(+3) is characteristic oxidation state +2/+4 for certain configurations

Show Variable Valence (extensive redox chemistry) control by environment ∼ ligands, pH etc…

Periodic trends Dominated by effective nuclear charge

Dominated by Lanthanide Contraction of Ln3+

Size changes are less marked

Occurence Widespread on earth Common mineralogy Diverse mineralogy Ligand field effect No effects Insignificant effects Substantial effects Nature Always ‘hard’ (prefer O, Hl,

N donors) Always ‘hard’ (prefer O, Hal, N donors)

Later(increasingly from Fe)/heavier metals may show a ‘soft’ side

Nature of organometallics

‘Ionic & ‘Covalent’ Organometallics

‘Ionic’ organometallics ‘Covalent’ Organometallics

Coordination number

Poor Coordination Properties (C.N. determined by size)

High Coordination Numbers – 6,7,8,9 (C.N. determined by size)

Extensive Coordination C.N. = 4 and 6 are typical maximum (but many exceptions)

Geometry Flexibility in Geometry Flexibilty in Geometry Fixed(by Ligand Field effects)Geometries

Magnetism No Magnetism from the metal ions due to noble gas configurations of ions

Free Ion ground state magnetism

Orbital Magnetism ‘Quenched’ by Ligand Fields Excited J-states poulated

Spectra/ Properties ‘Ionic’ compound formulations large HOMO-LUMO gaps UV spectra

Weak, Narrow Optical Spectra Forbidden, unfacilitated transitions

Stronger, Broader Optical Spectra Forbidden transitions vibronically-assisted

Table 19 : Comparison between lanthanoids and actinoids Property Lanthanoids Actinoids Valence shell configuration Filling of the 4f subshell 5f subshell Characteristic oxidation state +3, +2 and +4 are known but rare Although all the actinoids exhibit a +3

oxidation state, it is not the most stable one for most of them. Oxidation state as high as +7 are also achieved, especially in early actinoids

Contraction Lanthanoid curve consists of two very shallow arcs with a discontinuity at a spherically Gd3+

Actinoid contraction initially parallels that of lanthanides, the elements from curium are smaller than expected

Nature of f orbital 4f orbital are sufficiently low in energy that the e-s are seldom ionized or shared. They are unaffected by the environment to a great extent

5f orbitals have a radial node, which 4f orbital lack. 5f electrons are available in the early actinides

Ligand-metal interaction Less More Absorption spectra Spectra are sharp and line-like Am3+ and heavier actinides have

spectra resembling Ln3+ but P43+ and

lighter actinoids have broader spectra Magnetic properties Spin-orbit coupling is large but

ligand field effects are small. Spin-orbit coupling and ligand field effects are of comparable magnitude

Coordination chemistry Show high coordination Show high coordination