TRANSITION METALS AND

COORDINATION CHEMISTRY

knowledge, maturing into an in tegral (011011', become scientific knowledge. Science in an ordered

qcthal knowledge and theory.

JCTIONofmetals in the middle of the periodic table, whose inner d- orf-orbitals are not completely

.ansition elements (or d- or f-block) elements. Alternatively, a transition element is

or at least one of its ions has incompletelyfilled d- or f- orbitals. Alternatively, a transition

defined as "as elementwhQ$e at Icastpne simple ion contains 1 to 9 electrons in d-orbitals

in f-orbitags

: (i) Iron, chromium, nickel, zinc and copper ; (ii) Gold, silver, copper and platinum.

series : There are in allfour transition series :

transition series contain ten elements from scandium

'liplete ,orbitais.

Id transition series contain ten elements from yttrium

lave incomplete 4d-orbitals.

to zinc .(At. No. 30).

cadmium (At.

transition series contain ten elements with incomplete 5d-orbitaIs and constitute

'h transition series has incompletely filledf-group element'. This consists of lanthanideshus, transition elements are :

up elements :

cries vration 4s 2 2 2

31.1 1 2 3

I series Y Zrseries La

67up elements :

'La' ce P! Nd Pm sm.

Cr co cu1 2 2 2 2 1 2

5 5 6 7 8 10 10

Cd

w Re

Dy Ho Er Lu'Ac' ThPA U NP Pu Am cm Bk Cf Es Fm Md No Lr.

(1053)

3d (or •rst) 4d (or second)

Elejnent (At. No.) Con uration Element (At. No.) Con ration Element (nt. No.) consc (21) 2 Y (39) 461 1 2

Ti (22)

v (23)

Cr (24)

Mn (25)

Fe (26)

co (27)

Ni (28)

cu (29)

Zn (30)

3d2, 42

3d5, 451

3d5, 452

3616

, 4s 2

3d8,

36110

, 4s l

Zr (40)

Nb (41)

Mo (42)

Tc (43)

Ru (44)

Rh (45)

I'd (46)

Ag (47)

Cd (48)

, 58 La (57)4d2,

(72)

4d4, 58 1 Ta (73)

4d5, 551W (74)

4(15, Re (75)

4d7, 551

os (76)

4d8, 5s1(77)

Pt (78)

" 10, 55

1Au (79)

41 0, 552

Hg (80)

CHARACTERISTICS OF TRANSITION ELEMENTSSome of the characteristics of transition elements are discussed below :

5di

54

54,

5d10

(1) Atomic radii : (i) Their atomic radii lie in between those of s- and p- block elements,ralues in a series,first decrease with increase in atomic number, but the decrease is smaller

Table 2. Atonzic radii o/d-block elements (in pm)

Sc Cr•144 132 122 117

Zr162 145 134 128

La w169 144 134 130

Mn117

Re128

Fe117

124

124

co116

124

Ir120

115

128

129

cu

125

134

Au134 144

Reason : Initial radius is due to increase in nuclear charge. Since the electron

Iters the penultimate shell, thereby the added electron shields (or screens) the outermost electrons

lith the progressive increase in inner electrons, their screening effect counterbalances the opposing

(ect of increased nuclear charge, thereby the atomic radii remains almost same after Cr.

(ii) At the end of the series (or period), there is a slight increase in the atomic radius. For example

omic radius of Zn (= 125 pm) is higher than that of Cu (= 117 pm).

Reason : Near the end of the period (or series), the repulsions between the added electm

the same d-orbitals become higher than the attractive force, due to increased nuclear charge,

ereby resulting in the expansion of the electron-cloud and hence, the atomic radius increases atthk

d of the series.

(iii) Atoniic radii of transition metals the group. For example, atomic radius

transition series) = 117 pm ; MO (2nd transition series, below Cr) = 128 pm ; W (3rd transition

•ies, below MO) = IS(YÉm.

Reason : This is due to addition ofa new shell down the group from 1st to 2nd to 3rd transition

AND COORDINATION CHEMISTRY1055

dii : (i) Ionic radii decreases in a series. For example :

Cr Mn Fe co cu90 88 84 80 76 74 72 69

This is due to progressive increase in thg.effec/@ve nuclear charge.

of transition decrease t increase in oxidation state and vice versa. Thus,

. The effective nuclear charge in MA+ ion is greater than that in M2+of transition elements are smaller than the s- and p-block elements belonging

of transition elements are high.

: Since the atomic volumes of transition elements are low (due to filling of the innerelectrons, together with progressive increase in nuclear charge), so their densities

(4) Metallic character : Transition elements possess all the characteristic

and are

properties

changed

of

topositive

metals,lose one or two ns or ns electrons under appropriate conditions

Thus, they are solid (except Hg, which is a liquid at room temperature), hard, lustrous,able, Juctile and good conductor of heat as well as electricity and possess high tensile

igth,

: Transition metals have 1 or 2 electrons in tlleir outermost orbit (ns 1-2), and their ionization

relatively low, so they form ntetallic bonds, having hcp or ccp or bcc lattices. Moreover,d.electrons also cause the formation of metallic bonds. Consequently, greater the number

mpaired Il-electrons, stronger is the metallic bonding, because of overlapping of unpaired electronsatonts. Hence, Cr, MO and W having illaximunl number unpaired d-electrons (= 5); while Zn, Cd and Hg having no unpaired electrons are low melting and soft metals.

IS It(lllid, dile to the absence of unpaired d-electrons.

Note : Due to low value of its elasticity, copper possesses lotv yield-point (or crushing point). Consequently,pressure starts flowing and hence, exhibits ductility and lnalleability. Since zinc possesses comparatively

tv,llue of elasticity, so brass (an alloy of copper and zinc) is tenacious.

(5) Melting and boiling points of transition metals The melting point, how-vises to a maximum value and then falls as the atomienumberincreases in a series.Reason : High melting and boiling points of transition elements is due to strong metallic bonds

ir to . This is clear from their high values of enthalpies of atomization. Now thelgth of metallic bond depends upon the number of unpaired-electrons (or half-filled d-orbitals).e the number of unpaired electrons increases upto d5 configuration (e.g., Cr in 1st series) anddecreases upto d10 configuration (e.g., Zn in 1st series), consequently their melting points

upto Cr and then decrease to Zn. Hence, melting points of Zn, Cd and Hg (= 234 K) areit in their respective transition series.

(6) Ionization energies : (i) The values of ionization energies of d-block element lie betweens-block elements on their left and p-block elements on their rightEThey are less electropositivethe s-block elements and more electropositive than p-block elements.

(ii) The ionization energies increases, but irregularly with increase in atomic number of the ele-infirst transition series (see Table 3).

ENGINEERING CHEMISTRY1056

Table 3. First ionization energies offirst transilion series.

Element Cr Mn co cu

IE (kJ mol -I ) 631 656 650 652 717 762 758 745 905

Reason : As the atomic number increases from scandium to copper, following two opposing

forccs increase sinniltaneously :

(1) Due to increase in nuclear charge, the attraction between the nucleus and the -inner electrons

increases.

(ii) Due to screening effect, caused by the addition ofntore electrons in 3d-orbitals, the outer electrons

However, the screening effect is nearly equal _to attractiveforces, due to nucleus on the inner

electrons, so there is marginal and irregular variation in their ionization-energies.

(iii) First ionization energies of5d-ele111ents are higher than those of3d- alid 4d- elentents.

Reason : Due to the comparatively tveaker shielding (or screening) effect of the nucleus on

4f-electrons in case of 5d-elements, there is a greater förcedattraction between the nucleus and

the valence electrons. In other words, there is greater effective nuclear charge acting on outer valence

electrons in case of 5d-elements. Hence, the first ionization energies of 5d-elements are higher than

those of 3d- and 4d-elements.

(iv) Second ionization energies of Cr and Cu are exceptionally high.

Reason : The electronic configuration of : Cr+ = [Arl 3d5, and Cu + = [Arl 3d1() . Thus, both

Cr+ and Cu + have stable configuration of exactly half-filled [3d5] and fully-filled [3d10] d-orbitals.

Consequently, removal of one electron form these, to give Cr and Cu Ions, means change from

a Illore stable state to less stable configuration, [Ar] 3014 and [Ar] 3d9 respectively. Since such a change

is quite difficult, so second ionization energies of chromium and copper are sufficiently higher than

those of their neighbours on the left as well as right.

(v) The Illagnitude of ionization energies of the transition metals is inversely linked to the stability

of their conipounds. For example, first four ionization energies of nickel given

below :

(Ell + E12) MJ mol

2.49

2.66

From the above, it is clear that :

(E13 + MJ mol -1 Total

8.80 11.29 MJ mol -1

6.70 9.36 M] mol -1

(i) Second ionization energy of 'nickel is less than that of platinum, so N12+ compounds are

1/101? stable that Pt compounds.

(ii) Fourth ionization energy of platinum is less thall that of nickel, so Pt4+ compounds are

Illore stable than N14+ compounds.

It may be pointed that stability of contpounds depends upon their electrode potentials (EO), which

depends upon sum of the enthalpy of sublimation, ionization energy and hydration energy.

sublimation Ellhydration

M (s) Mg (g) M+ (g) M+ (aq)

—e

N METALS AND COORDINATION CHEMISTRY1057

EO values (M2+/M) for 1st series of transition metals is given below :

Cr coElement

-0.91 -1.18 — 0.44 -0.28cu

-1.18(V)

+ 0.34ECM"/M

Thus, there is no regular trend in EOM2+/M values as compared to (IEI + 1%). Moreover, subli-

energies of these elements also do not exhibit any regular trend. Hence, the of

ionization energies give a rough idea regarding the stabilities of their compounds.

(7) Oxidation states : One of the most important characteristics of transition elements is their

ability to show variable oxidation states, onds

which

to the group are related

nuntber. to their

Some electronic

of the common configurations.

oxidation Usually,

statesthe Iki$het oxidation state corresp

of first ransltion series are grven in Table 4.e

Table 4. Oxidation states offirst transition series (or 3d-series)

Elclncnt

Scandium (sc)

Titanium (Ti)

Vanadium (V)

Chromium (Cr)

Manganese (Mn)

Iron (Fe)

Cobalt (Co)

Nickel (Ni)

Copper (Cu)

Zinc (Zn)

At. No.

21

22

23

24

25

26

28

29

30

Con tration

1

[Arl 452, 3d

2[Arl 4s , 3d

3

3d5

5

[Arl 4s2, 3616

7

8

[Arl 4s , 3,1101

[Arl 4s2, 3d1()

Oxidation states

(+ 1), + 2, +3, (+ 4), (+ 5), (+ 16)

Reason : The variable oxidation states of these metals is due to theovailability of both (u — 1) d

and ns-electrons for bond ormation, as the energies of penultimate d-orbitals exhibit

and

+

ultimate

2 oxidation

s-orbitals

state.

are

Innearly equa ost of them have two Its-electrons, so generally they

addition, they may also utilize one or more (n — 1) d-electrons for bond formation, so oxidation

states of + 3 and higher also occurs. The sunt total of IIS- and unpaired (n — 1) d-electrons deternlines the

highest oxidation state shown by a transition element.

Examples : (i) The outer electronic configuration of scandium is 3dl, 4s2. It exhibits anoxidation state ofs+2by utilizing both 4s-electrons. It can also exhibit an oxidation state of + 3when it utilizes its two 4s- and one 3d-electronS.

(ii) Titanium (with outer electronic configuration 3d2, 4s2) shows an oxidation state of +2,when it utilizes both 4s-electrons; of + 3, when it uses both 4s-electrons and one 3d-eIectron; andof + 4, when it utilizes both 4s-electron and the two 3d-electrons.

(iii) Vanadium (with outer electronic configuration 3d3, 4s ) exhibits oxida ion states of+ 2, +3, + 4 and + 5, depending upon the number of 3d-electrons utilized for bond f rmation.

(i?) Chromium (with outer electronic configuration 3d5,4sl) shows oxiHation states equal to+ 2, + 3, +4, + 5 and + 6, depending upon the number of 3d-eIectrons utilized for bond formation.

1056

The maximum valency exhibited by chromium is + 6, since the total numberplus in 5 + 6. A few

chromium

Contpound Cro crß)g cr2(S04h

idalion state o! Cr

(o) Manganese (with outer electronic configuration 31, 492) exhibits2, + 3, + +6 and + 7.

(vi) Elements iron (3116, 4s2), cobalt (3117, 452), nickel (3d , 'Is copper (31110(3,110, in which 31/-electrons are more than five, exhibit maximum oxidation

(vii) If an element exists in more than one oxidation state, their relati ,predicted from their standard electrode potentials. For example :

Cuf ((111) + e - Cu (s) ; = 0.52 V

2 Cu (s) ; Eo = 0.344

Hence, Cu2f is ntore stable than Cu+. In other words, Cu is reduced more easily to Cu.(viii) The oxidation state of a transition metal is dependent on the nature

atonts. Thus, compounds of transition metals with F and O exhibit the highest oxidationelectronegativities of F and O are very Iligh.

(ix) The oxidation state of a transition metal depends on the nature of the solventFor example, Fe is unstable in aerated (vater, since it undergoes oxidation in it. Similarly,unstable in (Oilter, since undergoes oxidation ; while Cro is stable in water.

(x) Transition metals also exhibit + 1 and () oxidation states. For example, oxidationNi in is zero. It may be pointed that bonding in compounds like Ni(CO)4 is not

it will be discussed in next chapter.

(xi) Oxidation state increases vertically downwards with atomic number in a groupd

transition metals. For example, common oxidation states for Fe are + 2 and + 3; while forOs of the same group are + 4, +6 and + 8.

(xii) Ilighest oxidation state increase down the group. For example, highest oxidationstlt:

shown by Fe, Ru and os are + 2, + 3, (+ 6) ; + 2, + 3, +4, (+ 5), (+ 6), (+ 7), (+8) ;+2,

respe.ctively.

(8) Paramagnetism : A 'parantagnetic' substance is one, which is attractedänto a

Para mogne'tism is associated with the presence of unpaired electrons in the

Most 01 the compounds of transition elements are parmnagnetic in nature. This is due toll

presence. of unpaired electrons in the (n — 1) d- or (n — 2)f-orbitals. Thus, onl!LJJlQ$L'Jt0111i.lCJJß

Itavrng unpaired electrons show parantagnetistn. The greater the nuntber of unpaired electrons

(atonl or• ions), the "tore strongly parantagnetic it is. Thus, Cu+ (3d10, 450) anci

diantagnettc (i.e., repelled when placed in a magnetic field) and parantagnetic respectively,

they contain zero and one unpaired electron respectively. Similarly, Fe2+ (3d6

(Ad5, (so) having respectively •4 and 5 lunpaired electrons are both Pdrmnagnetic.

is "lore strongly paranmgnetic than Fe +, because the former possesses greater

elections than the latter.

gALS ANC C*QESTRY

a us expressed as

+2) Bmand rs tFæ

bekariour: A well as orE'1tal motxw a held, then

electron vs sum-rat to

e+ctnpt (a charged part«le) a duea paramagnettc substance (t.e„ havutg

magnetr moment (due to sptn and orbitalthe magnetic moment tnduv•d bv the apt'ttedCc•nseguently, such a suEstance expenenees attrxtra• tn the magnettc Oeld.

of 3å-orS1taLs m tons of Nl•settes ate as follows

2 3 4 4 3 2

paramagnetic moment (or paramagnetic behauour) t'tcreases from to Ntn andto Zn:- ; and only Z.n:• is dutma€netrc.

Formation of complexes : Tae cations of transttion metals ate almost untqtte in thettctgbendertcy to form complexes mth molecules (such as water, ammonta, cat+on ntonovde, et

•acc, ; ; .

Reason : zoa formation transuhon element tons is because trattsttten "tetats vteld.. t'.•ach ha:v co.-ant (n — I) d-.rFrtals 'f aprrcvmatelv the approprtate CttCtSV to

'lectrors, donated ether Stoups or "'telecules such as evantåc avtcr titelectde,monordz' molecules or tons which add themselves to the

of metals, are called For example, ferrous and ferrtc tons torm follow Ingions With 3 amde ions :

(i i - femxyarude ion. (u) FetCN)03 - -s ferrucyanrde ton,

the central ',ons. re-* and Fe are surrounded respecttvelv and six cyanide ions.Sm.larly, the central atom, mckel rs surrounded by CO molecules.

( 10j Formation of coloured ions : cemplcx (Y trattsttiort "teta.ls an', itsuallv,m form and m T'rv. coloured posttive ions itt

Reason : TYE colour trans.'tn•t metal assuateti tnc0'rpletelvfilled (n — 1) '1-01+1tals.d-ek•ctmns undergo electronic transittons front enc

'c another tlns d-d-transitton process. absorb certat't radtattons (from theand emit the remat*der as coloured light Se the of an Jen is cornplt"tcntary Oftlte

colour lt- Hence. coloåred •orts are formed due to transitions, C"luch fall V?ltht't theelements For example, I'lac tn colour, because It absorbs

colou.c (reedd for excitation) from the vvslble light, leavtng the complementary blite colouredavewngth to pass

exhibited by a trawsttton metal 'On depends upon the evutati0't state ot thei(blue); (dark (pale prnk); Mn (red); Feb

•joeo

(WI/ott'). ft may be pointed that the ltott-ntetalhc prt associated Withthe_go/our, e.g. Cuo (black), Cu(OH2) (pale blue), CuC1 the

CUSO,S (blue), etc Different colours exhibited by hydrated transition

Note : on white colour, complimentary

and purple, 01) orange and blue. yellow and indigo.

Table 5. Colours of different hydrated transilton metal ions

Ion

(111)

v (111)v (11)Cr (111)

Mn (111) •

Mn 01)

Fe (Ill)

co (Il)

cu (11)

Zn (Il)

()utercon uratton No. o un aired electrons

3rd)

3112

31

3<14

3d5

3d5

3,17

3,110

3,110

Nil

Nil

One

Two

Three

Three

Four

Five

Five

Four

Three

Two

One

Nil

Nil

cot

White

Purple

Green

Violet

Violet

Violet

Pink

Yellow

Green

Pink

Green

Blue

White (colourless)

White (colourles)

(Il) Catalytic properties : Many transition metals and their compounds are foundcatalysis. For e:xample :

(i) finely@ivided ntckel is used as a catalyst in the hydrogenation of oil tv fgt. duricfmanufacture of vegetable ghee ;

(n) Iron is used as a catalyst in Haber's_process for the manufacture ofintenaction of nityggen andYdyggen

(lit) platmtg_tn and vanadium pentoxide are employed as catalysts in the manufacture cir

phur1C acid by contact process ;

(IV) Mn02 is used as a catalyst, during decomposition of hydrogen peroxide solution;

(v) ztnc chromite is employed in catalyst in the synthesis of methyl alcohol;

(VI) decomposition of bleaching powder solution is catalysed by cobalt salts.

Reason for catalytic power of transition metals .6•The catalytic power of transibonö

probably though the use of the (n — 1) d-orbitals due to the formation

zvlltcll adsorb and activate the reacting stibstances, Thus, they acÜ as catalyst", due to

AND COORDINATION CHEMISTRY1061

Because of presence of incomplete d-orbitals, they can form gnstable intermediatethe reactants. Transition métatcompounds are believed to operate as catalys' byprotndmg

energy pathways, either by txtriatiotiin the oridati«m state of transition métal

F -E/ttfcrhlåatcs. during the conversGoRR31to the catalyst V202 is

, F to the ability of vanadium to have Ytrral oxidation states. The probable mec)nntsm

action is as follows :

tølyocOv}Eadsorbs a SC)2 molecule on its surface and gives it an oxyge•t atom to form

rtrøxde.

d,vanadium tetroxide is then converted back to by the reaction with oxygen2 V204 + 02 ---...-..-..-* 2 vz05

. It nay be pointed thatfinct the transition metal used, triter is its catalytic activity, stnce finely dividedsufføce area and more nunvtrr cffrre aalencu•s at Its surface for the odxrpf•on rectant

Formation of interstitial compounds : Jnterstitial compounds are those in snullH, C, N, B, etc) occupy the Interstitial sites (or empty spaces or touts) in tir crystal tattre.metals form a number of interstitial compounds With small-sized atoms hke H. B, C, N,

Example : Steel IS an antersutia) compound, in which voids among iron atcmts are

: Atoms of transition metals are quite large m st:-c and they posg•ss smatt txnds in thetrSo atoms of H, B, C, N, etc. can easily psgtjms m rx»ås prewnt

lattues of transition metals, thereby Yielding nearly allyproperties : ( I ) It makes the transition metals morr and nsul. (2) it fh••

trans:uon metals. For example, steel a less malleable and ductlk th&i pure iron. but Itsthan that Iron.

(13) Alloy formation : Alloy defined as the tuea rnctal.

of alloys formed by transition metals : Tranutum t'*t.als jorrn tw :

V) Interstitial alloys m h atonts of (hit 'f, B, C N'tntcrstJtutl spaces the mctal exampk•. an alloy

carbon

(a) Solid solution alloys an• formed When one as Nt. C', Ca. V,tmn. For example, sterl (an alloy of le. N/ and c t)

solid allov : (an alloy oi and tut).Reøoø of alloy format"'" : Transition metab ionn alloys very easaly.

*tab arr quite similar and they mutually substitute h"molten transition metals are mutually •n a maxtute

't gives an alloy. Fot evample. nn kel erm to tormin nickel to form art mangane« to

vanadium in to form aik'ys am.and more cortoason-restst.uuv than the m«ais. Truwt*M

CF. V, W. co, etc, ate in the manuia•tumhigh-speed tool steely. etc.