COORDINATION COMPOUNDS - :: Science Tuts :: Home COMPOUNDS Molecular or Addition compounds w w w The development in the field of co-ordination compound or complex ion and organometallics

COORDINATION COMPOUNDS

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Chapter Outline:


Prerequisites•Learning Objectives•Introduction•Werner’s theory•Important terms related to co-ordination compounds•Nomenclature of co-ordination compounds•Naming of the complex ion•Isomerism in co-ordination compounds•Geometrical Isomerism•Optical isomerism•Bonding in co-ordination compounds and valence bond theory•Magnetic Properties•Crystalfieldtheory•Limiations of Crystal Field Theory•Bonding in metal carbonyls•Stability of co-ordination compounds•Applications of co-ordination compounds•Summary•


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PREREQUISITES

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Transition elements form complex ions because of their smaller radii. The compound containing complex ion are called co-ordination compounds.

A complex ion is an electrically charged radical which is formed by the combination of a simple cation with one or more neutral molecule or one or more simple anions or in some cases positive groups also.

Every complex ion contains a central metal atom or ion to which ligands are attached. Ligands are ions or molecules that surround metal ion in a complex ion. These can be neutral, positive or negative possessing at least one lone pair.

Ligands are attached to metal atom or ion by means of a co-ordinate covalent bond.Ligands can be monodentate (donating lone pair from one donar atom) bidentate or polydentate, two or more atoms can donate lone pairs.

Polydentate ligands whose structure permits the attachment of two or more donar sites at the terminals of a chain to the same neutral ion simultaneously thus closing one ormore rings called chelating ligands and the complex called co-ordination chelate compound.

LEARNING OBJECTIVES

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In this topic we are going to discuss about Werner’s theory of co-ordination compounds.

About the different terms used in co-ordination chemistry.

Rule for writing the Nomenclature of co-ordination compounds and their formulas.Isomerism exhibited by co-ordination compounds.

Bonding in co-ordination compounds with respect to valence bond theory and crystal field theory.

Stabilities of co-ordination compounds.

Importance and applications of co-ordination compounds.

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INTRODUCTIONAll transition elements exhibit a characteristic property of complex ion formation, because of their smaller radii.

Transition elements

The complex which contain complex ions are called co-ordination compounds and the branch of chemistry under these compounds is called co-ordination chemistry.

Co-ordinate compounds

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Molecular or Addition compounds

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The development in the field of co-ordination compound or complex ion and organometallics has contributed a lot in the following ways.

a) It contributed to the growth of chemical industry, such as metallurgy, catalysis etc.

Chemical industry

Biological system

Metallurgy

b) It provided insight into the structures and functioning of biological systems.

c) It contributed in the development of new concepts of chemical bonding and molecular structures.

When solutions containing two or more stable salts in simple molecular proportions are evaporated, crystals of new compound may separate out. These new compounds are called molecular or addition compounds.

Addition Compounds

Double salts

Complex compounds

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i) Double salts:

ii) Complex compounds:

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Those which loose their identity in solution. In aqueous solutions these break down into simpler ions thus exhibit the properties of these constituent ions.Eg:

Carnalite(KCl.MgCl2.6H2O)

Potash alumK2SO4.Al2(SO4)3.24H2O

Mohr’s SaltFeSO4.(NH4)2SO4.6H2O

Those which retain their identity in solution. In aqueous solution these do not furnish all simple ions but instead give more complex ion having complicated structure.

Potassium ferrocyanide K4[Fe (CN)6]

Hexaammine cobalt (III) chloride [Co(NH3)6] Cl3

Double Salts Complex Compounds

They exist only in crystalline state. They exist both in crystalline and in aqueous solution.

In aqueous solution, they yield simple ions. In aqueous solution, they yield at least onecomplex ion.

They are made of two simple salts combined in equimolecular proportions.

They are made of two simple salts, which may or may not be combined in equimolecular proportions.

They lose their identity in solution. They retain their identity in solution.

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WERNER’S THEOARYOF

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Werner proposed a theory called Werner’s coordination theory for which he was awarded noble prize in 1913. The main postulates of his theory are

Werner

Metals posses

Primary or (principal) linkage

Secondary linkage

Primary linkages (valencies) are ionisable and are exhibited by a metal in the formation of its simple salt. Now a days primary valency is referred to as oxidation state of the metal ion.

Secondary linkages (valancies) are non-ionisable and are exhibited by a metal in the formation of its complex ion. Secondary valency is also referred as co-ordination number of the metal ion.

NH3NH3

NH3

NH3H3N

H3N

Cl- Cl-

Cl-

Co3+

Primary valency is satisfied by negative ions while secondary valency can be satisfied by neutral molecules, negative ion or positive ion (ligands).Every metal atom or ion has a fixed number of secondary linkages i.e., co-ordination number of the metal atom is fixed.

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i) Complex ion:

ii) Central metal atom or Ion:

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Every metal has a tendency to satisfy both its primary and secondary valencies. The ligands satisfying secondary valencies are always directed towards fixed positions in space about the central metal atom or ion.

NH3 NH3

NH3H3N

Cl-

Cl-

Cl-

Co+3

Thus the co-ordination compounds have a definite geometry.

Octahedral

IMPORTANT TERMS RELATEDTO

CO-ORDINATION COMPOUNDS

A complex ion may be defined as an electrically charged radical which is formed by the combination of a simple cation with one or more neutral molecules or one or more simple anions or in some cases positive groups also. It exists as a single entity and is usually indicated with in square brackets.[Cu(NH3)4]2+ is complex ion which is formed by the combination of four neutral molecules of ammonia with a simple Cu+2 cation.

The complex ion carrying a positive charge is called cationic complex.

Eg: [Cu(NH3)4]2+

The complex ion carrying a negative charge is called anionic complex.

Eg: [Fe(CN)6]4-

The complex with no charge is called neutral complex.

Eg: [PtCl2 (NH3)2] Complex ions do not give tests of their constituent ions.

Every complex ion contains a metal atom or ion to which one or more neutral molecules or ions are attached. This is known as the central metal atom or ion.

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Unidentate (or) Monodentate ligands:

Polydentate (or) Multidentate ligands:

iii) Ligands:

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K4[Fe(CN)6]

K4[Fe(CN)6]

It is sometimes known as the nuclear atom, in a complex the central metal atom or ion behaves as the electron pair acceptor or lewis acid.

The central metal ion is surrounded by number of the anions or the neutral molecules or sometimes positively charged ions possessing at least one lone pair. These surrounded ions are called ligands.The ligands are attached to the central metal ion or metal through coordinate bonds or dative linkages.

Ligands act as electron pair donors or Lewis bases. Ligands invariably contain one or more atoms having lone pairs of electrons.Depending upon the number of donor atoms (or sites). The ligands may be classified into various categories.

They are:

These ligands contain only one donor atom (or site) which is capable of donating an electron pair and thus attach to the central metal ion only at one point.

Eg: CN-, F-, Cl-, Br-, OH-, H2O, NH3, NO2-, NO+, NO2

+, NH2NH3+, etc...

Ligands which contain two or more such atoms which can simultaneously serve as donor atoms are called polydentate or multidentate ligands. Ligands with two donor sites called bidentate, and with three, four, five and six donor sites called tri, tetra, penta and hexa dentate ligands respectively.

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Chelating ligands

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Eg: H2NCH2CH2NH2,H2NCH2COO-, , SO4

-2, EDTA, Pn, tn etc.COO-

COO-

H2C N

N

CH2COO-

CH2COO-CH2COO-

CH2COO-H2C

H2C N

N

CH2COO-

CH2COO-CH2COO-

CH2COO-H2C

Some ligands like SO4-2 act as both monodentate as well as bidentate ligands.Certain polydentate

ligands have flexi character i.e., they do not use all the donor atoms to form a complex.

H3N

H3N

NH3

CO

O

O O

O

S

NH3

SO4-2 as bidentate ligands

SO4-2 as monodentate

H3N

H3N

NH3

COO

O

OO S

NH3 NH3

Br

Eg: EDTA (Ethylen ediamine tetraacetate ion) is a hexadentate ligand but it can also act as a pentadentate or a tetradentate ligand.

Polydentate ligands, the structures of which permits the attachment of two or more donor sites at the terminals of a chain to the same metal ion simultaneously, thus, closing one or more rings are called chelating ligands and the compounds are called chelate compounds.

Chelate complexes are more stable than ordinary complexes in which the ligand is monodentate. This increased stability of the compound due to chelation is called the chelate effect. The complex compound having maximum number of rings formed with the ligands is most stable.

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a) Ambidentate ligands:

iv) Co-ordination sphere:

Co-ordination number:

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CH2 CH2

CH2 CH2H2N

H2N NH2

NH2

Cu

2+

There are certain ligands which have two or more donor atoms but in forming complexes only one donor atom is attached to metal ion such ligands are called ambidentate ligands.

M NO2 (Nitro)

M ONO (Nitrito)

M CN (Cyano)

M NC (Isocyano) M SCN (Thiocyano)M NCS (Isothiocyano)

Phenoxide ion also serves as ambidentate ligand due to resonance, Ortho- and Para - positions of the benzene ring in the phenoxide ion also become electron donors.

d-

d-d-

d-

O

Phenoxide ion

The central atom and ligands which are directly attached to it are enclosed in square brackets called the co-ordination sphere. The ligands and the metal atom shown inside the square bracket actually behave as a single constituent unit.The bonding between the metal atom or ion and ligands inside the co-ordination sphere is non-ionisable.

The total number of ligands attached to the central metal ion through co-ordination bond is called the co-ordination number (CN) of the metal ion.

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Oxidation number or oxidation state

Charge on complex ion

Effective Atomic Number (EAN):

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In [Cu(NH3)4]2+ the co-ordination number of Cu+2 is 4In [Fe(CN)6]-4 the co-ordination number of Fe+2 is 6

The co-ordination number of most common metals are 4 and 6.Ag+, Pt+2, Cr+3, Fe+2, Co+3, and Pt+4 show only one co-ordination number

Ag+ show co-ordination number = 2, Pt+2 show co-ordination number = 4, Cr+3, Fe+2, Fe+3, Co+3, Pt+4, show co-ordination number = 6.

It is a number that represents an electric charge in which a metal atom or ion has or appear to have when combined with ligands.

In [Cu(NH3)4]2+ the oxidation number of Cu is +2.

In [Fe(CN)6]4- the oxidation number of Fe is +2.

The charge carried by a complex ion is the algebraic sum of the charges carried by central metal ion and the ligands co-ordinated to the central metal ion.

[Cu(NH3)4]2+

Eg: In [Cu(NH3)4]2+ the charge on Cu ion is +2, NH3 is neutral so the net charge on the complex is +2.Complex ion is represented as shown.

Sidgwick proposed effective atomic number (EAN) in order to explain the stability of the effective complex. It is defined as “The resultant number of electrons with the metal atom or ion after gaining electrons from the donor atoms of the ligand”. Generally EAN coincides with the atomic number of the next inert gas expect in some cases.

EAN = Z (atomic number of the metal) – number of e- lost in the ion formation + number electrons gained from the donor atoms of the ligands.

K4

Rn

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Order of naming ligands

Naming of Ligands

1) Negative Lignads

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NOMENCLATURE OF CO-ORDINATIONCOMPOUNDS

Nomenclature is important in co-ordination chemistry because of the need to have an unambiguous method of describing formulas and writing systematic names particularly when dealing with isomers. The formulas and names adopted for co-ordination entities are based on the recomendations of the IUPAC (International Union of Pure and Applied Chemistry).

If the complex compound is ionic, the positive ion whether simple or complex is named first followed by the negative ion.

Eg:

Eg:

Eg:

K[BF4] K+[BF4]-

(Potassiumtetrafluoroborate(III))

[Ag(NH3)2]Cl [Ag(NH3)2]+Cl-

(Diammine silver(I)chloride)

But one word name is given to non-ionic and molecular complexes.

[Ni(CO)4]Tetracarbonylnickel(0)

The number of each kind of ligand is specified using the Greek Prefixes like di, tri, tetra, pentaand hexa. If the name of the ligand is itself complex (i.e., it includes a numerical prefix like dipyridyl or ethylene diamine) then the term bis (for two), tris (for three), tetrakis (for four), pentakis(for five) and hexakis (for six) are used followed by the name of ligand.

1. [Cu(NH3)4]2+ Tetraammine copper(II)ion

Tri(ethylene diamine) cobalt(III) ion2. [Co(en)3]3+

The name of negative ligands normally ends with ‘O’. Examples are mentioned in the tables.

1 Mono dentate ligands with one unit negative charge

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Monodentate Ligands with two Negative Charges

Name Formula Charge

Name Formula Charge

Name of the ligand in the complex

Name of the ligand in the complex

Donor atom

Donor atom

Halide ion (X = Cl-, Br-, I-) -1 Halo XHydroxide ion HO- -1 Hydroxo OCyanide ion -C N -1 Cyano CIsocyanide N+ C- -1 Isocyano NNitro ion NO2

- -1 Nitro N Nitrito ion ONO- -1 Nitrito OThiocyanate ion SCN- -1 Thiocyanato S

Thioisocyanate ion NCS- -1 Isothiocyanato NHydride ion H -1 Hydrido HAmide ion NH2

- -1 Amido NAcetate ion CH3COO- -1 Acetato ONitrate ion NO3

- -1 Nitrato O

Cyanate ONC- -1 Cyanato O

Isocyanate NCO- -1 Isocyanato N

Sulphate ion SO4-2 -2 Sulphato O

Oxide ion O-2 -2 Oxo OPeroxide ion O2

-2 -2 Peroxo OCarbonate ion CO3

-2 -2 Carbonato OSulphide ion S-2 -2 Sulphido OSulphite ion SO3

-2 -2 Sulphito OThiosulphate ion S2O3

-2 -2 Thiosulphato SImide ion NH-2 -2 Imido N

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2. Neutral ligands

Bidentate Ligands with one or two Negative Charges

Polydentate Ligands having Negative Charges

Neutral Monodentate Ligands

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Name(symbol) Formula Charge Name of the ligand Donor atom


Oxalate ion (OX) COO-

COO--2 Oxalato Two-O-atoms

Acetyl acetonate

Glycinate ion

O

CH3- C - CH = C - CH3

-O

-1 Acetyl acetenato Two-O-atoms

CH2 - COO-

NH2

-1 Glycinate One-N-atom & One-O-atom

H2C N

N

CH2COO-

CH2COO-

CH2COO-

CH2COO-H2C

CH2 N

N

CH2COO-

HCH2COO-

CH2COO-CH2

Ethylene diaminetriacetate ion(pentadentate)

-3 EDTA3-

-4 EDTA4-

Two-N-atomsThree-O-atoms

Two-N-atomsFour-O-atoms

Ethylene diaminetetraacetate ion(Hexadentate)

Neutral ligands have no special endings

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3. Positive Ligands:



Ammonia NH3 Zero Ammine N

Water H2O Zero Aqua(old Aquo) O Phosphine PH3 Zero Phospine P

Nitrogen oxide NO Zero Nitrosyl N

Carbon monoxide CO Zero Carbonyl CPyridine(Py) C5H5N Zero Pyridine N

N

N N

Thiourea(tu) NH2-C-NH2 Zero Thio - urea S

S

Triphenyl phosphine (C6H5)3P Zero Triphenyl phosphine P

Thiocarbonyl CS Zero Tricarbonyl S

Neutral Bi, tri and Polydentate Ligands

Ethylene diamine (Bidentate)

NH2-CH2-CH2-NH2 Zero Ethylene diamine Two-N-atoms

2,2_dipyridyl(dipy) (Bidentate)

.. ..

Zero Dipyridyl Two-N-atoms

Diethylene triamine (tridentate)

NH2-(CH2)2-NH- (CH2)2-NH2

Zero Diethylene diamine Three-N-atoms

Triethylene tetramine (tetradentate)

NH2-(CH2)2-NH-(CH2)2-NH-(CH2)2-NH2

Zero Triethylene Four-N-atoms tetramine

These are very few and end with ‘-ium’.

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NAMING OF THE COMPLEX ION

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Nitronium ion NO2+ +1 Nitronium N

Nitrosonium ion NO+ +1 Nitrosonium N

Hydrazinium ion NH2NH3+ +1 Hydrazinium N

5. The order preference of the ligands in the complexes according to old system was as

i) Negative ligandsii) Neutral ligandsiii) Positive ligands

There should be no hyphen in between. In case there are more than one negative or positive ligands they are listed alphabetically.IUPAC rules 1991 recommend that all ligands whether anions, neutral or positive be arranged alphabetically without any preference order.6. Certain ligands like NO can act as a neutral as well as unipositive ligand. In such case it is very difficult to find the oxidation state and the charge on the ligand from the given formula of the complex.

In naming the complex ion, number and name of the ligands are given first, then the central metal atom followed by its oxidation state indicated by roman numerical in parenthesis.

[ Pt(NH3)2Cl4 ] Diammine tetrachloroplatinum(IV)

If the complex ion is a cation, the name of the central metal ion is given as such followed by its oxidation state indicated by numerals (such as II, III, IV) in the parenthesis at the end of the name of the complex without a space between the two. If the complex ion is an anion, the name of the central atom is made to end in ‘ate’ followed by the oxidation number in brackets without any space between them. For an acid we have a characteristic ending (ic).

Eg: [Pt(NH3)2Cl4]

Na3[ AlF6 ]

H4[Pt (CN)6]

Diammine tetrachloroplatinum(IV)

Sodium hexafluoroaluminate(III)

Hexa cyanoplatinic(II)acid

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Formula writing:

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If the complex contains two or more metal ion it is turned polynuclear complex. The ligandswhich link the two metal atoms are called bridge groups and are separated from the rest complex by hyphens and denoted by the prefix ‘µ’ placed before their names.

[(en)2 Co Co(en)2](SO4)2

NH2

OH

Eg:

bis(ethylene diamine)cobalt (III) - m - amido - m - hydroxo bis(ethylene diamine) cobalt(III)sulphate

Common names are used in place of IUPAC names if

a) Structure is not certain

Eg: Zincate ion [ZnO2]-2

b) They are more convenient than IUPAC names

Eg: Ferro cyanide for [Fe(CN)6]-4 Ferri cyanide for [Fe(CN)6]-3

Following rules have to be followed while writing the formula of the complex from IUPAC names.

1. Cation whether simple or complex is written first followed by the anion.2. The order of formulating a complex ion is reverse to that adopted in naming i.e., the central metal atom is written first followed by the ligand.3. When there are more than one type of ligands present, they are arranged in the reverse order to that adopted in naming the complexes i.e., in the order as anionic ligands first, then neutral and then cationic ligands in the last, but this is not yet being fully followed. If there are a number of anionic ligands they are written alphabecally according to the first symbol of their formulae.Same for neutral and positive also.4. Whole of the complex ion is enclosed in a square bracket. 5. The charge on the complex species is equal to the charge on the metal atom i.e., its oxidation state plus the total charge carried by all the ligands coordinated to the metal atom i.e., thus the formula of tetraamine copper(II) chloride is [Cu(NH3)4]Cl2

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ISOMERISM

Ionisation isomerism

Hydrate isomerism

Linkage Isomerism

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Coordination compounds show mainly two types of isomerism

(i) Structural isomerism(ii) Stereo isomerism

It is of the following types

Ionisation isomers yield different ions in solution although they have same composition. This type of isomerism is due to the exchange of groups between the complex ion and the ions outside it.

Eg:1. [Co(NH3)5Br]SO4 is red-violet and in solution gives a precipitate of BaSO4 with BaCl2 confirming the presence of SO4

-2 on the other hand [Co(NH3)5SO4]Br is red and does not give test for sulphate ion in the solution, but instead gives a precipitate of AgBr with AgNO3.

2. [ Pt(NH3)4(OH)2SO4] and [Pt(NH3)4SO4](OH)2

3. [Pt (NH3)4Cl2]Br2 and [Pt(NH3)4Br2]Cl2

Isomers in which they differ in the number of water molecules in the coordination sphere are called hydrate isomers.

Eg: [Cr(H2O)6]Cl3 – violet ; [Cr(H2O)5Cl]Cl2.H2O – Bluish green;

[Cr(H2O)4Cl2]Cl2.H2O – Dark green

[Co(NH3)4H2O Cl]Cl2 and [Co(NH3)4Cl2]Cl.H2O

[Co(NH3)4H2O Cl]Br2 and [Co(NH3)4Cl.Br]Br.H2O

This type of isomerism is seen in compounds where monodentate ligands are having two or more donor atoms.

Eg:

H3N

H3N

NH3

NH3

2+

CoONO

NH3

H3N

H3N

NH3

NH3

2+

CoNO2

NH3

Penta ammine nitrito cobalt (III) ion Penta ammine nitro cobalt (III) ion

i) Structural isomerism

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Coordination position isomerism:

Ligand Isomerism

Coordinate isomerism

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GEOMETRICAL ISOMERISM

This type is due to the difference in the distribution of ligands in two coordination centers. Generally bridged complexes involving different ligands show this isomerism.

(NH3)4Co Co(NH3)2Cl2 Cl(NH3)3Co Co(NH3)3ClOH OH

OH OH2+

&

2+

Unsymmetrical Symmetrical

This arises in those complexes in which the two ligands are isomers themselves. This type is very rare.

CH2 _ CH _ CH3

NH2 NH2

CH2 _ CH2

_ CH3

NH2 NH21,2- diamino propane 1,3- diamino propane

This type is possible when both positive and negative ions of a salt are complex ions. The two isomers differ in the distribution of ligands in the cation and the anion.

Eg:

[Co(NH3)6] [Cr(CN)6] and [Cr(NH3)6] [Co(CN)6]

[Cr(NH3)6] [Co(C2O4)3] and [Co(NH3)6] [Cr(C2O4)3]

Geometrical isomers differ in the spatial distribution of atoms or groups about the central atom or atom in polynuclear compounds. Those complexes in which the two same ligands occupy adjacent positions to each other or opposite to each other are able to show this isomerism. cis (or) trans respectively

PtNH3

NH3Cl

ClPt

NH3

NH3 Cl

Cl

cis trans

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Square Planar Complexes

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This isomerism cannot be exhibited by coordination compounds having 2 (or) 3 coordination numbers as it is not possible to have more than a single arrangement of ligands in space around the central ion.

NH3

NH3NH3

NH3

+

CoCl

ClNH3

NH3 NH3

+

Co

Cl

ClNH3

cis

cis

cis

trans

trans

trans

Geometrical isomerism is not found among tetrahedral complexes of the type MA4 (or) MA3B (or) MA2B2 because all the four ligands are equidistant from each other.

Only square planar and octahedral complexes show this type of isomerism.

Square planar complexes of the type [M(AA)2], MA4, MA3B and MAB3 do not show this type of isomerism.Square planar complexes of the type MA2B2, MA2BC, and MABCD show this type of isomerism i.e., cis-trans isomerism.

Eg: (1) MA2B2 ---- Dichloro diammine platinum(II)

Eg: (2) MA2BC ---diammine chloro nitro platinum(II)

Eg: (3) MABCD ---ammine bromo chloro pyridine platinum(II)

PtNH3

NH3Cl

ClPt

NH3

H3N Cl

Cl

PtNH3

NH3

O2N

O2NPt

ClH3N

O2N NH3

PtPy

BrCl

H3NPt

Cl

H3N

Br

PyPt

Cl

H3N Br

Py

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This type of complex show three geometrical isomers

Geometrical isomerism can also occur in square planar complexes if the chelate group is not symmetrical.

Eg: [Pt(gly)2] Diglycinato platinum(II)

Eg: MA4B2 ______> [Co(NH3)3Cl3] tetraammine dichloro cobalt(III) ion

Eg: MA3B3 ____> [Co(NH3)Cl3] triammine trichloro cobalt (III)

PtNH2 NH2 CH2CH2

CoCov OOPt

NH2

NH2

CoCH2

CH2Co

O

OCis Trans

Geometrical isomerism is also shown by bridged binuclear planar complexes of the type M2A4X2 Eg: [PtCl2 PEt3]2

Pt PtEt3P PEt3

Cl Cl Cl

Cl Et3P

PEt3Pt Pt

Cl Cl

ClCl

cis trans

Octahedral complexesOctahedral complexes of [MA5B] or M(AA)3 [ where AA = Symmetrical tridentate ligand ] do not exhibit geometrical isomerism.

cis

cis

trans

trans

CoNH3

NH3

H3N

H3N Cl

Cl

CoNH3H3N

H3N

Cl

CoNO2

NO2

NH3

NH3

O2N

H3N

Cl

NH3

CoNH3

NH3

NH3

NO2

O2N

O2N

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Eg: Another type is facial (fac) and meridional(mer) isomers.

Facial (fac) Meridional (mer)

M(AA)2B2 where A-A can be any symmetrical tridentate like ethylene diamine(en) and B can be any anionic ligand such as Cl-, CN- ,NO2

- etc...

Pt

Cl

Cl

en

en

Pt

Cl

Cl

en en

Eg: [Pt(en)2cl2] bis(ethylene diamine)dichloro platinum( II )

Complexes having six different ligands[M(ABCDEF)] shall exhibit geometrical isomerisms theoretically fifteen different isomers should be possible. In practice, three isomers of the complex. [Pt(C5H5N)(NH3)(NO2)(Cl)(Br)(I)] have actually been isolated.

As in square planar complexes unsymmetrical bidentate ligands also give rise to geometrical isomerism in octahedral complexes of the type [M(AB)3].

OPTICAL ISOMERISMThis isomerism arises when a compound can be represented by two asymmetrical structures (called optical isomers).The two isomers are structurally the mirror images of each other and non-super imposableand do not possess the plane of symmetry.Optical isomerism is common in octahedral complexes involving 2 (or) 3 symmetrical bidentate groups of the type [M(AA)2X2 ], and [M(AA)3 ] and [M(AA)B2X2]

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Eg: Cis-dichloro bis(ethylene diamine) Cobalt (III) cis-form is optically active (d and l forms) where as trans - forms is optically inactive due to plane of symmetry.

Co CoCl Cl

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Cl Cl

[M (AA)2x2] [Co(en)2Cl2]

[M (AA)3] [Cr (en)3 ]3+

[M (AA)3] [Cr (C2O4)3 ]3-

Mirror

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Tris ethylenediamine chromium (III) ion

Trioxaloto chromate (III) ion

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[M (AA)B2X2 ] [CoCl2(NH3)2en]3+

Diammine dichloro(ethylene diamine) cobalt (III) ion

Complexes containing hexadentate ligand(EDTA) show optical activity Eg: [Co(EDTA)]-------ethylene diamine tetra aceto cobaltate(III) ion.

Complex of the type [MABCEDF] type also exhibit iptical isomersm.

Eg: [Pt(Py)(NH3)(NO2)(Cl)(Br)(I)]

The optical activity has been observed for chelated tetrahedral and square planar complexesbut only rarely. In tetrahedral complexes only the type which occur in bis chelates with unsymmetrical ligand has been detected. These have been found in Be(II), B(III), Zn(II) andCu(II) complexes.

Eg: Bis (salicyl aldehyde) boron (III) cation has been found to be racemic and resolution has been accomplished.

C=O

O=CO

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Valence bond theory

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Eg: The two enantiomers of bis (benzoyl acetonato) beryllium (II)

Eg: Square planar complexes are seldom optically active.

BONDING In Co-ordination COMPOUNDSThe formation of co-ordination compounds can be explained electronically by the numberof theories like

This was given by Linus Pauling.

Linus Pauling

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The main postulates are

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In this approach, the basic assumption made is that the metal ligand bonds arise by way of donation of pairs of electron from ligands to the metal i.e., through forming the co-ordinate covalent bond. In order to accomidate these electrons the metal ion must posses requisite number of vacant orbital of similar energy.

Orbitals of Co3+ion

3d 4s 4p 4d

These metal orbitals undergo hybridization to give a set of hybrid orbitals of equal energy with the approach of the ligands.

d3sp3 hybridised orbitals of Co3+

3d sp3d3 hybrid 4d

The ion of the transition elements have some incompletely filled d-orbital.Some or all the electrons in the incompletely filled d-orbital are unpaired.Sometimes the unpaired (n-1)d-electrons couple as fully as possible prior to bond formation. In this process some (n-1)d-orbitals become vacant. Thus central metal atom makes available a number of empty orbitals equal to co-ordination number of for the formation of co-ordinate bonds with suitable ligand orbitals.Since the energy of (n-1) d-orbital is only slightly less than that of s and p-orbitals of the n-orbit these vacant orbitals mix together to form new equivalent orbitals called hybrid orbitals.Each ligand has atleast one sigma orbital containing a lone pair of electrons.The hybrid orbitals formed take part in the formation of hybrid bonds with the ligands by overlapping. The bond formed takes place by accepting lone pair of electrons by the hybrid orbital of metal from the ligands. When (n-1)d-orbitals is used for bonding only two d-orbitals are used remaining do not participate in bonding and the complex called inner orbital complex.Some times in place of (n-1)d-orbitals, outer nd-orbitals are used for hybridization. The complex thus formed is called outer orbital complex.If there are unpaired electrons in d-orbital then the complex will be paramagnetic and no unpaired electrons diamagnetic.Inner orbital complex or low spin or spin paired complex. Outer orbital complex or high spin or spin free complex.

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Colour of complexes

Drawbacks of valence bond theory

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It describes bonding in co-ordination compounds only qualitatively.It does not offer any explanation for the optical absorption spectra of complexes.It does not describe the detailed magnetic properties of co-ordination compounds.

MAGNETIC PROPERTIESThe diamagnetic and paramagnetic behavior of co-ordination compounds can be explained on the basis of crystal field theory.Eg: Co (III) show diamagnetic properties in [Co(NH3)6]3+ and paramagnetic properties in [CoF6]3- complex which can be explained on the basis of crystal field theory. D0 is small, the energy required to pairup the 4th and 5th electrons with the lower d-orbitals (t2g) is higher as compared to that required to place the electron in higher d-orbital (eg) but D0 is higher in so 4th and 5th electron pairs up.

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The coloured nature of solutions of co-ordination compounds can also be explained on the basis of crystal field theory, because the energy difference between two sets of d-orbital is usually small thus excitation of electron from lower energy to higher energy is very easy and can be achieved even by the absorption of low energy radiations of visible region.

As a result of the absorption of such selected wave lengths of visible light, the complexes appeared coloured.

(a) Ruby: this gems stones was found in marble frome Mogo, Myanmar.

(b) Emerald: this gems stones was found in Muzo, Columbia.

EmeraldRuby

[Ni(H2O)6]2+(aq) + en (aq) = [Ni(H2O)4(en)]2+(aq) + 2H2OGreen Pale blue

[Ni(H2O)4(en)]2+(aq) + en (aq) = [Ni(H2O)2(en)2]2+(aq) + 2H2OBlue/Purple

[Ni(H2O)2(en)2]2+(aq) + en (aq) = [Ni(en)3]2+(aq) + 2H2OVoilet

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CRYSTAL FIELD THEOARY (CFT)This was developed by H.Bethe and V.Vleck in 1935.

V.Vleck H.Bethe

This theory is based upon the fact that the degenerate d-orbitals of metal ions are splitted energy wise due to concentration of point charges in certain specific positions.

This is seen because the energy of the orbitals lying in the direction of point charges increases more in comparision to orbitals lying in between the point charges as the electrons in former case experience greater repulsion than in latter case. So two groups of orbitals are seen with energy wise i.e., one with higher and other with lower energy. This splitting of 5 degenerate orbitals into two sets is called crystal field splitting.In case of octahedral complexes, eg set (dx2 - y2, dz2) is of higher energy while in case of tetrahe-dral complexes, t2g set (dxy,dyz,dxz) has higher energy.

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Average energy of the d-orbitals in spherical crystalfield

Splliting of d-orbitals in octahedralcrystalfield

The difference in energy levels is orbitrarily taken to be D. The set of lower energy stabilizes the complex ion by 0.4 units and that with higher energy destabilizes the ion by 0.6 units. The gain in energy achieved by preferential filling of electrons in orbitals is called crystal field stabilization energy(CFSE).

Average energy of the d orbitals in spherical crystalfield

Splliting of d orbitals intetrahedrelcrystalfield

d-orbitals free ion

The tetrahedral crystal field splitting is only 4/9 of the octahedral splitting and there is no known low spin tetrahedral complex. The number of unpaired electrons in octahedral and tetrahedral complexhaving different configuration can be summarized as here one point must always be kept in mind that crystal field theory considers purely electrostatic attractions between central metal ion and ligands.

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Nature of ligands:

Oxidation state of metal ion:

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Factors which affect the magnitude of crystal field splitting are

The ligand with smaller size, large negative charge with a good s donor and p acceptor properties will give large crystal field splitting.The spectrochemical series of ligands based on their crystal field splitting is i- < Br- < Cl- < F- < OX-≈H2O<P4≈NH3 < en < NO2

- < CN- < CO.

Crystal field splitting will be more in metal ion with higher oxidation states.Larger the size of d-orbitals, more crystal field splitting is seen.Geometry of the complex-splitting is different for tetrahedral and octahedral complexes.

LIMITATIONS OF CRYSTAL FIELD THEORY

CFT can explain the spectra of the metal ions and complexes on the assumption that these arise from the transition of electrons from lower energy d-orbital to higher energy d-orbital. However the position and intensities of spectral bands calculated on the basis of CFT do not always coincide with those determined experimentally.Apart from this a pure electrostatic interaction between central metal ion and ligand fails to explain the relative positions of ligands in spectro chemical series. So it is evident that covalent bonding too makes a significant contribution towards the metal ligand bonding.

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The strength of a complex ion depends upon the following factors

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Based on the above facts CFT was modified so as to include the contribution of covalent bonding in the metal ligand bond and is called ligand field theory which assumes that the extent of covalent character in metal ligand bond is generally low as compared to the extent of the ionic character of the bond that means the conclusions arrived at by the crystal field theory are still valid to a large extent.

Higher charge of the central metal ion i.e., greater ionic potential (ionic charge/ionic radius) and greater is the stability.Greater base strength of the ligand, greater will be the stability.Ring formation (chelation) in structure of the complexes is the chief factors, which increases the stability of the complex in solution.If a multidentate ligand happens to be cyclic without any steric effects, a further increase in stability occurs. This is called as near cocyclic effect.

BONDING IN METAL CARBONYLSThe homoleptic carbonyls (ligands are only carbonyls) are formed by most of the transition metals. These carbonyls have simple well defined structures. Tetra carbonyl nickel (0) is tetrahedral, penta carbonyl iron (0) is trigonal bipyramidal while hexa carbonyl chromium(0) is octahedral.

Tetrahedral [Ni(CO)4] Octahedral [Cr(CO)6] Trigonal bipyramida [Fe(CO)5]

Deca carbonyl dimanganese(0) is made up of two square pyramidal Mn(CO)5 units joined by a Mn-Mn bond. Octacarbonyl dicobalt(0) has a Co-Co bond bridged by two Co groups.

[Mn2(CO)10] [Co2(CO)8]

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The metal carbon bond in metal carbonyl possess both s and p character. The metal carbon s bond is formed by the donation of lone pair of electrons on the carbonyl carbon into a vacant orbital of the metal.The metal carbon p bond is formed by the donation of a pair of electrons from a filled d-orbital of metal into the vacant antibonding p-orbital of carbon monoxide.The metal to ligand bonding creates a synergic effect which strengthens the bond between CO and the metal.

Synergic bonding

STABILITY OF CO-ORDINATION COMPOUNDSThe stability of a complex in solution refers to the degree of association between the two speciesinvolved in the state of equilibrium. The magnitude of the equilibrium constant for the association, quantitatively expresses the stability.

For the reaction

M + 4L ML4

M + L ML K1 = [ML]/[M][L]

larger the stability constant, higher the proportion of ML4 that exists in solution.

Free metal ions rarely exist in solution because generally metal is surrounded by solvent moleculeswhich will compete with the ligand molecules.

By ignoring solvent molecules the stability constants of the reactions are

ML + L ML2 K2 = [ML2]/[ML][L]

ML2 + L ML3 K3 = [ML3]/[ML2][L]

ML3 + L ML4 K4 = [ML4]/[ML3][L]

K1, K2, K3 and K4 are called stepwise stability constants

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Estimation of hardness of water:

Analytical chemistry or Qualitative and Quantitative Estimation of metal ions:

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Eg: In the addition of four ammine groups to copper show stability constants as

logK1 = 4.0, logK2 = 3.2, logK3 = 2.7, logK4 = 2.0 and logK = 11.9

The instability constant or the dissociation constant of coordination compounds is defined as the reciprocal of the formation constant.

APPLICATIONS OF CO-ORDINATION COMPOUNDS

EDTA a hexadentate ligand form complex with various metal ions. So it is used in the estimation of hardness of water by a volumetric method.Hardness of water is due to Ca+2 and Mg+2 ions. Since stability constant values of EDTA complex with Ca+2 and Mg+2 are 107 and 108 respectively, it helps the selective estimation of different ions.

H2C N

N

CH2COO-

CH2COO-CH2COO-

CH2COO-H2C

(a) Nickel detected and estimated as its red dimethyl glyoxime complex which is a chelate complex.(b) Mg and Al are estimated as complexes of 8-hydro-oxy quinoline(oxime)

(c) The separation of Ag+ from Hg22+ in the first group of analysis is based on the fact that while

silver chloride soluble in aqueous ammonia, Hg2Cl2 forms a black insoluble material.

AgCl + 2NH4OH [Ag(NH3)2]Cl + 2H2O

Hg2Cl2 + NH4OH [Hg(NH2)Cl] + Hg + HgCl + H2OBlack insoluble

(d) The separation of II B group sulphides from II A group sulphides is based on the fact that sulphides of II B group from complex sulphides with yellow ammonium sulphide which are soluble while sulphides of II A group do not react.

Ag2S5 + 3(NH4)2S 2(NH4)3[AsS4]

Sb2S5 + 3(NH4)2S 2(NH4)3[SbS4]

SnS2 + (NH4)2S (NH4)2[SnS3]

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(e) The detection of Cu+2 is based on the formation of a blue tetra ammine copper sulphate complex which give a deep blue coloured solution upon addition of NH3 to Cu+2 ions.

CuSO4 + 4NH3 [Cu(NH3)4]SO4

It also forms a chocolate coloured precipitate with potassium ferrocyanide.

2CuSO4 + K4[Fe(CN)6] Cu[Fe(CN)6] + 2K2SO4

(f) Fe+3 is detected by formation of a blood red coloured complex with KSCN.

Fe+3 + KSCN [Fe(SCN)]2+ + K+

(g) S-2 is detected by the formation of a violet-coloured complex with sodium nitro prusside.

Na2S + Na2[Fe(Cn)5NO] Na4[Fe(Cn)5NOS]Sodium nitro prusside Violet colour

Animal and plant world:

Electroplating of metals:

Extraction of metals:

In chemotherapy:

Cyclic ligand impact extra stability to co-ordinate compounds. Thus Mg in chlorophyll and iron in hemoglobin are stable towards dissociation due to the formation of coordination compounds. Hemoglobin of the blood gets oxygenated through the binding of di-oxygen, O2 to the ferrous ions in hemoglobin.

Co-ordination compounds are used electroplating industry.

Eg: Gold and silver are electroplated from their coordination compounds such as

K[Ag(CN2)] or K[Au(CN)4]

The formation of complex compounds is used in the extraction of some metals.

Eg: Ag and Au are extracted.

2K[Ag(Cn)2]+Zn K2[Zn(Cn)4] + 2Ag

This is due to more electropositive nature of Zn as compared to Ag and Au.Nickel tetra carbonyl complex is used for the extraction and purification of nickel.

Ni+4CO [Ni(CO)4] Ni+ 4COD

Platinum complex, cis[PtCl2 (NH3)2] called cis-platin is used in cancer chemotherapy. Vitamin B12 is a coordination compound of cobalt and is essential to prevent anaemia.

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SUMMARY

The chemistry of co-ordination compounds is an important and challenging part in modern inorganic chemistry.Developments in this part have given new concepts of bonding, structure in chemical industry and biological systems.The first explanation about formation, bonding, structure of co-ordination compounds was given structure of co-ordination compounds was given by A.Werner.He postulated that metal in co-ordination compound shows two types of linkages (primary and secondary) or valencies (ionisable and non- ionisable) or (ionic and covalent).The valence bond theory (VBT) explains the formation of magnetic behaviour and geometrical shapes of co-ordination compounds, but it failed to give a quantitative interpretation of magnetic behaviour and their optical properties.The crystal field theory (CFT) is based on the effect, degeneracy of d-orbitals provide different electronic arrangements in strong and weak crystal fields. This theory quantitatively explained about orbital separation energies, magnetic moment, spectral and stability parameters.The metal carbon bond in metal carbonyls possesses both s and p characters. The ligand to metal is s bond and metal to ligand is p bond. This unique synergic bonding provides stability to metal carbonyls.The stability of co-ordination compounds is measured in terms of step wise stability constants or over all stability constant.The stability due to chelation is called chelate effect.Co-ordination compounds are of great importance. They have extensive applications in chemotherapy, photography, analytical and medicinal chemistry, metallurgical process, as catalyst, in electroplating in pigments etc.

As catalysts:

In pigments:

A coordination complex of Ti and Al is used as a catalyst to convert ethylene and propylene into polythene. Ziegler-Natta catalyst is TiCl4 + (C2H5)3Al or (CH3)3 Al. Wilkinson Catalyst is used for hydrogenation of alkenes.

Many pigments such as phthalocyanines used in paints which are co-ordination compounds.

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COORDINATION COMPOUNDS - :: Science Tuts :: Home COMPOUNDS Molecular or Addition compounds w w w The development in the field of co-ordination compound or complex ion and organometallics