Isomerism

Coordination Compounds

2

Isomers have the same molecular formula, but their atoms are arranged either in a different order (structural isomers) or spatial arrangement (stereoisomers).

1. Ionization isomerism2. Hydrate isomerism3. Coordination isomerism4. Coordination Position isomerism5. Linkage isomerism6. Polymerisation

In this type of isomerism the difference arises from the positions of groups within or

outside the coordination sphere. Therefore, these isomers give different ions in solution,

hence the name ionisation isomerism. For example, there are two distinct compounds of

the formula Co(NH3)5 BrSO4. Position of groups differ within and outside coordination

sphere.e.g. – [CoBr(NH3) 5]SO4 and [Co SO4 (NH3) 5] Br

1.[CoBr(NH3) 5]SO4 [CoBr(NH3) 5 ]2+ + SO4 2-

Red – Violet Gives precipitate with Barium chloride2. [Co SO4 (NH3) 5] Br [Co SO4

2- (NH3) 5 ]+ + Br-

RedGives precipitate with silver nitrate

ii. [Co Cl2 (NH3) 4] NO 2 and [Co Cl NO 2 (NH3) 4] Cl

Structural Isomerism - 1. Ionization Isomerism

Structural Isomerism - 2. Hydration Isomerism•This type of isomerism arises from replacement of a coordinated group by water of hydration. example - CrCl3. 6H2O:

Three isomers are [Cr(H2O)6]Cl3, [Cr Cl(H2O)5]Cl2-H2O and [CrCl2(H2O)4]Cl.2H2O

They differ largely from one another in their physical and chemical properties as illustrated below.

[Cr(H2O)6]Cl3 - It does not lose water when treated with conc.H2SO4

Violet - three chloride ions are precipitated with AgNO3

[CrCl(H2O)5]Cl2.H2O - It loses one water molecule when treated with conc.H2SO4

Green - two Cl—ions are precipitated with AgNO3

[CrCl2(H2O)4]Cl. 2H2O - It loses one water molecules on treatment with conc. H2SO4

Green - one Cl—ion is precipitated with AgNO3

Example - 2 [CoCl(en)2(H2O)]Cl2 and [CoCl2 (en)2] Cl.H2O

Structural Isomerism - 3. Coordination Isomerism

Structural Isomerism - 4. Coordination Position Isomerism

Structural Isomerism - 5. Polymerisation Isomerism

Structural Isomerism - 6. Linkage Isomerism

Same complex ion structure but point of attachment of at least one of the ligands differs.

I [Co(NH3)4(NO2)Cl]Cl and II [Co(NH3)4(ONO)Cl]Cl

I Nitro group - (NO2) linkage through N , II Nitrito group -(ONO) link thru O

Stereoisomerism - Geometrical Isomerism

Coordination no. 4

1. Tetrahedral complexes - Cis-trans isomerism is not possible in tetrahedral complexes because all the four lignads are adjacent to one another.

2. Square Planar Complexes - cis-trans isomerism is very common amongst square planar complexes of the following types.

Geometrical isomerism is due to ligands occupying different positions around the central ion. The ligands occupy positions either adjacent to one another or opposite to one another. These are referred to as cis form and trans form, respectively. This type of isomerism is, therefore, also referred to as cis-trans isomerism .

1. Complexes of the type Ma2b2 can exist in cis and trans forms. Amongst the best known examples are the complexes of platinum (II) and palladium (II), such as [PtCl2 (NH3)2],

[Pd(NO2 )2(NH3)2 ] and [PtCl2(py)2].

[PtCl2 (NH3)2],

Geometrical Isomerism2. Complex of the type Ma2bc - there are cis and trans isomers depending upon whether the two ‘a’ groups are adjacent or opposite to each other. Ex. Pt(py)2 NH3Cl]

3. Complexes of the type Mabcd produce three isomers. The structures of these isomers can be written by fixing the position of one ligand (say ,a ) and placing the other ligands b, c and d trans to it

Pt Pt Pt

PyPyPy NO2

NO2NO2

NH3

NH3NH3NH2OH

NH2OH

NH2OH

PtPy

Cl

NH3

Py

PtPy

Cl

NH3

Py

Geometrical IsomerismCoordination no. - 6 Octahedral complexes

The complexes having coordination number 6 adopt octahedral geometry. The octahedral complexes also exhibit geometrical isomerism. The complexes of the type Ma6 and Ma5b would not show geometrical isomerism.

1. Complexes of the type Ma4b2 or Ma2b4 or Ma4bc - A large number of other octahedral complexes of these types where M is Co(III) Cr(III), Rh(III), Pt(IV), have been prepared.

Geometrical Isomerism

2. Complexes of the type Ma3b3 exist in two isomeric forms. Example - [RhCl3(py)3]

It is seen in the cis form, like groups occupy the corners of one of the triangular faces of the octahedron while in the trans form., they do not.

Cl

Cl

Cl

Cl

Cl

Cl

Rh Rh

Py

PyPy

Py

Py

Py

Cis - Trans -

3. Complexes of the type [Mabcdef], where all the ligands are different, there is a

possibility for the existence of 15 different geometrical forms. However, the only

compound of this type that has been prepared so far is [PtBrCII(NO2) (C5H5N)(NH3) ]

It has been possible to isolate only three different forms of this compound.

4. Complexes of the type (M(aa)2b2] or [M(aa)2bc] containing bidentate ligands (aa) show

geometrical isomerism. The other two ligands may be the same (b2) or different (bc). The

common examples are [M(en)2bc] where M is Co (III), Cr(III), Ir(III), Rh(III), Pt(III), Pt(IV) and

b and c are ligands such as NO2, I , Br, Cl , NH3, py, etc. For example , the complex

[Co(en)2Cl2]+ where en is H2NCH2CH2NH2, exists in cis and trans forms.

Geometrical Isomerism

en

en

Co

Cl

Cl

Cl

en

en

Co

Cl

Cis -Trans -

en

en

Co

Cl

Cis -

en

en

Co

Cl

Cl

en

en

Co

Cl

Trans -

[M(aa)2bc]

Py Py

(M(aa)2b2]

Geometrical Isomerism

Simple substances which show optical isomerism exist as two isomers known as enantiomers.•A solution of one enantiomer rotates the plane of polarisation in a clockwise (left) direction. This enantiomer is known as the (+) form or Laevoritatory (l-) The solution of the other enantiomer rotates the plane of polarisation in an anti-clockwise direction. This enantiomer is known as the (-) form or Dextrorotatory (d-) •If the solutions are equally concentrated the amount of rotation caused by the two isomers is exactly the same - but in opposite directions, mixed in equal amounts they form Racemic mixture.

if you rotate molecule B.

Stereoisomerism - Optical Isomerism

• Optical isomers have non-super imposable mirror images.•A molecule which has no plane of symmetry is described as chiral. The carbon atom with the four different groups attached which causes this lack of symmetry is described as a chiral centre or as an asymmetric carbon atom. The molecule with a plane of symmetry is described as achiral. Only chiral molecules have optical isomers.

If one enantiomer of a chiral compound is placed in a polarimeter and polarized light is shone through it, the plane of polarization of the light will rotate.

If one enantiomer rotates the light 32° to the right, the other will rotate it 32° to the left.

Optical activity

Optical IsomerismMetal complexes not having any elements of symmetry - Centre, Plane of symmetry etc can show optical isomerism. Theoritically all such complexes must show optical activity but not many have been resolved into d- and l- isomers.

Complexes with coordination number 4

1. Tetrahedral complexes - Not many tetrahedral complexes show optical activity. Complexes of Be (II), B(III) and Zn(II), containing two symmetrical bidentate ligands have been resolved into optically active forms. For example the complex bis(benzoylacetonato)beryllium(II) has been shown to exist in two optical forms as shown below.

2. Square planar complexes are seldom optically active. This is because all the ligands and the metal atom lie in the same plane. The molecule has a plane of symmetry and hence cannot exhibit optical isomerism. One such platinum compound which has , however been resolved into two forms is as shown below.

Optical Isomerism

HHMe

Me Ph

Ph

Complexes with coordination number 6 1. Complexes of the Type [M (aa)3 ]where aa= bidentate ligand. The complexes such as [Co(en)3]Cl3, [Cr(ox)3]3- etc., exist as optical isomers because they form non- superimposable mirror images,

Optical Isomerism

en

en

en

en

en

en

Co Co

[Cr(ox)3]3-

2. Complexes of the type [M(aa)2 b2] The cis isomer is asymmetric and can be resolved into d- and I- isomers. For example , the complex ion dichlorobis (ethylenediamine)rhodium (III) [RhCl2 (en)2]+ exists in the form of three isomers (trans optically inactive and cis shows d- and l- isomers.

Complexes with coordination number 6 Optical Isomerism

[CoCl2 (en)2]+

Cl

en

Rh

Cl

Trans -

en

Cis-

d-

en

en

Co +2

Cl

Cl

Complexes with coordination number 6

Optical Isomerism

en

en

en

en

Co +2 Co +2

NH3 NH3

Cl Cl

3. Complexes of the type [M(aa) 2bc] The complexes containing two different unidentate ligands and two symmetric bidentate ligand also show optical isomerism. For example, the complex ion [CoCl (en) 2(NH3)] +2 exists in d- and I -forms.

4. Complexes of the type [M(aa)b2 c2] The complexes containing only one symmetric bidentate ligand also show optical isomerism. For example, the complex ion [CrCl2(en)(NH3)2]+ exists in d- and I -forms.

Optical IsomerismComplexes with coordination number 6

en

en

en

Cr + Cr +

NH3 NH3

Cl Cl

Cl Cl

NH3NH3

Complexes with coordination number 6

Optical Isomerism

5. Complexes containing hexadentate ligands - The complexes containing hexadentate ligands such as ethylenediaminetetraacetato (EDTA) also show optical activity. Fox example, the complex [Co(EDTA) exists in two forms d- and I- .

6. Complexes containing unidentate ligands - The complexes of the type [Mabcdef] are also expected to show optical isomerism. A complex of this type can theoretically have 15 geometrical forms, each of which should be optically active.