Chemistry Isomerism in Coordination Compounds
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### Topics Covered :

● Isomerism
● Geometrical Isomerism
● Optical Isomerism
● Coordination Isomerism
● Ionisation Isomerism
● Solvate Isomerism

### Isomerism in Coordination Compounds :

color{green}("Definition ") : Isomers are two or more compounds that have the same chemical formula but a different arrangement of atoms.

● Because of the different arrangement of atoms, they differ in one or more physical or chemical properties.

● Two principal types of isomerism are known among coordination compounds. Each of which can be further subdivided.

(a) color{green}("Stereoisomerism ") : Stereoisomers have the same chemical formula and chemical bonds but they have different spatial arrangement.

(i) Geometrical isomerism

(ii) Optical isomerism

(b) color{green}("Structural Isomerism ") : Structural isomers have the same chemical formula but different bonds.

(ii) Coordination isomerism

(iii) Ionisation isomerism

(iv) Solvate isomerism

### Geometric Isomerism :

=> This type of isomerism arises in heteroleptic complexes due to different possible geometric arrangements of the ligands.

=> Important examples of this behaviour are found with coordination numbers 4 and 6.

=> In a square planar complex of formula color{red}([MX_2L_2]) (color{red}(X) and color{red}(L) are unidentate), the two ligands color{red}(X) may be arranged adjacent to each other in a c is isomer, or opposite to each other in a trans isomer as depicted in Fig. 9.2.

=> Other square planar complex of the type color{red}(MABXL) (where color{red}(A, B, X, L) are unidentates) shows three isomers-two cis and one trans.

=> Such isomerism is not possible for a tetrahedral geometry but similar behaviour is possible in octahedral complexes of formula color{red}([MX_2L_4]) in which the two ligands X may be oriented cis or trans to each other (Fig. 9.3).

=> This type of isomerism also arises when didentate ligands color{red}(L – L) [e.g., color{red}(NH_2 CH_2 CH_2 NH_2 (en))] are present in complexes of formula color{red}([MX_2(L – L)_2]) (Fig. 9.4).

=> Another type of geometrical isomerism occurs in octahedral coordination entities of the type color{red}([Ma_3b_3]) like color{red}([Co(NH_3)_3(NO_2)_3]).

=> If three donor atoms of the same ligands occupy adjacent positions at the corners of an octahedral face, we have the facial (fac)
isomer. When the positions are around the meridian of the octahedron, we get the meridional (mer) isomer (Fig. 9.5).
Q 3031701622

Why is geometrical isomerism not possible in tetrahedral complexes having two different types of unidentate ligands coordinated with the central metal ion ?

Solution:

Tetrahedral complexes do not show geometrical isomerism because the relative positions of the unidentate ligands attached to the central metal atom are the same with respect to each other.
Q 3031801722

Draw structures of geometrical isomers of [Fe(NH_3)_2(CN)_4]^–

Solution:

### Optical Isomerism :

=> Optical isomers are mirror images that cannot be superimposed on one another. These are called as enantiomers.

=> The molecules or ions that cannot be superimposed are called chiral.

=> The two forms are called dextro (d) and laevo (l) depending upon the direction they rotate the plane of polarised light in a polarimeter (d rotates to the right, l to the left).

=> Optical isomerism is common in octahedral complexes involving didentate ligands (Fig. 9.6).

=> In a coordination entity of the type color{red}([PtCl_2(en)_2]^(2+)), only the cis-isomer shows optical activity (Fig. 9.7).
Q 3021001821

Out of the following two coordination entities which is chiral (optically active)?
(a) cis-[CrCl_2(o x)_2]^(3–) (b) trans-[CrCl_2(o x)_2]^(3–)

Solution:

The two entities are represented as

Out of the two, (a) cis - [CrCl_2(o x)_2]^(3-) is chiral (optically active).

=> Linkage isomerism arises in a coordination compound containing ambidentate ligand.

=> A simple example is provided by complexes containing the thiocyanate ligand, color{red}(NCS^–), which may bind through the nitrogen to give color{red}(M–NCS) or through sulphur to give color{red}(M–SCN).

=> Jørgensen discovered such behaviour in the complex color{red}([Co(NH_3)_5(NO_2)]Cl_2), which is obtained as the red form, in which the nitrite ligand is bound through oxygen (color{red}(–ONO)), and as the yellow form, in which the nitrite ligand is bound through nitrogen (color{red}(–NO_2)).

### Coordination Isomerism :

=> This type of isomerism arises from the interchange of ligands between cationic and anionic entities of different metal ions present in a complex.

color{red}("Example ") : color{red}([Co(NH_3)_6][Cr(CN)_6]) : ● In this the color{red}(NH_3) ligands are bound to color{red}(Co^(3+)) and the color{red}(CN^–) ligands to color{red}(Cr^(3+)).

● In its coordination isomer color{red}([Cr(NH_3)_6][Co(CN)_6]), the color{red}(NH_3) ligands are bound to color{red}(Cr^(3+)) and the color{red}(CN^-) ligands to color{red}(Co^(3+)).

### Ionisation Isomerism :

=> This form of isomerism arises when the counter ion in a complex salt is itself a potential ligand and can displace a ligand which can then become the counter ion.

=> An example is provided by the ionisation isomers color{red}([Co(NH_3)_5SO_4]Br) and color{red}([Co(NH_3)_5Br]SO_4).

### Solvate Isomerism :

=> This form of isomerism is known as ‘hydrate isomerism’ in case where water is involved as a solvent.

=> This is similar to ionisation isomerism.

=> Solvate isomers differ by whether or not a solvent molecule is directly bonded to the metal ion or merely present as free solvent molecules in the crystal lattice.

=> An example is provided by the aqua complex color{red}([Cr(H_2O)_6]Cl_3) (violet) and its solvate isomer color{red}([Cr(H_2O)_5Cl]Cl_2 .H_2O) (grey-green).