Isomerism in Coordination Compounds

Isomerism in Coordination Compounds

Stereoisomerism

It arises due to the different arrangement of atoms or groups in space. Two different types of stereoisomerism are described as follows:

Geometrical isomerism

It arises in heteroleptic complexes due to different possible geometrical arrangements of ligands. In square planar complex of formula [MX2L2] (where X and L are unidentate), the two ligands X may be arranged adjacent to each other in a cis isomer, or opposite to each other in a trans isomer.

For example: Geometrical isomers of Pt(NH3)2Cl2) are shown below:

Similarly, in octahedral complex of formula [MX2L4], two ligands X may be oriented cis or trans to each other.

For example: Geometrical isomers of [Co(NH3)4Cl2] are shown below:

Optical isomerism

It arises due to the presence of non-super imposable mirror images. The non-superimposable mirror images are called enantiomers. The enantiomers react differently with the plane polarised light.

The enantiomer which rotate the plane polarised light in a clockwise direction is called dextrorotatry (d) or (+) and the enantiomer which rotate the plane polarised light in anticlockwise direction is called levorotatory (l) or (‒).

For example: Optical isomers of [Co(en)3]3+ are shown below:

Structural isomerism

It arises due to the difference in structures of coordination compounds. Four different types of structural isomerism are described as follows:

Linkage isomerism

Isomers having the same molecular formula but different linking atoms are named as linkage isomers. This arises due to the presence of ambident ligands.

For example: [CO(NH3)5(NO2)]2+ and [CO(NH3)(ONO)]2+ are linkage isomers.

Coordination isomerism

This type of isomerism is possible when a cation and anion both are complex and isomerism arises due to the complete exchange of coordination sphere.

For example: [Pt (NH3)4] [Ni (CN)4] and [Ni (NH3)4] [Pt (CN)4] are coordination isomers.

Ionisation isomerism

These are the isomers having the same molecular formula but give different ions in solution.

For example: [Cr(NH3)Br]SO4 and [Cr(NH3)(SO4)]Br are ionization isomers.

Solvate isomerism

Isomers having the same molecular formula but different water molecules of hydration are known as hydration isomers of each other.

For example: [Cr (H2O)5Cl]Cl2.H2O and [Cr(H2O)4Cl2]Cl.2H2O are hydration isomers.