Linkage isomerism

Linkage isomerism is the existence of co-ordination compounds that have the same composition differing with the connectivity of the metal to a ligand.

History
The first reported example of linkage isomerism had the formula [Co(NH3)5(NO2)]Cl2. The cationic cobalt complex exists in two separable linkage isomers. In the red-coloured isomer, the nitrite ligand is bound through nitrogen. In the yellow linkage isomer, the nitrite is bound through one oxygen atom. The O-bonded isomer is often written as [Co(NH3)5(ONO)]2+. Although the existence of the isomers had been known since the late 1800’s, only in 1907 was the structural difference explained. It was later shown that the red isomer converted to the yellow isomer upon UV-irradiation. In this particular example, the formation of the nitro isomer (Co-NO2) from the nitrito isomer (Co-ONO) occurs through the rearrangement of the molecular structure. Thus, no bonds are broken during isomerization.

Scope of linkage isomerism
Typical ligands that give rise to linkage isomers are:
 * thiocyanate, SCN-
 * selenocyanate, SeCN-
 * nitrite, NO2-
 * sulfite, SO32-

Examples of linkage isomers are violet-colored [(NH3)5Co-SCN]2+ and orange-colored [(NH3)5Co-NCS]2+. The isomerization of the S-bonded isomer to the N-bonded isomer occurs intramolecularly.