Linkage Isomerism

Linkage isomerism arises in a coordination compound that contains an ambidentate ligand — a ligand which possesses more than one kind of donor atom, so that it can attach itself to the central metal through either one of them.

The thiocyanate example

A simple, commonly cited example is provided by complexes containing the thiocyanate ligand, $NCS^{-}$. This ligand may bind to the metal:

• through its nitrogen atom, giving an $M\text{–}NCS$ linkage, or
• through its sulphur atom, giving an $M\text{–}SCN$ linkage.

The nitrite example

Jørgensen discovered this behaviour in the complex $[Co(NH_3)_5(NO_2)]Cl_2$, which is obtained in two distinct forms depending on how the nitrite ligand is bonded:

• a red form, in which the nitrite ligand is bound to the metal through oxygen — written $\text{–ONO}$, and
• a yellow form, in which the nitrite ligand is bound through nitrogen — written $\text{–NO}_2$. …