What is linkage isomerism in coordination compounds, and give an example?

An ambidentate ligand is a ligand that has more than one atom capable of forming a coordinate bond with the central metal, but only one atom forms the bond at a time.

For example, the nitrite ion ($\text{NO}_2^-$) is an ambidentate ligand.

• It can bind through the nitrogen atom, forming a nitro complex (e.g., $\text{Co}-\text{NO}_2$). This is shown in $[\text{Co}(\text{NO}_2)(\text{NH}_3)_5]\text{Cl}_2$, which is yellow and stable.
• It can also bind through an oxygen atom, forming a nitrito complex (e.g., $\text{Co}-\text{ONO}$). This is shown in $[\text{Co}(\text{ONO})(\text{NH}_3)_5]\text{Cl}_2$, which is red and less stable.

These two complexes are linkage isomers because they have the same chemical formula but differ in the atom through which the $\text{NO}_2^-$ ligand is attached to the cobalt metal.