In transition metal chemistry, this is a form of isomerism which applies to metal complexes. Among the ligands which form such complexes, some are capable of binding through more than one of their atoms - such ligands are called ambidentate. Linkage isomerism is the phenomenon whereby the ligands in a metal complex may bind through one of their atoms or through another.

Although research into this topic continues, at present only two examples of such ligands are well understood.

One of these is the thiocyanate ligand, SCN-. This can bind to a metal centre either through the sulphur atom, S, to give the thiocyanato isomer, or through the nitrogen atom, N, to give the isothiocyanato isomer.

The other is the nitro ligand, NO2-. This can bind through the nitrogen atom to give the nitro isomer, or through the oxygen atom to give the less stable nitrito isomer (represented as ONO-M, where M is the metal).

Different linkage isomers have different physical properties. For example, cobalt complexes containing NO2- and NH3 have a yellow colour when nitro and a red colour when nitrito.

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