In chemistry, linkage isomerism or ambidentate isomerism is a form of isomerism in which certain coordination compounds have the same composition but differ in their metal atom's connectivity to a ligand.
Typical ligands that give rise to linkage isomers are:
Examples of linkage isomers are violet-colored and orange-colored . The isomerization of the S-bonded isomer to the N-bonded isomer occurs intramolecularly.[1]
The complex cis-dichlorotetrakis(dimethylsulfoxide)ruthenium(II) exhibits linkage isomerism of dimethyl sulfoxide ligands due to S- vs. O-bonding. Trans-dichlorotetrakis(dimethylsulfoxide)ruthenium(II) does not exhibit linkage isomers.
Linkage isomerism was first noted for nitropentaamminecobalt(III) chloride, . This cationic cobalt complex can be isolated as either of two linkage isomers. In the yellow-coloured isomer, the nitro ligand is bound through nitrogen. In the red linkage isomer, the nitrito is bound through one oxygen atom. The O-bonded isomer is often written as . Although the existence of the isomers had been known since the late 1800s, only in 1907 was the difference explained.[2] 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 from the nitrito isomer occurs by an intramolecular rearrangement.[3]