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:

• cyanide, CN⁻ – isocyanide, NC⁻
• cyanate, OCN⁻ – isocyanate, NCO⁻
• thiocyanate, SCN⁻ – isothiocyanate, NCS⁻
• selenocyanate, SeCN⁻ – isoselenocyanate, NCSe⁻
• nitrite, NO−2
• sulfite, SO2−3

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.^[1]

The complex cis-dichlorotetrakis(dimethylsulfoxide)ruthenium(II) (RuCl₂(dmso)₄) exhibits linkage isomerism of dimethyl sulfoxide ligands due to S- vs. O-bonding. Trans-dichlorotetrakis(dimethylsulfoxide)ruthenium(II) does not exhibit linkage isomers.

History

Linkage isomerism was first noted for nitropentaamminecobalt(III) chloride, [Co(NH₃)₅(NO₂)]²⁺. 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 [Co(NH₃)₅(ONO)]²⁺. 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 (Co-NO₂) from the nitrito isomer (Co-ONO) occurs by an intramolecular rearrangement.^[3]