Linkage isomerism

Linkage isomerism is the existence of coordination compounds that have the same composition differing with the connectivity of the metal to a ligand.

Typical ligands that give rise to linkage isomers are:

• thiocyanate, SCN⁻ – isothiocyanate, NCS⁻
• selenocyanate, SeCN⁻ – isoselenocyanate, NCSe⁻
• nitrite, NO₂⁻
• sulfite, SO₃²⁻

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] In the complex, dichlorotetrakis(dimethyl sulfoxide)ruthenium(II), linkage isomerism of dimethyl sulfoxide ligands can be observed in the NMR spectrum due to the effect of S vs. O bonding on the methyl groups of DMSO. The proper notation for linkage isomerism is the kappa notation where the atom directly bonding to the metal is proceeded by the lowercase Greek letter kappa; κ. For example, NO₂⁻ is represented as nitrito-κ-N and nitrito-κ-O, replacing the old system of trivial names such as nitro and nitroso.^[2]

History

The first reported example of linkage isomerism had the formula [Co(NH₃)₅(NO₂)]Cl₂. The cationic cobalt complex exists in two separable 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 structural difference explained.^[3] 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.^[4]

Structures of the two linkage isomers of [Co(NH₃)₅(NO₂)]²⁺.

References