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

[edit] 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 1800’s, only in 1907 was the structural difference explained.^[1] 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 through the rearrangement of the molecular structure. Thus, no bonds are broken during isomerization.

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

[edit] Scope of linkage isomerism

Typical ligands that give rise to linkage isomers are:

• thiocyanate, SCN^-
• selenocyanate, SeCN^-
• nitrite, NO₂^-
• sulfite, SO₃^2-

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.^[2] In the complex Dichlorotetrakis(dimethyl_sulfoxide)_ruthenium(II), linkage isomerism of dimethylsulfoxide ligands can be observed in the NMR spectrum due to the effect of S vs. O bonding on the DMSO methyl groups.

[edit] References

1. ^ Werner, A. “Über strukturisomere Salze der Rhodanwasserstoffsäure und der salpetrigen Säure” Berichte der deutschen chemischen Gesellschaft 1907, volume 40, 765-788. DOI: 10.1002/cber.190704001117
2. ^ Buckingham, D. A.; Creaser, I. I.; Sargeson, A. M. "Mechanism of Base Hydrolysis for Co^III(NH₃)₅X²⁺ Ions. Hydrolysis and Rearrangement for the Sulfur-Bonded Co(NH₃)₅SCN²⁺ Ion" Inorganic Chemistry (1970), volume 9, pages 655-61.