Linkage isomerism is the existence of co-ordination compounds that have the same composition differing with the connectivity of the metal to a ligand. 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. Scope of linkage isomerism Typical ligands that give rise to linkage isomers are:
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. 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
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