Linkage isomerism is the occurrence of coordination compounds having the same composition but differing in the connectivity of the metal to a ligand. An ambidentate or ambident ligand is a ligand molecule containing more than one atom that could donate an electron pair. Such ligands can coordinate with the metal atom through any of their donor atoms and hence are given different names corresponding to the nature of donor atoms linked to the metal atom. For example, each of NO2–, SCN– and CN– has more than one possible site of attachment. In the case of NO2–, either the nitrogen or the oxygen atom may act as the electron donor, and hence there is a possibility of isomerism.
Examples of Linkage isomerism
Jorgensen prepared two complexes of [Co(NH3)5NO2]Cl2 each containing a single NO2– ligand in the complexion. One isomer, which is red in colour, is easily decomposed by acid to give nitrous acid. It contains Co-ONO and is a nitrito complex. The other isomers, which are of a yellowish-brown colour, are stable to acids. It contains the Co-NO2 group and is a nitro compound. Hence, these two isomeric modifications are repeated as follows.
The linkage isomers are easily identified by means of infrared spectroscopy. The symmetrical NO2 stretching frequencies are different in nitro and nitrilo groups attached to a metal ion.
The thiocyanate ion can attach itself to the metal through either the nitrogen, M-N=C-S or the sulphur, M-S-C≡N. The following linkage isomers with thiocyanate ions have been prepared.
[Mn(CO)5SCN]+ and [Mn(CO)5NCS]+