Structural isomerism: linkage isomerism

    Linkage isomers may arise when one or more of the ligands can coordinate to the metal ion in more than one way, e.g. in [SCN]^- (1.1), both the N and S atoms are potential donor sites.

(1.1)

   Thus, the complex [Co(NH₃)₅(NCS)]²⁺‏ has two isomers which are distinguished by using the following nomenclature:

• in [Co(NH₃)₅(NCS-N)]²⁺, the thiocyanate ligand coordinates through the nitrogen donor atom;
• in [CoNH₃₅NCS-S]²⁺, the thiocyanate ion is bonded to the metal centre through the sulfur atom. Scheme 1.1 shows how linkage isomers of [CoNH₃₅NO₂]²⁺ can be prepared.

         (1.1)

     In this example, the complexes [Co(NH₃)₅(NO₂-O)]^2+‏ and [Co(NH₃)₅(NO₂-N)]^2+‏ can be distinguished by using IR spectroscopy. For the O-bonded ligand, characteristic absorption bands at 1065 and 1470 cm^-1 are observed, while for the N-bonded ligand, the corresponding vibrational wavenumbers are 1310 and 1430 cm^-1.