Some aspects of the coordination chemistry of cobalt(III)

Department of Chemistry, Utkal University, Bhubaneswar-751 004, India

Manuscript received 8 November 2000

Octahedral cobalt(III) complexes have played a central role in the elucidation of mechanisms of inorganic reactions. Some aspects of ligand substitution and redox reactions of low-spin octahedral cobalt(III) complexes in solution are discussed. The reactions include : (i) acid and base hydrolysis of halogenoaminecobalt(m) complexes, (ii) anation of aqua-aminecobalt(iiI) ions, (iii) acid, base and metal ion catalysed hydrolysis of carboxylatoaminecobalt(iii) complexes, and (iv) oxidation of ligands coordinated to cobalt(III). The halogenoaminecobalt(III) complexes derived from weakly basic amines have proved to be useful models for the elucidation of SN₁CB mechanism. The metal ion catalysed hydrolysis of carboxylatoaminecobalt(III) complexes derived from dicarboxylic and phenolic acids have demonstrated the kinetic and thermodynamic implications of binuclear complexes of several substitution labile metal ions. Such entities are also models for the innersphere electron transfer reac­tions. Extensive studies of the ligand substitution reaction of a wide variety of cobalt(III) complexes have shown that the Co^III X bond breaking generally takes precedence over the bond making between the incoming ligand and the cobalt(iii) centre in the transition state. However, the ligand envelope of this metal ion mediates the reactivities of the complexes and further dictates the Co^III-X bond cleavage in the limits of interchange dissociative and/or fully dissociative modes.