Some aspects of the coordination chemistry of cobalt(III)

Octahedral cobalt(III) complexes have played a central role in the elucidat ion of mechanisms of inorganic reactions. Some aspects of ligand substituti on and redox reactions of low-spin octahedral cobalt(III) complexes in solu tion are discussed. The reactions include : (i) acid and base hydrolysis of halogenoaminecobalt(III) complexes, (ii) anation of aqua-aminecobalt(III) ions, (iii) acid, base and metal ion catalysed hydrolysis of carboxylatoami necobalt(III) complexes, and (iv) oxidation of ligands coordinated to cobal t(III). The halogenoaminecobalt(III) complexes derived from weakly basic am ines have proved to be useful models for the elucidation of SN1CB mechanism . The metal ion catalysed hydrolysis of carboxylatoaminecobalt(III) complex es derived fro i dicarboxylic and phenolic acids have demonstrated the kine tic and thermodynamic implications of binuclear complexes of several substi tution 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- bond breaking generally takes precedence over the bond making between the incoming ligand and the cobalt(III) centre in the transition state. How ever, the ligand envelope of this metal ion mediates the reactivities of th e complexes and further dictates the Co-III-X bond cleavage in the limits o f interchange dissociative and/or fully dissociative modes.