Some reactions of carboxylate anions with dioxalatodiaquocobaltate(III) in aqueous solution

Abstract

The retardation observed in the rate of decomposition of Co(C₂O₄)₂(H₂O)₂^– to Co^II and CO₂ with increase in the concentration of carboxylate buffers R·CO₂^––R·CO₂H is interpreted in terms of the relatively rapid formation of more stable Co^III complexes, Co(C₂O₄)₂(R·CO₂)(H₂O)^2–. The stability constants for the formation of such monocarboxylato-complexes from the diaquo-complex have been obtained from the kinetic results in acetate, formate, chloroacetate, and glycine buffers.

By contrast, the rate of disappearance of the Co^III complex is increased with increase in the concentration of oxalate buffers. This is ascribed to oxidation of the buffer oxalate in a two-stage mechanism, consisting of the replacement of a ligand water molecule by oxalate, followed by a three-centre redox reaction involving the intermediate so formed and a second Co(C₂O₄)₂(H₂O)₂^– ion. The oxidations of oxalate by IrCl₆^2– and by Mo(CN)₈^3– have also been briefly studied and the rates of these three oxidation of oxalate are compared in the light of their probable mechanisms.