KINETICS AND MECHANISM OF THE REVERSIBLE FORMATION OF THE BINUCLEAR SPECIES BETWEEN PYRIDINE-2-CARBOXYLATO(PENTAAMMINE)COBALT(III) AND COBALT(II) IN AQUEOUS-SOLUTION

The kinetics of formation and dissociation of the binuclear complex of cobalt(II) with pyridine-2-carboxylato(pentaammine) cobalt(III) have been studied in the temperature range 10-25-degrees-C and at I = 0.3 m ol dm-3 (ClO4-). The formation of (NH3)5CoO2CC5H4NCo4+ in the pH range 2.35-3.20 involves the reaction of Co(OH2)62+ with the protonated (py ridine-N) and the deprotonated forms of the complex. The rate and acti vation parameters for the formation reaction are consistent with an I( d) mechanism. The binuclear species undergoes dissociation to yield th e parent cobalt(III) substrate and cobalt(II) via spontaneous and acid -catalysed paths. The rate data at 20-degrees-C for the spontaneous di ssociation of the binuclear complex and of mono(pyridine-2-carboxylato ) cobalt(II) are comparable indicating the chelating nature of the bin uclear species.