Complexation of cobalt(II) with histidinato(pentaammine)cobalt(III) in aqueous medium. A kinetic and mechanistic study

Abstract

The kinetics of formation and dissociation of the binuclear complex of Co-II with histidinato(pentaammine)Co-III have been studied at 10.0 degrees C less than or equal to t degrees C less than or equal to 25 degrees C and I = 0.3 mol dm(-3) (ClO4-). The formation of the binuclear complex, [(NH3)(5)Co(III)LCo(II)](4+) (L = histidinate), in the 5-7-6.8 pH range involves the reaction of the Co(OH2)(6)(2+) with the deprotonated, (NH3)(5)CoL(2+), and monoprotonated, (NH3)(5)CoLH(3+), forms of the complex. The rate and activation parameters for the formation are consistent with an I-d mechanism. The binuclear species undergoes dissociation to yield the parent Co-III substrate and Co(OH2)(6)(2+) via spontaneous and acid-catalysed paths. Comparison of spontaneous dissociation rate of the binuclear complex with other related systems indicated the chelate nature of the binuclear species.