Kinetic studies of the reduction of hexaaquacobalt(III) perchlorate by a cobaloxime, [Co(dmgBF2)2(H2O)2]

The reaction of Co(OH2)63+ with Co(dmgBF2)2(H2O)2 in 1.0 M HClO4/LiClO4 was found to be first-order in both reactants and the [H+] dependence of the second-order rate constant is given by k2obs = b/[H+], b at 25 °C is 9.23 ± 0.14 × 102 s−1. The [H+] dependence at lower temperatures shows some saturation effect that allowed an estimate of the hydrolysis constant for Co(OH2)63+ as Ka = 9.5 × 10−3 M at 10 and 15 °C. Marcus theory and the known self-exchange rate constant for Co(OH2)5OH2+/+ were used to estimate an electron self-exchange rate constant of k22 = 1.7 × 10−4 M−1 s−1 for Co(dmgBF2)2(H2O)2+/0.

The kinetics of the reaction of aqueous cobalt(III) with Co(dmgBF2)2 have been studied. The possible reaction mechanism involves a rapidly maintained equilibrium between Co(OH2)63+ and its hydrolyzed form Co(H2O)5(OH)2+ and oxidation of Co(dmgBF2)2(H2O)2 by both of these species.Co(OH2)63+⇄KaCo(OH2)5(OH)2++H+Co(OH2)63++Co(dmgBF2)2(H2O)2→K1Co(OH2)62++Co(dmgBF2)2(H2O)2+Co(OH2)5(OH)2++Co(dmgBF2)2(H2O)2→K2Co(OH2)5(OH)++Co(dmgBF2)2(H2O)2+The rate has an inverse dependence on [H+], indicating that Co(H2O)5(OH)2+ is the only active oxidant.Figure optionsDownload as PowerPoint slide