KINETICS OF THE REACTION OF COPPER(II) WITH COBALT(II) SEPULCHRATE - CATALYSIS BY CHLORIDE-ION AND IMIDAZOLE

The kinetics of the oxidation of cobalt(II) sepulchrate by aqueous copper(II) have been studied in the presence of chloride ion, imidazole, and acetonitrile at 25-degrees-C. The reaction rate increases with increasing concentrations of chloride ion (0.05-0.2 M in 0.50 M HClO4/LiClO4) and imidazole (0.025-0.09 M at pH 6.5, in 0.15 M LiClO4), but is unaffected by 0.4 M acetonitrile (0.50 M HClO4/LiClO4). The reaction of Cu2+(aq) and Co(sep)2+ is complicated by the rapid formation of copper metal, and it was necessary to use 02 as a scavenger for Cu+(aq) in order to determine the rate constant of 5.0 +/- 0.25 M-1 s-1 (0.02 M HClO4, 0.48 M LiClO4, 25-degrees-C). This value and earlier results for reductions of Ru(III) complexes by Cu+(aq) give a self-exchange rate constant of 5 X 10(-7) M-1 s-1 for Cu2+/+(aq) from the Marcus cross relationship. The Cu(II)(Cl)n complexes have rate constants of 1.6 X 10(3), 1.5 X 10(4), and 4.5 x 1O(5) M-1 s-1 for n = 1-3. The change in reactivity can be accounted for in terms of Marcus theory by the increased driving force and reduced charge, with a self-exchange rate constant for the Cu(II)/I(Cl)n species of 2 x 10(-4) M-1 s-1. The Cu(II)(Im), complexes show a much smaller change in reactivity (35, 70, and 120 M-1 s-1 for n = 2-4) and a smaller self-exchange rate constant of approximately 1 X 10(-7) M-1 s-1.