| Abstract: | The violet superoxo complex, [(H2O)4(OH)RhIII(O2)RhIII(OH)(H2O)4]3+, formed by treatment of (RhII)24+ with O2 in HClO4, is converted to a le? reduction product, the corresponding μ-peroxo complex, by the reductants I?, IrCl63?, and the trinuclear aquamolybdenum(III) cation, (MoIII)3. Each reaction is first-order in both redox partners, and the le? reduction by IrCl63? is followed by a much slower conversion to a peroxide-free complex. Among the rapid reductions of the superoxo derivative examined here and in a previous study, only that by IrCl63? is accelerated by increases in acidity; the rate law for this reaction features both an acid-independent and a [H+]-proportional component, the latter stemming from partial conversion of the oxidant to its conjugate acid (pKA < ?1.0). Rate laws for reductions by other metal-center reagents generally exhibit inverse-[H+] terms, reflecting deprotonation of the reductant. All reductions thus far observed involving this superoxo species appear to be outer-sphere. Treatment of acid-independent rate constants within the framework of the Marcus model, allows estimates of the self-exchange rate, k11, for the (RhIII)2-bound superoxo-peroxo couple. Because values of k11 calculated from the several reductions span a range of 104.5, reductions of the superoxo complex cannot be taken to conform satisfactorily to the Marcus treatment, being in this respect comparable to the systems VO(OH)+/2+, Mn2+/3+, Eu2+/3+, and Ti(OH)2+/3+, each of which exhibits similar divergences. The wide range of calculated self-exchange rates appears to invalidate an earlier suggestion that reduction of the superoxo complex by Fe2+ proceeds primarily through a bridged path. © 1994 John Wiley & Sons, Inc. |
