LARGE REORGANIZATION ENERGIES IN ELECTRON-TRANSFER - OXIDATION OF HYDROXYLAMINE BY HEXACHLOROIRIDATE(IV)

Oxidation of NH2OH (designated N(-1)) by [IrCl6](2-) in aqueous solution is catalyzed by Cu2+ and Fe2+, but the effects of these ions can be thoroughly inhibited by C2O42-. In the presence of this inhibitor and with excess N(-1), the rate law is -d[IrCl62-]/dt = k(1)K(a)[IrCl62-][N(-1)]/([H+] + K-a) with k(1) = 24 +/- 5 M(-1) s(-1) and K-a = (6.7 +/- 1.7) x 10(-7) M at mu = 0.1 M, 25 degrees C, and pH (4.23-6.78). The K-a value refers to the deprotonation of NH3OH+, while k(1) is inferred to be the rate constant of electron transfer from NH2OH to [IrCl6](2-). An analysis of k(1) in terms of the cross relationship of Marcus theory leads to an estimated self-exchange rate constant of 5 x 10(-13) M(-1) s(-1) (lambda = 530 kJ mol(-1)) for the NH2OH+/NH2OH redox couple, which is in agreement with the predictions of ab initio calculations.