KINETICS AND MECHANISM OF THE REDUCTION OF TRANS-DIOXORUTHENIUM(VI) BY IRON(II)

The kinetics of the reduction of trans-[Ru(TMC)(O)2]2+ ,11-tetramethyl -1,4,8,11-tetraazacyclotetradecane) has been studied in aqueous acidic solution. The reaction has the following stoichiometry: trans-[RuVI(T MC)(O)2]2+ + 2Fe2+ + 2H+ --> trans-[RuIV(TMC)O(OH2)]2+ + 2Fe3+. The ra te law is -d[Ru(VI)]/dt=k2[Ru(VI)][Fe2+] with k2=27.4 M-1 s-1 at 25.0- degrees-C and 1.0 M ionic strength, and DELTAH(double dagger)=1.3+/-0. 3 kJ mol-1, DELTAS(double dagger)=-(210+/-20) J mol-1 K-1. A mechanism that is consistent with the rate data is [RuVI(TMC)(O)2]2+ + Fe2+ hal f arrow right over half arrow left [RuV(TMC)(O2)]+ + Fe3+, followed by [RuV(TMC)-(O2)]+ + Fe2+ + 2H+ --> ([RuIV(TMC)O(OH2)]2+ + Fe3+ (fast). Using the Marcus cross relation the rate constant for the outer-spher e electron transfer between Ru(VI) and Fe2+ is calculated to be 10 M-1 s-1, which compares favorably with the experimental value. However th e low DELTAH(double-dagger) and the large negative DELTAS(double dagge r) is more consistent with an inner sphere mechanism.