Kinetics and mechanisms of the electron-transfer reactions between uranium(III) and some ruthenium(III) ammine complexes

Abstract

The rates of reduction of [Ru(NH₃)₆]³⁺ and [RuX(NH₃)₅]³⁺(n= 2 or 3; X = H₂O, OH^–, Cl^–, Br^–, and I^–) by U³⁺ have been measured with a stopped-flow spectrophotometer. The reductions of [Ru(NH₃)₆]³⁺ and [Ru(NH₃)(OH₂)]³⁺ proceed by an outer-sphere mechanism. The mechanism for [Ru(NH₃)₅(OH)]²⁺ is less certain, but is probably also outer sphere. The penta-amminehalogeno-complexes have a reactivity order I > Br > Cl, probably by an inner-sphere mechanism. The rates of reduction and reactivity patterns of equivalent cobalt(III) and ruthenium(III) complexes with a common reducing ion are compared. From the results, it is concluded that differences in the kinetic reactivity of the two metal complexes are due to the basic difference in the electronic structure of Co^III and Ru^III. These differences are manifested as influences of intrinsic factors on the rates and reactivity, and affect not only the rates of the reactions but also the type of mechanism often preferred. On the other hand, a comparison of rates of oxidation of U³⁺ with those of other aqua-ions employing Marcus theory leads to the conclusion that the very low reduction potential of U³⁺ is responsible for its very high redox reactivity.