Electron transfer. 50. Reduction of carboxylato-bound chromium(V) with hydrazine

Article abstract of DOI:10.1021/ic50224a010

Solutions of the chromium(V) complex sodium bis(2-ethyl-2-hydroxybutyrato)oxochromate(V) (I) oxidize hydrazinium ion smoothly. Each mole of N₂H₅⁺ consumes 2 mol of Cr(V), yielding N₂ and a Cr(III) product exhibiting ion-exchange behavior and an electronic spectrum consistent with a chelated monocarboxylato derivative of (H₂O)₄Cr^III. Kinetic results, obtained at pH 3.3-4.6 in the presence of added carboxylato ligand, point to a pair of two-electron transactions. The first of these, an oxidation to N₂H₂, is rate determining, whereas the second, the conversion to N₂, is rapid. Reactions, carried out in trimethylacetate buffers, are inverse first order in added 2-ethyl-2-hydroxybutyrate and are also inhibited by H⁺. The algebraic form of the rate law (eq 3 in text) is consistent with a two-path mechanism (eq 5-9), both components of which involve preliminary rapid and reversible loss of a carboxylato ion from the Cr(V) oxidant. The resulting monocarboxylato intermediate then may undergo reduction either directly through (outer-sphere) reaction with N₂H₅⁺ or, alternatively, via formation of a precursor complex, assembled with loss of H⁺. The appearance of only one Cr(III) reaction product suggests that the Cr(V) species attacking N₂H₂ in the final rapid step(s) has the same ligand environment as that attacking N₂H₅⁺ in the rate-determining steps.