Kinetics of oxidation of hydrazine by a dioxo-bridged dimanganese(III,IV) complex ion

In aqueous acidic media containing an excess of Hbipy⁺–bipy buffer in the pH 3.5–4.5 range, the complex ion (bipy)₂Mn^III(-O)₂Mn^IV(bipy)₂]³⁺ (1) coexists in rapid equilibrium with its diaqua derivative Mn^III,IV ₂ (-O)₂(bipy)₃(H₂O)₂]³⁺ (1a) (bipy = 2,2-bipyridine). An excess of N₂H₅ ⁺ quantitatively reduces the mixture to Mn^II, itself being oxidised to N₂. The first order rate constant, k _o decreases with increasing C _bipy (C _bipy = Hbipy⁺] + bipy]) but increases with increasing N₂H₅ ⁺] and H⁺]. The observed kinetic dependence can be explained in terms of a reaction between (1a) and N₂H₅ ⁺. Replacement of solvent H₂O with D₂O decreases k _o substantially and the effect suggests simultaneous transfer of an electron and a proton in the rate-determining step. The relevance of this observation to the delayed oxidation of H₂O in the hydrazine-treated photosystem II is discussed.