Stepwise chromium(II) and vanadium(II) reductions of the µ-amido-µ-superoxo- and µ-amido-µ-peroxo-bis[bis(ethylenediamine)cobalt(III)] complexes

Abstract

The five-equivalent Cr²⁺ reduction of the µ-amido-µ-superoxo-complex, [(en)₂Co·µ(NH₂,O)·Co(en)₂]⁴⁺, has been studied in aqueous perchlorate media, I= 2·0M(LiClO₄). Fast outer-sphere reduction to the µ-amido-peroxo-complex [(en)₂Co·µ(NH₂,O₂)·Co(en)₂]³⁺ is observed in the initial step, k 1·4 × 10⁵ l mol^–1 s^–1 at temperatures in the range 4–35 °C. The first stage of reduction of the µ-amido-µ-peroxo-complex is inner sphere and Co²⁺, but not hexa-aquochromium(III), is obtained following ion-exchange separation of the products of the 1 : 1 reaction. Reaction occurs predominantly via the unprotonated form of the complex : k(25 °C)= 2100 ± 60 l mol^–1 s^–1, ΔH‡= 7·75 ± 0·25 kcal mol^–1, and ΔS‡=–17·5 ± 0·9 cal K^–1 mol^–1. From our observations it is concluded that a mixed µ-peroxo-cobalt(III)–chromium(III) complex (NH₃)(en)₂Co·O₂·Cr(H₂O)₅⁴⁺ is formed. Other subsequent stages, some of which are independent of [Cr²⁺] and involve isomerisation, have been studied and kinetic data obtained. The remaining cobalt(III) chromophore dominates the u.v.–visible spectrum until the peroxo-bridge has been fully reduced to give a µ-hydroxo-cobalt(III)–chromium(III) complex. The latter is reduced in the final slow step which is outer sphere, rate constant (1·09 ± 0·07)× 10^–3 l mol^–1 s^–1 at 35 °C, with no dependence on [H⁺] in the range 0·08–0·50M investigated. Less extensive studies on the V²⁺ reduction of the µ-amido-µ-superoxo- and µ-amido-µ-peroxo-complexes are also reported.