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.