The Cr2+ and V2+ reduction of µ-carboxylato-dicobalt(III) ammine complexes. Part III. The mechanism of reduction of µ-benzoato- and µ-o-chlorobenzoato-complexes

Abstract

The µ-benzoato-di-µ-hydroxo- and µ-o-chlorobenzoato-di-µ-hydroxo-bis[tri-amminecobalt(III)] complexes are stable to bridge cleavage in aqueous perchlorate solutions [H⁺]= 0·01–1·0M, I= 1·0M(LiClO₄), during 3 days at 25°. Reduction of the µ-benzoato-complex with Cr²⁺ gives a rate law (i). At 30°k_Cr=(1·52 ± 0·06)× 10^–3 Rate =(k_Cr[Cr²⁺]+k₁[H⁺])[(Co^III)₂](i) l mol^–1 s^–1, ΔH^‡_Cr= 9·46 ± 0·28 kcal mol^–1 and ΔS^‡_Cr=–40·3 ± 0·9 K^–1 mol^–1, and for the path in which bridge cleavage is rate determining k₁=(1·8 ± 0·4)× 10^–5 l mol^–1 s^–1, ΔH^‡₁= 18·6 ± 1·9 kcal mol^–1 and ΔS^‡₁=–19·2 ± 6·0 cal K^–1 mol^–1. The rate law for the V²⁺ reduction is (ii) and at 30°k_v=(6·74 ± 0·06)× Rate =kv [V²⁺][(CO^III)₂](ii) 10^–2 l mol^–1 s^–1. Rate constants (25°, l mol^–1 s^–1) for the corresponding reductions of the µ-o-chlorobenzoato-complex are k_Cr=(1·62 ± 0·04)× 10^–3, k₁=(0·30 ± 0·10)× 10^–5 and k_v=(7·9 ± 0·2)× 10^–2. The ratios k_Cr/k_v of 0·023 and 0·021 respectively are consistent with outer sphere Cr²⁺ and V²⁺ reductions. No obvious enhancement in rate results from the presence of the benzene ring or the remote chlorine atom.