Kinetics of the reduction of the µ-amido-µ-sulphato- and µ-amido-µ-selenato-bis[tetra-amminecobalt(III)] and sulphatopenta-amminecobalt(III) complexes with vanadium(II)

Abstract

The reaction of the µ-amido-µ-sulphatobis[tetra-amminecobalt(III)] complex with vanadium(II) 2V^II+ 2(Co^III)₂→ 2V^III+ 2Co^II(i) has been studied in perchlorate media, µ= 2·0M(LiClO₄). The reaction proceeds in two stages each involving reduction of cobalt(III) by V²⁺. The second stage, the reaction of Co(NH₃)₅SO₄⁺ with V²⁺(k₂), is faster than the first (k₁), and has to be studied independently. The rate law is of the form –d(ln[Complex])/dt=k[V²⁺] in both cases, and there is no hydrogen-ion dependence. At 25°k₁= 7·94 l mol^–1 s^–1 and k₂= 25·5 l mol^–1 s^–1, and the activation parameters are ΔH₁^‡= 11·2 ± 0·1 kcal mol^–1, ΔS₁^‡=–16·8 ± 0·3 e.u., ΔH₂^‡= 11·6 ± 0·1 kcal mol^–1, ΔS₂^‡=–13·1 ± 0·2 e.u. The first stage of the reaction of the µ-amido-µ-selenato-complex with V²⁺(k₃) has also been studied. It conforms to the same rate law and at 25°k₃= 15·3 l mol^–1 s^–1, ΔH₃^‡= 11·7 ± 0·1 kcal mol^–1 and ΔS₃^‡=–14·1 ± 0·3 e.u.