Assignment of inner- and outer-sphere mechanisms to the vanadium(II) reductions of halogenopenta-amminecobalt(III) complexes

Abstract

Although rate constants (l mol^–1 s^–1) at 25°, I= 2.0M(LiClO₄), for the V²⁺ reduction of Co(NH₃)₅F²⁺(3·95), Co(NH₃)₅Cl²⁺(10·2), Co(NH₃)₅Br²⁺(30·0), and Co(NH₃)₅I²⁺(127) show a systematic trend, the activation parameters (ΔH^‡ kcal mol^–1; ΔS^‡ cal K^–1 mol^–1) indicate a different mechanism for Co(NH₃)₃F²⁺(11·1, –18·5) compared with Co(NH₃)₅Cl²⁺(7·5, –28·8), Co(NH₃)₅Br²⁺(7·2, –27·6), and Co(NH₃)₅I²⁺(7·3, –24·6). An inner-sphere substitution-controlled mechanism is proposed for the reduction of Co(NH₃)₅F²⁺, whereas Co(NH₃)₅Cl²⁺, Co(NH₃)₅Br²⁺, and Co(NH₃)₅I²⁺ react by an outer-sphere mechanism. These assignments are supported by linear free-energy plots using data for the V²⁺, Cr²⁺, Ru(NH₃)₆²⁺, and Cr(bipy)₃²⁺ reductions of common oxidants. Reasons for the different behaviour are considered.