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.
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