One-equivalent reactions in the reduction of the decammine-µ-amido-dicobalt(III) complex with vanadium(II)

Abstract

The kinetics of the reaction of vanadium(II) with the binuclear complex (NH₃)₅Co·NH₂·Co(NH₃)₅⁵⁺ have been studied in aqueous perchloric acid solutions of ionic strength µ= 2·0 and 0·4M at temperatures from 10·4 to 31·2°. The reaction is independent of the hydrogen-ion concentration (indicating an outer-sphere reaction), and is at least predominantly by a mechanism involving successive one-equivalent reactions: V²⁺+(NH₃)₅Co·NH₂·Co(NH₃)₅⁵⁺ [graphic omited] V³⁺+ Co²⁺+ Co(NH₃)₆³⁺+ 5NH₄⁺(i), V²⁺+ Co(NH₃)₆³⁺ [graphic omited] V³⁺+ Co²⁺+ 6NH₄⁺(ii) At µ= 0·4M activation parameters for (i) are ΔH^‡= 9·5 (±0·8) kcal. mole^–1 and ΔS^‡=–31 (±3) e.u., and at 25° reaction (i) is some thirty times faster than (ii). The activation enthalpies for the two reactions are essentially the same and the difference in rates is due to the difference in activation entropies.

Reaction (i) is catalysed by anions Cl^– < SO₄^2– < F^–, the relative rates being similar (they are 2–4 times greater) to those for reaction (ii). The nature of the activated complex is discussed.