Reactions of µ-hydroxo-dicobalt complexes. Part V. Equilibrium and kinetic studies on the reaction of the µ-amido-µ-hydroxo-bis[tetra-amminecobalt(III)] complex with sulphate in aqueous perchloric acid solutions

Abstract

Equilibrium and kinetic studies on the interconversion of the µ-amido-µ-hydroxo- and µ-amido-µ-sulphato-complexes are described, at µ= 2·0M. From spectrophotometric studies at 25° the overall equilibrium con [graphic omitted] stant K12= 2·24 (±0·05)× 103 l2 mol–2. In the forward reaction the conversion of (I) into (II) is rate determining, and pseudo-first-order rate constants kf may be expressed as:, kf=k1a[H+][SO42–]+k1b[H+][HSO4–], with further contributions from a term in [SO42–]2. At 25°k1a= 2·15 × 10–2 l2 mol–2 s–1, ΔH1a‡= 17·7 ± 0·05 kcal mol–1 and ΔS1a‡=–6·6 ± 0·15 e.u.; k1b= 1·45 × 10–4 l2 mol–2 s–1ΔH1b‡= 17·9 ± 0·25 kcal mol–1 and ΔS1b‡=–16·1 ± 0·8 e.u. The reverse reaction has been studied at temperatures in the range 40–60°. The rate constant, kr(s–1), shows an [H+] dependence of the form: kr=a+b[H+]. With the assumption that stationary-state kinetics apply and that k2k–1, kr is a composite term k–2k–1/k2. At 25°(extrapolated) values of a and b are 4·4 × 10–6 s–1 and 7·4 × 10–7 l mol–1 s–1 respectively. Activation parameters for the [H+]-independent term a are ΔHa‡= 17·9 ± 0·15 kcal mol–1 and ΔSa‡=–23·0 ± 0·4 e.u., and for b, ΔHb‡= 15·4 ± 0·35 kcal mol–1 and ΔSb‡=–34·9 ± 1·1 e.u. The mechanism of the reaction is discussed.