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 k_f may be expressed as:, k_f=k_1a[H⁺][SO₄^2–]+k_1b[H⁺][HSO₄^–], with further contributions from a term in [SO₄^2–]². At 25°k_1a= 2·15 × 10^–2 l² mol^–2 s^–1, ΔH_1a^‡= 17·7 ± 0·05 kcal mol^–1 and ΔS_1a^‡=–6·6 ± 0·15 e.u.; k_1b= 1·45 × 10^–4 l² mol^–2 s^–1ΔH_1b^‡= 17·9 ± 0·25 kcal mol^–1 and ΔS_1b^‡=–16·1 ± 0·8 e.u. The reverse reaction has been studied at temperatures in the range 40–60°. The rate constant, k_r(s^–1), shows an [H⁺] dependence of the form: k_r=a+b[H⁺]. With the assumption that stationary-state kinetics apply and that k₂k_–1, k_r 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 ΔH_a^‡= 17·9 ± 0·15 kcal mol^–1 and ΔS_a^‡=–23·0 ± 0·4 e.u., and for b, ΔH_b^‡= 15·4 ± 0·35 kcal mol^–1 and ΔS_b^‡=–34·9 ± 1·1 e.u. The mechanism of the reaction is discussed.