Reactions of µ-hydroxo-dicobalt(III) complexes. Part X. Kinetic studies of the reaction of the µ-amido-µ-hydroxo-bis[tetra-amminecobalt(III)] complex with phosphate ions in aqueous perchloric acid solutions and the characterisation of the µ-amido-µ-phosphato-bis[tetra-amminecobalt(III)] complex

Abstract

The nitrate salt of the µ-amido-µ-hydrogenphosphato-bis[tetra-amminecobalt(III)] complex and the bromide salt of the corresponding µ-amido-µ-phosphato-complex have been prepared and characterized. First and second acid dissociation constants of H₃PO₄ have been determined by potentiometric titration and pK₁= 0·9 and pK₂= 5·8 at 25 °C and I= 2·0M(NaClO₄). Acid dissociation constants for the phosphato-complexes [equation (i)] have also been determined and, at 25 °C, pK₃= 1·5₂[I= 2·0M(NaClO₄)] and pK₄= 6·0 ± 0·1 [I= 0.1M(LiClO₄)]. [graphic omitted] The kinetics of the interconversion of complexes (I) and (III)[equation (ii)] have been studied at I= 2·0M(NaClO₄). [graphic omitted] The intermediate (II) has not been isolated and behaves as a transient. Rate constants k_r(=k_–1k_–2/k₂) for the reverse reaction (40–55 °C) give an [H⁺] dependence of the form k_r([H⁺]+K₃)/[H⁺]K₃=p+q[H⁺]. At 25 °C (extrapolated) values of p and q are 0·23 × 10^–3 l mol^–1 s^–1 and 0·57 × 10^–4 l² mol^–2 s^–1 respectively, and activation parameters have been obtained for both constants. For the forward reaction (20–35 °C), the conversion (I)→(II) is rate determining and pseudo-first-order rate constants, k_f, may be expressed as k_f=k_1a[H⁺][H₂PO₄^–]+k_1b[H⁺][H₃PO₄], with small contributions from a term in [H₂PO₄^–]². At 25 °C: k_1a= 1·18 × 10^–3 l mol^–1 s^–1, ΔH_1a^‡= 19·5 ± 0·7 kcal mol^–1, and ΔS_1a^‡=–6·5 ± 2·3 cal K^–1 mol^–1; and k_1b= 8·54 × 10^–5 l² mol^–2 s^–1, ΔH_1b^‡= 14·3 ± 0·8 kcal mol^–1, and ΔS_1b^‡=–29·2 ± 2·8 cal K^–1 mol^–1.