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 kr(=k–1k–2/k2) for the reverse reaction (40–55 °C) give an [H+] dependence of the form kr([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, kf, may be expressed as kf=k1a[H+][H2PO4–]+k1b[H+][H3PO4], with small contributions from a term in [H2PO4–]2. At 25 °C: k1a= 1·18 × 10–3 l mol–1 s–1, ΔH1a‡= 19·5 ± 0·7 kcal mol–1, and ΔS1a‡=–6·5 ± 2·3 cal K–1 mol–1; and k1b= 8·54 × 10–5 l2 mol–2 s–1, ΔH1b‡= 14·3 ± 0·8 kcal mol–1, and ΔS1b‡=–29·2 ± 2·8 cal K–1 mol–1.