The uncatalysed and mercury(II)- and thallium(III)-catalysed elimination of chloride from the µ-amido-µ-chloro-bis[tetra-amminecobalt(III)] complex

Abstract

The kinetics of the mercury(II)- and thallium(III)-catalysed elimination of chloride from the title complex (I), equation (i), have been studied in aqueous HClO₄ solution, I= 2·0M(NaClO₄). Both reactions are independent [graphic omitted] of [H⁺] in the range 0·5–2·0M. With mercury(II) there is a less than first-order dependence on [Hg²⁺] which cannot be satisfactorily accounted for in terms of a 1 :1 adduct. Instead the dependence can be explained by considering Hg²⁺-catalysed conversion of (II) into (III) and applying the steady-state approximation for (II). From the treatment (at 25 °C), k₁= 3·02 × 10^–3 s^–1, ΔH^‡₁= 19·2 ± 0·5 kcal mol^–1, ΔS^‡₁=–5·7 ±1·8 cal K^–1 mol^–1, and k₂/k_–1= 0·028. At 25 °C ratios k₃/k₂= 1·57 × 10³ I mol^–1 and k₄/k₂= 1·3 × 10³ I mol^–1 are measures of the effectiveness of Hg²⁺ and HgCl⁺ as catalysts for (II)→(III). Thallium(III) produces only a mild catalytic effect, a first-order dependence on [Tl³⁺] is observed, and k₅/k₂= 15 I mol^–1 is a measure of the effectiveness of Tl³⁺ as a catalyst. It is concluded that Tl³⁺ is a relatively weaker catalyst compared to Hg²⁺ for (II)→(III) than for the catalysed aquation of mononuclear chloro-complexes.