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.