Reactivity of the tri-µ-hydroxo-bis[triamminecobalt(III)] ion in halide media

Abstract

The kinetics of acid hydrolysis of trans-[(H₂O)(NH₃)₃Co(µ-OH)₂Co(NH₃)₃(H₂O)]⁴⁺ in acidic chloride and bromide solutions was studied using stopped-flow and conventional spectrophotometric techniques, at an ionic strength of 1.0 mol dm^–3 and over the ranges 0.10 ⩽[H⁺]⩽ 0.50 mol dm^–3, 0.10 ⩽[X^–]⩽ 0.50 mol dm^–3, 25 ⩽T⩽ 35 °C and in binary aqueous cosolvent mixtures containing up to 40% methanol by volume. The rate equation observed was of the form k_obs=(k_c[H⁺]+k′_c)[X^–]. At 25 °C the rate constants k_c were 1.10 × 10^–2 and 1.22 × 10^–3 dm⁶ mol^–2 s^–1 in chloride and bromide solutions respectively for the [H⁺]-dependent term. The corresponding values for the [H⁺]-independent pathway, k′_c, were 4.5 × 10^–4 and 0.7 × 10^–4 dm³ mol^–1 s^–1. The activation parameters in chloride solution were ΔH^‡(k_c)= 51 ± 14 kJ mol^–1, ΔS^‡(k_c)=–110 ± 48 J K^–1 mol^–1; ΔH^‡(k′_c)= 132 ± 32 kJ mol^–1 and ΔS^‡(k′_c)= 136 ± 105 J K^–1 mol^–1. At constant [H⁺](0.5 mol dm^–3) the reaction was first order with respect to [Cl^–] and [Br^–] and the rate constants, k_d, were 4.5 × 10^–3 and 3.3 × 10^–3 dm³ mol^–1 s^–1 respectively.