Dinuclear complexes of transition metals containing carbonate ligands. Part 3. Kinetics and mechanism of bridge cleavage in the di-µ-hydroxo-bis[bis(ethylenediamine)cobalt(III)] ion in weakly basic aqueous solution containing carbonate ions

Abstract

The kinetics of the reaction of the title dinuclear ion [(en)₂Co(µ-OH)₂Co(en)₂]⁴⁺ with aqueous carbonate have been followed spectrophotometrically at 340 nm over the ranges 8.26 ⩽ pH ⩽ 9.4, 35 ⩽θ_c⩽ 45 °C, and 0.05 ⩽[CO₃^2–]⩽ 0.2 mol dm^–3 at an ionic strength of 0.5 mol dm^–3(LiClO₄). During the reaction the complete cleavage of the hydroxo-bridges occurs and the sole reaction product is [Co(en)₂(CO₃)]⁺. The rate of the bridge cleavage is completely independent of pH as well as total carbonate in solution. The rate data required a consecutive reaction treatment to resolve the k_obs. values into the components k₁ and k₂ which have been assigned to the first and second bridge cleavages of the title complex. At 25 °C and I= 0.5 mol dm^–3, the values of k₁ and k₂ are (1.7 ± 0.3)× 10^–3 and (0.46 ± 0.05)× 10^–3s^–1 with the Eyring–Polyani parameters ΔH₁^≠= 101 ± 9 kJ mol^–1, ΔS₁^≠= 28.5 ± 28.0 J K^–1 mol^–1 and ΔH₂^≠= 90.8 ± 5.4 kJ mol^–1, ΔS₂^≠=–13.0 ± 18.4 J K^–1 mol^–1 respectively. The reaction has also been studied without any added carbonate over the range 8.21 ⩽ pH ⩽ 9.25 at 40 °C and I= 0.5 mol dm^–3. The rate constants are found to be independent of pH here also and have average values of (1.05 ± 0.06)× 10^–3 and (2.98 ± 0.03)× 10^–4s^–1 respectively. The higher values for the rate constants in carbonate-containing solutions indicate a mechanism involving some form of carbonate catalysis during the bridge cleavage.