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 ⩽θc⩽ 45 °C, and 0.05 ⩽[CO32–]⩽ 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 kobs. 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.