Dinuclear complexes of transition metals containing carbonate ligands. V. Kinetics and mechanism of the reactions of the μ-Amido-μ-hydroxo-bis-[(tetraamminecobalt(III)] complex ion in aqueous solutions

Abstract

The kinetics of the reaction of the title complex with aqueous carbonate has been investigated by spectrophotometry at 440 nm over the range 8.00 <pH<9.30, 30<θ t<45 °C and 0.05<[CO 3] T < 0.20 M (where θ t = temperature and [CO 3] T = total carbonate concentration), at an ionie strength of 0.5 M (LiClO 4). The reaction observed under these conditions is the unexpectedly effective substitution of CO 3 2− for NH 3 on one of the cobalt centres of the dinuclear species. The rate data revealed that the reaction follows a consecutive reaction pathway involving the loss of two ammonia molecules in the rate-determining steps - the value of k 1 and k 2 at 40 °C and pH = 8.62 being 5.25 × 10 −3 s −1 and 1.48 × 10 −3 s −1, respectively. The rate of base hydrolysis in the absence of any carbonate also follows a consecutive reaction pattern, and the values of the rate constants are 1.36 × 10 −2 s −1 and 2.71 × 10 −3 s −1, respectively. In carbonate solution the reaction product is the complex μ-amido-μ-hydroxo[tetra- amminecobalt(III)] [biscarbonatocobalt(III)] which has been characterized by a previous X-ray diffraction study. The kinetic data for the formation of this compound have been explained by a mechanism involving extensive ion-pairing in solution.