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 - Koshy, K.; P. Dasgupta, T. (Elsevier)
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 ^oC 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 ^oC and pH = 8.62 being 5.25 x 10^-^3 s^-^1 and 1.48 x 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 x 10^-^2 s^-^1 and 2.71 x 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.
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