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Kinetic Study of the Acid-catalysed Hydrolysis of Cobalt(III) Carbonato Complexes with Nonlabile Tetrakis(methylamine), Tetrakis(ethylamine), Fac(N)-(ethylenediamine)(glycinato), Mer(N)-(ethylenediamine) (glycinato) or Bis(1,3-propanediamine) Ligands

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Acid-catalysed hydrolysis kinetics were studied for 14 cobalt(III) carbonato complexes involving the title five at oxonium ion concentration of 0.01-1.0 M, ionic strength of 2.0 (NaClO4) and 15-45°C (M=mol dm−3). Preprotonation constant, about 1 M−1, was estimated from a non-linear acidity dependence of an experimental rate constant for the ring-opening of the didentate hydrogencarbonato intermediate. Acidity constant, 0.003-0.7 M, and basicity constant, 0.1-0.9 M−1, of the ring-opened aquahydrogencarbonato intermediate were estimated from an acidity dependence of an experimental rate constant for the decarboxylation. Concerning the widely ranging ring-opening rate constants (0.05-126 s−1 at 25°C), we demonstrate labilizing effects of a reduced charge and of small tetrakis(amine) ligand, correlating with a high activation entropy, as well as of the trans five-membered chelate correlating with a low activation energy. The charge effect may be understood by the solvational change during the dissociative activation. Activation parameters of the uniform decarboxylation rate constant (0.2-4 s−1 at 25°C) from the aquahydrogencarbonato intermediate are higher than those of the decarboxylation from carbonic acid. This may be relevant to a Co-O bond rotation prior to the decarboxylation. The isometric structures of carbonato(ethylenediamine)(glycinato)cobalt(III) are studied by X-ray crystallography.

Keywords: Carbonato chelates; Charge effect; Cobalt(III); Hydrolysis kinetics; Stopped-flow; X-ray crystal structure

Document Type: Research Article


Publication date: January 1, 2002

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