Effect of Nonlabile Quadridentate Ligand on the Acid-catalysed Hydrolysis Velocity of Cobalt(III) Carbonato Chelates

Acid-catalysed hydrolysis kinetics were studied for 10 cobalt(III) complexes bearing a bidentate carbonato ligand at oxonium ion concentration of 0.02-1.0 u M, ionic strength of 2.0 (NaClO 4 ) and 15-45°C (M=mol u dm m 3 ). Preprotonation constant, about 1 u M m 1 , was estimated from a non-linear acidity dependence of the ring-opening velocity of the bidentate hydrogencarbonato intermediate. Acidity constant, 0.005-0.4 u M, and basicity constant, about 0.4 u M m 1 , of the ring-opened aquahydrogencarbonato intermediate were estimated from an acidity dependence of the decarboxylation velocity. Between analogous complexes, ring-opening is faster; activation parameters of ring-opening are lower; and acidity of an aquahydrogencarbonato intermediate is weaker, when the nonlabile quadridentate forms five-membered chelates on the Co(III) center. We believe that a five-membered chelate, especially trans to the hydrogencarbonato, labilizes the hydrogencarbonato chelate by reinforcing the steric strain and that it...