Acid hydrolysis of carboxylatopentaamminecobalt(III) complexes Kinetics and mechanism of aquation of malonato, succinato, o-phthalato and o-methoxy benzoato pentaamminecobalt(III) complexes

Abstract

The kinetics of aquation of malonato, succinato, o-phthalato and o-methoxybenzoato pentaamminecobalt(III) complexes have been studied in aqueoys acidic media adjusted to 0.3 M ionic strength. At [H⁺] = 0.005 - 0.3 M and 55 - 70°C, the observed pseudofirst-order rate constants are described by the relationship: k_obs = k₁ + k₂ [H⁺] where k₁ and k₂ stand for the rate constants of spontaneous and acid catalysed equation of (NH₃)₅CoCO₂(CH₂)nCO₂H₂+ (n = 1,2), (NH₃)₅CoCO₂C₆H₄CO₂H₂⁺ and (NH₃)₅CoCO₂C₆H₄OCH₃₂⁺ species respectively. The rate and the activation parameters for the k₁ and k₂ paths have been determined. The results indicate that the unbound carboxyl group does not exert a specific catalytic effect on the spontaneous and acid catalysed paths of aquation of the cobalt(III) substrates derived from the dicarboxylic acids. Co---O bond fission appears to be rate limiting in both k₁ and k₂ paths of all complexes. The possibility of C---O bond breaking, however, can not be ruled out in the k₂ path of the o-phthalato complex.