Kinetic investigations of complexes of cobalt (III) containing co-ordinated malonate

Abstract

The acid-catalysed deuteration of the active methylene group in the complexes: [Coen2mal]+, [CoBipy2mal]+ and [Comal2en]-, (where en = ethylenediamine, mal = malonate and Bipy = 2,2'bipyridyl), has been studied in the temperature range 20-45°C.The reaction was found to be of the first order in both acid and complex concentrations in the range pD2-3. The relative rates of deuteration are discussed in terms of a keto-enolate cation mechanism. The stereospecific deuteration of the methylene protons in [Comalen] is discussed in terms of the conformational flexibility of the six membered chelate ring. Absolute assignment of the magnetically inequivalent geminal protons in the latter complex followed the interpretation of the pmr spectra of the complex anions, [CoEthylmal2en] and [Comalbipy] in acidic and in basic solution. Part Two. The kinetics and mechanisms of the base and of the acid hydrolysis of the OC -monosubstituted complex cations [Coen2Ethylmal]+ and [Coen2Benzylmal]+ have been investigated spectrophotometrically in the temperature range 18-60°C. In basic solution the reaction proceeds in two identifiable stages. Both obey the rate law: 2Rate = K[complex] + k2[complex][OH ] + k[complex][OH-] .The second term is dominant in dilute base ([OH 1M), and the combined first and third in concentrated base ([OH ]>1.50M), at 25°C. In acidic solution, the reaction proceeds in two stages. In the pH range studied (2-4), the first shows half order dependence on acid, whilst the second is independent of acid. Mechanisms consistent with the kinetics are discussed.