Kinetics of the formation of iron(III) monochelates with substituted malonic acids

Abstract

The first and second dissociation constants of malonic, methylmalonic, n-butylmalonic, benzylmalonic, and cyclobutane-1,1-dicarboxylic acids have been determined in aqueous solution by pH measurements at ionic strength 0·50M and 25 °C. The kinetics of formation of iron(III) monochelates with these bidenate ligands have been studied by the temperature-jump method under the same conditions of ionic strength and temperature over the acidity range 0·040–0·45M. Kinetic measurements made at constant acidity have enabled the evaluation of the stability constants of all the iron(III) monochelates examined. The kinetic data obtained at various acidities have been interpreted in terms of the reactions of the FeOH²⁺ ion with the undissociated acid ligands and the corresponding monoanions. The estimated rate constants for both reactions are consistent with a mechanism in which the rate-determining step is the loss of the ‘first’ water molecule from the inner co-ordination sphere of the metal ion.