Effect of solvent structure on the kinetics of ligand substitution in pentacyano(N,N-dimethylethylenediamine)ferrate(II)

Abstract

The rate constants and activation parameters for the dissociation of N,N-dimethylethylenediamine from its complex with the pentacyanoferrate(II) moiety have been determined in aqueous methanol mixtures from a methanol mole fraction x₂= 0 to 0.114. There is a negative correlation between the logarithm of the rate constant and Grunwald–Winstein Y values. Changes in the excess Gibbs free energy of mixing are reflected in the Gibbs free energy of activation, indicating a transition state more hydrophobic than the initial state. The increase in enthalpy and entropy of activation with increasing mole fraction of methanol is interpreted by considering the greater requirements for exchanging a ligand for a water molecule in a more structured solvent, thus pointing to an I_d mechanism. Entropy variations seem to control the free-energy changes as in the case of the excess thermodynamic parameters of mixing.