Leaving group effects on ligand substitution reactions of pentacyanoferrate(II) complexes: Rate constant and activation volume correlations

The kinetics of the reaction of cyanide ions with pentacyanoferrate(II) complexes have been studied spectrophotometrically at pressures of 1 bar and up to 1 kbar, at 298.2 K. An excess of cyanide ions was employed and first-order kinetics were observed both in aqueous solution and in aqueous-mono-ol mixtures. For several pyridine derivative leaving groups, neutral or mono-positively charged, the rate constant variation in aqueous medium is only over one half-order of magnitude, although thiourea and quinoxaline are much more labile, dissociating with rate constants about ten and three hundred times greater than this range, respectively. Very modest changes in rate constant are observed upon addition of 40% methanol, and in a few examples studied, kinetic differences become significant only in cosolvent-rich mixtures. Volumes of activation, Delta V*, are all positive, for reaction in water, confirming the expected bond extension of the leaving group in a D mechanism. Solvation changes and ligand differences do not wholly explain the variation in Delta V* values, or the changes in this parameter found when cosolvents are added. Reasonably good correlations are found for the logarithms of rate constants both with the pK, of the ligand and with Delta V*. Other potential correlations of the leaving group property and kinetic parameter are discussed.