Kinetic and equilibrium properties of pentacyano(3,5-dimethylpyridine)-iron(II) and related anions in mixed aqueous solvents

Abstract

The kinetic pattern for reaction of [Fe(CN)₅(3,5-Me₂py)]^3– and [Fe(CN)₅,(3-CNpy)]^3– with a range of incomlng groups, in water and in 40% glycol and 40% t-butyl alcohol, is consistent with the operation of a D mechanism under all the conditions studied. Rate constants for the reaction of [Fe(CN)₅,(3.5-Me₂py)]^3– with cyanide ion have been determined in binary aqueous mixtures containing up to 40% by volume of methanol, ethanol, t-butyl alcohol, glycerol, or tetrahydrofuran. Correlation of these rate constants with Grunwald–Winstein Y values gives a value of m of –0.1; this small value reflects the low sensitivity of the rate to solvent composition. Free energies of activation determined from the observed rate constants have been plotted against excess Gibbs free energies of mixing (G^E) for the respective solvent mixtures. The resulting pattern indicates how far the thermodynamic characteristics of mixing of the solvent are reflected in the kinetics of this substitution reaction. The relation between the relative stabilities of [Fe(CN)₅(3,5-Me₂py)]^3– and its pyridine analogue, and solvent mixture G^E values, has also beenexamined. Thesolvatochromic behaviourofthecharge-transfer band of [Fe(CN)₅,(N-methylpyrarinium)]^2– has been investigated, and contrasted with that of organic analogues.