Journal of Solution Chemistry, Vol. 12, No. 2, 1983 Hydrophobic Interaction Effects on the Kinetics of Ligand Substitution in Binary Aqueous Mixtures at Pentacyano (piperidine)ferrate (11) Ion 1 Julio A. Salas,2 Miguel Katz, 2 and Nestor E. Katz 3 Received December 7, 1982; In Final Form March 1, 1983 The kinetics of piperidine replacement by pyridine at the pentacyano(piperidine)- .ferrate(ll) complex ion was studied under pseudo:first order conditions in binary aqueous mixtures of methanol, t-butanoL p-dioxane, and glycerol ,from a mole .fraction of co-solvent .from 0 to about O.15. The observed variations can be explained considering the degree of hydrophobic interaction between released ligand and water molecules which changes according to the structure:forming or struucture-breaking effect of added co-solvent on water. KEY WORDS: Solvent effects on kinetics; binary mixtures; hydrophobic interaction. 1. INTRODUCTION Ligand substitution processes in water + co-solvent mixtures are an area of active research./~ The importance of solute-solvent and solvent-solvent interactions on the kinetics of ligand replacements has recently been assessed for the dissociation of piperidine and N,N- dimethylethylenediamine from their corresponding complexes with the pentacyanoferrate(II) moiety in aqueous acetonitrile ~2) and in aqueous methanol, (3) respectively. In order to investigate further how far the thermodynamic properties of aqueous binary mixtures are reflected on IPresented at the XVI Congreso Argentino de Qurmica, C6rdoba, Argentina, September 1982. 2C~'tedra de Fi'sico-Qui'mica, Instituto de lngenieri'a Quimica, Facultad de Ciencias Exactas y Tecnologi~a, Universidad Nacional de Tucum~n, Tucum~n, Argentina. 3Catedra de Fisir 111, Instituto de Quim~'ca Fisi'ca, Facultad de Bioqu~'mica, Qua'mica y Farmacia, Universidad Nacional de Tucumg.n, Ayacucho 491, (4000) Argentina. 115 0095-9782/83/0200-0115503.00/0 9 1983 PlenumPublishing Corporation