Study of the Ligand Substitution Reaction Fe(CN) 5 H 2 O 3 - + Pyrazine in Micellar Solutions MARÍA DEL MAR GRACIANI, MARÍA AMALIA RODRÍGUEZ, MARÍA LUISA MOYÁ Departamento de Química Física, Universidad de Sevilla, C/ Profesor García González s/n, 41012 Sevilla, Spain Received 24 June 1996; accepted 7 October 1996 ABSTRACT: The ligand substitution reaction Fe(CN) 5 H 2 O 3- + pyrazine : Fe(CN) 5 pyrazine 3- + H 2 O has been studied in sodium dodecyl sulfate, SDS, hexadecyltrimethylammonium bro- mide, CTAB, and salt aqueous solutions at 298.2 K. Kinetics were studied in dilute and con- centrated salt solutions and in SDS and CTAB solutions at surfactant concentrations below and above the critical micelle concentration. Experimental results show that salt effects can be explained by considering the interaction between the cations present in the working me- dia, which come from the background electrolyte, and the Fe(CN) 5 H 2 O 3- species in the vicin- ity of the cyanide ligands. This interaction makes the release of the aqua ligand from the inner-coordination shell of the iron(II) complex to the bulk more difficult, resulting in a decrease of the reaction rate when the electrolyte concentration increases. Kinetic data in surfactant solutions show that not only micellized surfactants are operative kinetically, but also nonmicellized surfactants are influencing the reactivity. © 1997 John Wiley & Sons, Inc. Int J Chem Kinet 29: 377 – 384, 1997. INTRODUCTION The influence of organized media; micelles, vesicles, and membranes, on the course of a large variety of chemical and biochemical reactions is now well doc- umented. Our interest has been focused on the study of micellar effects on electron transfer and on substi- tution reactions [1 – 6]. In many cases, the study of re- actions at surfactant concentrations below (or close to) the c.m.c. is precluded because of solubility prob- lems with one or both reactants. However, consid- ering the large difference found in the reaction rate for numerous processes in micellar systems com- Correspondence to: M. L. Moyá Contract grant Sponsor: D.G.C.Y.T. Contract grant number: PB95-0527 © 1997 John Wiley & Sons, Inc. CCC 0538-8066/97/050377-08 pared to pure water, the study of changes in the rate constants by changing surfactant concentrations in the dilute region below the c.m.c. seems interesting. With this idea in mind, the substitution reaction between aquopentacyanoferrate(II), Fe(CN) 5 H 2 O 3- , and pyrazine was chosen. The high molar extinction coefficient of the product, Fe(CN) 5 pyrazine 3- ( max = 452 nm,   5000 mol dm -3 cm -1 in pure water [7]), permit the use of a low iron(II) reactant concentration and, therefore, solubility problems at low surfactant concentrations can be avoided. The reaction Fe(CN) 5 H 2 O 3- + pyrazine has been studied in several aqueous dilute and concentrated salt solutions as well as in sodium dodecylsulfate, SDS, and hexadecyltrimethylammonium bromide, CTAB, solutions at surfactant concentrations below and above the c.m.c. In all experiments temperature was kept at 298.2 K.