Kinetics of the dissociation reaction of ferricenium tri-iodide in benzene solution

Abstract

The dissociation of ferricenium tri-iodide in benzene solution has been studied by a photochemical technique under conditions where the concentration of the ion pair is many times greater than the equilibrium value. Kinetically the reaction is found to be of ca. three-halves order, with variations towards first-order or towards second-order behaviour, depending on the concentrations of iodine and ferrocene. The Arrhenius activation energy lies in the range 95–125 kJ mol^–1, showing lower values at low concentrations. More rapid dissociation is found for phenylferrocene and slower reaction for 1,1′-diphenyl-, methyl- and 1,1′-dimethyl-ferrocene and for [3]ferrocenophane. A mechanism has been proposed which involves the intermediates Fn⁺I^–₄, Fn⁺I^–₂ and Fn⁺I^–, and it is suggested that the variations in reaction order are a consequence of the variable importance of the reverse processes of some of these reaction steps.