Acid solvolysis of some low-spin tris(α,α′-di-imine)iron(II) complexes in dimethyl sulphoxide: kinetic evidence for competitive ion-pair formation

Abstract

The kinetics of acid solvolysis of some low-spin complexes of the type [Fe(L–L)₃]²⁺[L–L = 1,10-phenanthroline(phen), 5NO₂-phen, or 2,2′-bipyridyl (bipy)] have been examined in dimethyl sulphoxide (dmso). The rates of solvolysis in this solvent are about an order of magnitude faster than in water, and, unlike the behaviour in aqueous solution, the dependence of k_obs. on [acid] is similar for the three complexes studied. It is concluded that species containing unidentate protonated bipy are not formed to a significant extent during the acid solvolysis of [Fe-(bipy)₃]²⁺ in dmso. Addition of chloride ion to solutions of [Fe(phen)₃]²⁺ containing H₂SO₄ dramatically enhances the rate of solvolysis of the complex. This effect is less marked at higher acid concentrations, and these observations are interpreted in terms of a mechanism involving competitive ion-pair formation between the complex and Cl^– and [HSO₄]^–.