Evidence for contribution of intermediate steps to the limiting rate in iron(III) monomandelate complex formation

Abstract

The temperature-jump and stopped-flow methods have been used to study the kinetics of the chelation of iron(III) with mandelic acid at 25.0 °C and ionic strength 1.0 mol dm^–3 over the acidity range 0.0200 ⩽[H⁺]⩽ 0.900 mol dm^–3. The observed dependence of the overall forward rate constant on [H⁺] suggests that only the reaction between the Fe(OH)²⁺ ion and the neutral ligand (HL–LH) contributes significantly to the iron(III) chelate formation and that this reaction occurs through the formation of the intermediate protonated monodentate complex [Fe(OH)(L–LH)]⁺. At the lowest acidities examined the rate-determining step is the release of the first co-ordinated water molecule, while at high acidities ([H⁺] > 1.0 mol dm^–3) the slow step is the deprotonation of the intermediate monodentate complex. Over the acidity range covered in the present study both steps contribute to the limiting rate in the overall chelate-formation process.