Examination of activation parameters for the dissociation of iron(III) complexes as a means of assessing mechanistic ambiguities. Data for phenolic complexes

Abstract

Activation parameters for the dissociation (aquation) of iron(III) monophenolate complexes in aqueous solution have been estimated at ionic strength 0.1 mol dm^–3 by the temperature-jump method and examined together with those of other iron(III) complexes. The activation entropies for the pathways involving the FeOH²⁺ species in the transition state appear to be markedly higher than those for the pathways involving Fe³⁺. The correponding differences in the activation enthalpies are generally less significant. The dissociation activation parameters (ΔS^*_d much better than ΔH^*_d) can be considered a useful tool, in both the complex formation and dissociation processes, for establishing the actual reaction path for the two kinetically indistinguishable reactions Fe(H₂O)³⁺₆+ A^–⇌(H₂O)₅FeA²⁺+ H₂O and Fe(H₂O)₅OH²⁺+ HA ⇌(H₂O)₅FeA²⁺+ H₂O.