The formation of FeL2+ (L−=N3−, SCN−) in H2O-dimethylsulphoxide (DMSO) mixtures was studied by equilibrium, stopped flow and temperature jump observations. The acid dissociation constants associated with LH and Fe(H2O)63+ and the formation constants associated with FeL2+ were determined at various solvent compositions. In kinetic studies, the solvent composition dependence of the overall equilibration rate was explained in terms of equations predicted by a reaction scheme which includes DMSO coordinated species. Analysis of results on the basis of these equations identified Fe·OH2+·HN3 and Fe3+·SCN− as the dominant transition states contributing to pH-independent paths of formation of FeL2+ in the cases of N3− and SCN− respectively. Thus, the results which support the Eigen-Tamm-Wilkins mechanism have resolved the mechanistic ambiguity in the pH-independent paths of formation of FeL2+. It is also shown that we cannot neglect species such as FeOH·HN32+, Fe·OH·N3+ and Fe·HN33+ while accounting for the kinetic behaviour.
Volume 134, 2022
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