Kinetic Studies of the Electron Transfer Reaction in Iron(II) and Iron(III) Systems. VIII. The Effect of Hexamethylphosphoric Triamide on the Electron Transfer Reaction in Aqueous Solution

The kinetics and mechanism of the electron transfer reaction between Fe(II) and Fe(III) in mixed solvents of water and hexamethylphosphoric triamide (HMPA) were studied. The apparent rate constant k varies depending upon [HMPA] and [H+]; k exhibits a minimum value at [HMPA]=1.5 M at constant [H+] and increases linearly with respect to [H+]−1 at constant [HMPA]. Judging from the basicity constant of HMPA, the complex formation constant of Fe(hmpa)3+, and the absorption spectra of aqueous solutions containing Fe(II), Fe(III), HMPA, and HClO4, the existing species in the reaction systems are mostly Fe2+aq, Fe3+aq, Fe(OH)2+, Fe(hmpa)3+, and Fe(hmpa)(OH)2+ at low concentrations of HMPA. Therefore, the probable reaction paths are Fe2+–Fe3+, (k0); Fe2+–Fe(OH)2+, (kH); Fe2+–Fe(hmpa)3+, (kL); and Fe2+–Fe(hmpa)(OH)2+, (kHL) at the concentration range of [HMPA]=0–1 M and [HClO4]=0.03–0.2 M. The occurrence of a minimum value of k vs. [HMPA] is reasonably understood by taking the complex formation between Fe(III) and ...

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