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Analysis of a quasi-reversible two-electron cyclic voltammetric wave for an organometallic ir(III)/ir(I) couple at platinum and mercury electrodes
Institution:1. Photonics Research Center, University of Malaya, 50603 Kuala Lumpur, Malaysia;2. Visiting Professor at the Department of Physics, Faculty of Science and Technology, Airlangga University, Surabaya 60115, Indonesia;1. Carlson School of Chemistry and Biochemistry, Clark University, 950 Main Street, Worcester, MA 01610, United States;2. Department of Physics, Clark University, 950 Main Street, Worcester, MA 01610, United States;3. Department of Chemistry, University of Canterbury, Private Bag 4800, Christchurch, New Zealand
Abstract:*CpIr(η-C6Me6)2+/0 (*Cpη5-C5Me5) displays Nernstian two-electron voltammetry at a Hg electrode, but quasi-reversible charge transfer kinetics at solid electrodes. Cyclic voltammetry (CV) peak shapes and separations change drastically from one solvent to another at Pt, ΔEp values being as small as 170 mV in acetone and as large as 350 mV in CH3CN (v = 0.03 V/s). These variations arise from changes in the relative E° values of the one-electron Ir(III)/Ir(II) and Ir(II)/Ir(I) couples, and from changes in charge-transfer rates. It is concluded that the Ir(II)/Ir(I) couple has a significantly lower charge-transfer rate than the Ir(III)/Ir(II) couple at platinum electrodes. The sensitivity of the CV curves to the relative E° values allows the approximate determination of the individual E° values for each one-electron process. In contrast, Nernstian conditions allow only the average of the two one-electron E° potentials to be determined. Solvents with higher solvating power are shown to facilitate the thermodynamics of the two-electron transfer process by moving E°2 positive with respect to E°1. Possible reasons for the abnormally slow charge transfer rates at Pt electrodes are discussed.
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