Electron-transfer Kinetics of Transition-metal Complexes in Lower Oxidation States. Part II. The Electrochemical Electron-transfer Rate of the Cr(bipy)3+/Cr(bipy)3 System as Measured by the Galvanostatic Double-pulse Metho

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The kinetic parameters of the electron-transfer reaction, Cr(bipy)3++e\rightleftharpoonsCr(bipy)3, were measured by the galvanostatic double-pulse method at a dropping mercury electrode in N,N-dimethylformamide. The apparent and the corrected standard rate constants were 1.0±0.2 and 0.3 cm s−1 respectively, and the transfer coefficient was 0.6±0.1 at 25 °C. These rate constants were discussed in comparison with the corresponding homogeneous rate constant on the basis of the theoretical equation of Marcus. They were quite inconsistent with the value, 103 cm s−1, predicted from the corresponding homogeneous rate constant. The polarograms of Cr(bipy)3+ and Cr(bipy)3 exhibited three waves with half-wave potentials of −0.21, −0.65 and −1.19 V vs. a cationic glass electrode. The diffusion coefficients of these complexes, as determined polarographically, were identical: 3.2×10−6 cm2 s−1 at 25 °C.
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