Solvent effects on the Co(NH3)4(pzCO2)2+–Fe(CN)64– reaction. An interpretation based on spectroscopic data

Abstract

The reduction of Co(NH₃)₄(pzCO₂)²⁺(pzCO₂= pyrazinecarboxylate) by Fe(CN)₆^4– has been studied in several isodielectric binary aqueous mixtures (water–tert-butyl alcohol, water–ethylene glycol and water–glycerol) at 298.2 K. Spectroscopic data have been obtained for the metal-to-metal charge-transfer band within the ion-pair, Co(NH₃)₄(pzCO₂)²⁺/Ru(CN)₆^4–. From these data, the Gibbs energies of activation for the thermal electron-transfer reaction have been calculated and compared with those obtained from a kinetic study. The two series of data corresponding to the different water–cosolvent mixtures are of the same order of magnitude but quantitative agreement between calculated and experimental data is not found. This discrepancy is discussed with the main conclusions being: (i) The classical model of the solvent, with a linear response, does not give correct predictions for the Gibbs energy of reorganization: the trend in changes of this magnitude calculated from the model is opposite to that found experimentally. (ii) Differences between the experimental and calculated Gibbs energies of activation arise from (1) the distance effect and (2) a failure of Hush's formulae based on linear response of the medium.