Electron Exchange between Bis(eta(6)-Arene) Chromium(I) and Bis(eta(6)- Arene) Chromium(O). Comparisons between Experimental and Calculated Kinetics Parameters

Abstract : Comparisons have been made between kinetics parameters for the self exchange of (eta(6)-arene)2Cr(I)/(eta(6)-arene)2Cr(O) (where arene = benzene, toluene, methoxylbenzene, biphenyl, ethylbenzoate and chlorobezene) measured by ESR line broadening in dimethylsulfoxide, with the predictions from contemporary electron-transfer theory. The biphenyl system was additionally studied in a number of other solvents. These reactions provide especially tractable systems with which to test theories of outer-sphere electron transfer since the work terms should be essentially zero, and the inner-shell contributions. Delta G to the free energy barrier are small and can be estimated from infra-red spectroscopy combined with crystallographic data. These were found to be somewhat (ca. 5-20 fold) larger than the corresponding frequency factors derived from the experimental activation enthalpies combined wtih the dielectric continuum estimates of the activation entropies. These 'experimental' frequency factors are compared with the estimates obtained from the 'reactive collision' and the 'ion-pair pre-equilibrium' models. The majority of the experimental frequency factors were found to be numerically closer to the predictions of the former model.