Electron Transfer Reactivity in Some Cobalt(III)-Cobalt(II) Couples. Franck-Condon Vs. Electronic Contributions
Electron-transfer rates involving the C~(sep)~+*~+ (sep = 1,3,6,8,10,13,16,19-octaazabicyclo[6.6.6]eicosane) and C~(en)~~+,~+ couples have been examined for any differences in contributions of Franck-Condon and electronic terms to the rate constants. Rate constants for cross-reaction oxidations of Co(sep)*+ follow the classically predicted dependence of the Franck-Condon factor on AGab0. Far-infrared and Raman vibrational spectra of M(~ep)~+/M(en)~,+ (M = Co, Rh) are in very close correspondence. As a consequence, the force constants of the M-N stretching modes must also be comparable (differing by less than 10%) in the C~(sep)~+ and C~(en)~~+ complexes. Strain energy calculations based on these and other published force constants are consistent with the differences in self-exchange rates and indicate upper limits on the Franck-Condon term in the rate constants consistent with significant contributions from electronic factors in both exchange reactions.
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Endicott, John F.; Brubaker, George R.; Ramasami, T.; Kumar, Krishan; Dwarakanath, Kaddi; Cassel, Jonathan; and Johnson, David W., "Electron Transfer Reactivity in Some Cobalt(III)-Cobalt(II) Couples. Franck-Condon Vs. Electronic Contributions" (1983). Chemistry Faculty Publications. 25.