Intervalence Electron Transfer in Bicobaltocene Cations: Comparison with Biferrocenes

摘要

The near-infrared absorption parameters for bicobaltocene monocation, (Cp2Co)2+, evaluated in several solvents, are compared with corresponding data for biferrocene cation, (Cp2Fe)2+, in order to ascertain the consequence of metal substitution upon the degree of redox-site electronic coupling. From the markedly (ca. 5 fold) larger intervalence band intensities, narrower bandwidths, and milder solvent dependence of the band energy observed for the former system, the degree of Co(III)-Co(II) electronic coupling is deduced to be substantially greater than for Fe(III)-Fe(II). A similar conclusion is reached from a comparison of near-infrared spectra for bis(fulvalene)-dicobalt and -diiron monocations. These differences are also reflected in more negative comproportionation free energies for the cobalt mixed-valence analogs, as derived from electrochemical data. These findings are consistent with orbital symmetry considerations, since electron transfer for the Co(III)-Co(II) system is expected to involve a ligand-centered 4e1g orbital, as compared with strongly metal-localized 4e2 or 8a1g orbitals that are apparently utilized in the Fe(III)-Fe(II) metallocene case.