Orbital Interactions in Fe(II)/Co(III) Heterobimetallocenes:  Single versus Double Bridge

Ferrocenyl cobaltocenium hexafluorophosphate (1) and ferrocenylene cobaltocenylenium hexafluorophosphate (2) are investigated by a range of spectroscopic methods. Both compounds are diamagnetic, in contrast to an earlier report indicating a temperature-dependent paramagnetism of 2. Electronic absorption spectra of 1 and 2 are presented and fully assigned up to 50 000 cm-1 on the basis of electronic structure (DFT) calculations and spectral comparisons with ferrocene and cobaltocenium. The lowest-energy bands, I, of both 1 and 2 correspond to metal-to-metal CT (MMCT) transitions; further intermetallocene charge-transfer bands are identified at higher energy (bands III and V). On the basis of the spectroscopic properties, a trans geometry and a twisted structure are derived for 1 and 2, respectively, in solution. Analysis of the I bands gives orbital mixing coefficients, α, electronic-coupling matrix elements, V AB, and reorganization energies, λ. Importantly, α and V AB are larger for 1 than for 2 (0.07 and 1200 cm-1 vs 0.04 and ∼600 cm-1, respectively), apparently in contrast to the presence of one bridge in 1 and two bridges in 2. This result is explained in terms of the respective electronic and geometric structures. Reorganization energies are determined to be 7600 cm-1 for 1 and 4600 cm-1 for 2, in qualitative agreement with the analogous Fe(II)−Fe(III) compounds. The general implications of these findings with respect to the spectroscopic and electron-transfer properties of bimetallocenes are discussed.