Intermetal Coupling in [(η5-C5R5)Fe(dppe)]2(μ-CHCHCHCH) and in Their Dicationic and Monocationic Mixed-Valence Forms

The butadienediyl-bridged complexes [(η5-C5R5)Fe(dppe)]2(μ-CHCHCHCH) (R = H, Me) and their radical cationic and dicationic forms have been prepared and characterized by cyclic voltammetry, electronic spectroscopy (UV−vis and near-IR), EPR, and X-ray crystallography (R = Me). Comparisons with each other and with the literature complexes [(η5-C5H5)Fe(dppm)]2(μ-CHCHCHCH), [(η5-C5Me5)Fe(dppe)]2(μ-C⋮CC⋮C), and [(η5-C5Me5)Fe(dppe)]2(μ-C(OMe)CHCHC(OMe)) have allowed for systematic evaluation of several structural variations (C5H5 vs C5Me5, dppm vs dppe, μ-CHCHCHCH vs μ-C⋮CC⋮C, and μ-CHCHCHCH vs μ-C(OMe)CHCHC(OMe)) and their effects on spectra and electronic intermetal coupling. Some of the structural changes have opposite effects on H ab, the effective coupling parameter, and K c, the comproportionation constant. The data for the mixed-valence cations are most consistent with electronic delocalization.