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

The butadienediyl-bridged complexes [(η 5 -C 5 R 5 )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 [(η 5 -C 5 H 5 )Fe(dppm)] 2 (μ-CH CHCH CH), [(η 5 -C 5 Me 5 )Fe(dppe)] 2 (μ-C⋮CC⋮C), and [(η 5 -C 5 Me 5 )Fe(dppe)] 2 (μ-C(OMe) CHCH C(OMe)) have allowed for systematic evaluation of several structural variations (C 5 H 5 vs C 5 Me 5, 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.