Diphosphine Mobility at a Binuclear Metal Center: A Concerted Double Trigonal-Twist in Bis(dithiolate) Complexes [M₂(CO)₄(μ-dppm){μ-S(CH₂)_nS}] (M = Fe, Ru; n = 2, 3)

Heating [M₂(CO)₆{μ-S(CH₂)_nS}] (M = Fe, Ru; n = 2 (edt), 3 (pdt)) with bis(diphenylphosphino)methane (dppm) in toluene affords the bridged-diphosphine complexes [M₂(CO)₄(μ-dppm){μ-S(CH₂)_nS}]. At room temperature, all show two separate environments for the methylene protons of the diphosphine ligand, while at higher temperatures these coalesce to a single peak. This behavior, which interconverts the two sulfur atoms, is ascribed to a concerted double trigonal-twist of the M(CO)₂P moieties. No such fluxional behavior was observed for the nonlinked dithiolate complexes [Fe₂(CO)₄(μ-dppm)(μ-SR)₂] (R = Me, Ph, p-tolyl). The X-ray structures of [M₂(CO)₄(μ-dppm)(μ-edt)] (M = Fe, Ru) and [Fe₂(CO)₄(μ-dppm)(μ-SMe)₂] are presented in order to compare them to the previously reported [M₂(CO)₄(μ-dppm)(μ-pdt)].