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A series of dinuclear metal σ-acetylides of the type trans-[Cl(P−P)2MC⋮CRC⋮CM(P−P)2Cl] (M = Fe, Ru, Os; P−P = 1,2-bis(diphenylphosphino)methane (dppm), 1,2-bis(diethylphosphino)ethane (depe), 1,2-bis(dimethylphosphino)ethane (dmpe); R = 1,4-benzenediyl, 1,3-benzenediyl, 2,5-xylenediyl, 2,5-pyridinediyl, 2,5-thiophenediyl) have been formed. Electrochemistry of these complexes shows that there is a metal−metal interaction which is dependent upon the metal and the π-conjugated bridging ligand. Coulometry and optical absorbance spectroscopic studies show the presence of mixed-valence oxidized species which possess a delocalized allenylidene structure and can be classified as Robin and Day “class II” mixed-valence species. Theoretical calculations have been carried out to optimize the geometric structure of the bridging acetylide ligand and indicate that the conjugated system undergoes a structural change upon oxidation to give a quinoid-like geometry. Two mononuclear metal−allenylidene complexes, [Cl(dppm)2MCCCHPh][PF6] (10, M = Ru; 11, M = Os), have also been synthesized and an X-ray crystal structure determination on 10 undertaken. This shows a distorted-octahedral coordination about ruthenium and distinct double-bond character extending along the M−C−C−C chain, which is essentially linear. Structural and spectroscopic details on the metal allenylidenes have been compared to those of the mixed-valence oxidized dinuclear metal acetylides and show that the latter exist in a delocalized allenylidene form.

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