Structure of (2,2′-biallylene)hexacarbonyldiiron

Article abstract of DOI:10.1021/om00004a062

The crystal structures of the title compound [C₁₂H₈Fe₂O₆, orthorhombic, Iba2 (No. 45), a = 22.762(15) ?, b = 12.229(11) ?, c = 9.615(8) ?, Z = 8] and its triphenylphosphine substitution product [C₂₉H₁₃Fe₂O₅P, triclinic, P1 (No. 2), a = 10.7506(20) ?, b = 13.3657(23) ?, c = 9.5827(16) ?, α = 92.803(11), β = 94.022(11), γ = 73.444(14)°, Z = 2] are reported. They show systematic deviations from C_2v, symmetry, with the biallylene ligand being nonplanar due to pyramidalization and internal rotation about its central C-C bond; also, the Fe-Fe and central C-C axes are not parallel, being twisted by 11° relative to each other. Low-temperature NMR studies at -95° indicate similar distortions in solution, with very low activation energy for interconversion of equivalent lower-symmetry forms. In the crystal structures, these distortions of the biallylene ligand are accompanied by relief of eclipsing interactions among the equatorial carbonyl ligands of the Fe₂(CO)₅L fragment. Pyramidalization is found in all published (2,2′-biallylene)diiron structures, but the twisting and rotation are not, consistent with the small energy change which accompanies these latter deformations.