Syntheses, Structures, and Molecular-Orbital Calculations of Chromium Chevrel-Type Cluster Complexes [Cr₆E₈(PR₃)₆] (E=S, PR₃=PEt₃, PMe₃; E=Se, PR₃=PEt₃, PMe₃, PMe₂Ph)

Hexanuclear chromium cluster complexes [Cr₆E₈(PR₃)₆] (E = S, PR₃ = PEt₃, PMe₃; E = Se, PR₃ = PEt₃, PMe₃, PMe₂Ph) were prepared by the reaction of anhydrous chromium dichloride, a trialkylphosphine, and either NaSH or Na₂Se in methanol. The complexes comprised the Cr₆ octahedra with eight face-bridging chalcogen atoms and six terminal phosphine ligands. Though the overall ligand arrangements were the same, the symmetry of the central Cr₆ octahedra was O_h in PEt₃ complexes, but distorted to D_3d and D_2h in PMe₃ and PMe₂Ph complexes, respectively. Because of a weakness of the metal-metal interaction, the steric and electronic effect complementarily caused a variation in the chromium octahedra. The electronic effect was analyzed by molecular-orbital calculations using the DV-Xα method.