PYRAMIDALITY AND METAL-METAL MULTIPLE BONDING - STRUCTURAL CORRELATIONS AND THEORETICAL-STUDY

An analysis of structural data for more than 350 dinuclear compounds o f the types M2X8, M2X8L, and M2X8L2 (M = Cr, Mo, W, Tc, Re, Os, or Rh) , with a multiple bond between the transition-metal atoms M, shows the existence of a clear correlation between the average pyramidality ang le (M-M-X, or alpha) and the metal-metal bond distance. Although the p resence of axial ligands (L) favors elongation of the M-M bond, this e ffect is also intimately connected with changes in the pyramidality an gle. The metal-metal bond distance also varies with the internal rotat ion angle, being shorter for the eclipsed conformation in quadruply bo nded complexes, while the opposite trend appears for the triply bonded ones. These regularities may help in understanding some apparent inco nsistencies previously found in bond order-bond length relationships i n multiply bonded metal systems. Electronic structure calculations car ried out on simplified model compounds at different levels of sophisti cation (extended Huckel, Hartree-Fock SCF, and multiconfiguration CASS CF) nicely reproduce the experimental trends and allow us to explain t he effect of the different structural parameters on the M-M bond dista nces.