AB-INITIO STUDIES OF LIGAND EFFECTS ON THE METAL-METAL BOND IN DIMOLYBDENUM COMPLEXES

Quadruply bonded dimolybdenum complexes with both symmetric and asymme tric ligand configurations were studied by ab initio quantum theory. T he ligands studied were chloride, phosphine, and hydroxy groups. The e ffects of the different ligands, with their different pi bonding prope rties, on the known metal-metal quadruple bond were examined. In two t ypes of complexes, one with weak pi-donating chloride ligands and the other with strong pi-donating hydroxy ligands, the metal-metal bonding is better described as a triple bond for some ligand configurations. Since correlation is necessary for a proper description of the quadrup le bond, the complexes were studied at the multiconfigurational self-c onsistent field (MCSCF) level of theory. The active space of orbitals was straightforward to choose for eclipsed conformations but less so f or staggered conformations. Our results agree with the experimental ph otoelectron spectroscopy data in that we have obtained good agreement in the separation between peaks. For comparisons with ultraviolet spec troscopy data, we performed some configuration interaction as well as MCSCF calculations on the low-lying excited states. The two different calculations provided similar results for the excitation energy of the state corresponding to the first important peak of the spectrum but d id not give quantitative agreement with experimental results from a co mplex with trimethylphosphine rather than phosphine ligands.