Theoretical calculations on the electron density and electronic structure of chromium benzene tricarbonyl, triply bonded dimolybdenum, and quadruply bonded dichromium compounds

Abstract

Hartree-Fock-Roothaan (HFR) and generalized molecular orbital with configuration interaction (GMO-CI) calculations are reported for several group VIB transition metal compounds. These calculations demonstrate the feasability of computational chemistry to help examine and explain problems that are difficult or impossible to examine experimentally. HFR calculations were performed on Cr(C(,6)H(,6))(CO)(,3) to determine its electron density distribution. These results were compared to two experimental electron density studies. Comparison of the results shows that the experimental studies still contain problems, particularly in the region right around the chromium atom and the carbonyl ligands. The theoretical deformation density maps help interpret the electron density changes in simple chemical terms. GMO-CI calculations were used to calculate the dissociation energy of the molybdenum-to-molybdenum triple bond, by comparing it to the dissociation energies of a series of Group VA diatomics, which also contain triple bonds. The results predict a triple bond energy of 284 kJ mol('-1) in Mo(,2)H(,6). The results also show the importance of including the differential correlation energy when calculating the dissociation energies in transition metal complexes. GMO-CI calculations were also used to predict the equilibrium bond length for Cr(,2). The predicted value of 1.73 (ANGSTROM) agrees well with the experimental value of 1.68 (ANGSTROM). Finally GMO-CI calculations were used to examine bridging ligand effects in quadruply bonded dichromium(II) compounds. The results show that the nature of the bridging ligand strongly affects the electronic structure of the quadruple bond. Bridging ligands with donor atoms other than oxygen donate more electron density to the Cr atoms thereby expanding the Cr 3d orbitals and causing better overlap of Cr-Cr orbitals and a stronger quadruple bond.