THEORETICAL INVESTIGATIONS OF CLASSICAL AND NONCLASSICAL STRUCTURES OF MH7L2 POLYHYDRIDE COMPLEXES OF RE AND TC

The geometries, energies, and electronic properties of ReH7L2 and TcH7L2 complexes have been calculated using ab initio theoretical techniques including relativistic effective core potentials for Re and Tc. Classical nine-coordinate MH7L2 polyhydride forms and nonclassical M(H2)H5L2 and M(H2)2H3L2 structures containing molecular dihydrogen ligands have been investigated. Molecular geometries were optimized at the Hartree-Fock SCF level with L = PH3 model ligands for the various structures of Re and Tc. Electron correlation effects were treated using configuration interaction (CI) techniques at the optimized geometries. Results are compared with experimental structural determination and with solution NMR studies of Re complexes. The classical structure is found to be more stable than the eight-coordinate structure by 2-4 kcal/mol for the case of Re, depending on the level of calculation, and 7-10 kcal/mol lower than seven-coordinate forms. The nonclassical seven-coordinate structures, by contrast, are predicted to be 2-12 kcal/mol lower in energy than the classical form for Tc depending on the basis set and treatment of electron correlation effects. Although the small differences in energy between these forms make predictions of the most stable form of MH7P2 species with bulkier phosphine ligands in solution more difficult, these results indicate a greater tendency for Tc to display nonclassical structures relative to Re. The positive ions of ReH7L2 and Re(H2)H5L2 are also investigated, with the IP of the latter form calculated to be 2.1 eV lower in energy than the classical structure.