THEORETICAL-STUDY OF ELECTRONIC-STRUCTURES AND SPECTRA OF CHALCOGENIDO COMPLEXES OF MOLYBDENUM, TRANS-MO(Q)(2)(PH3)(4) (Q=O,S,SE,TE)

Electronic structures of the title complexes have been studied using q uantum chemical computations by different methods. It is shown that th e results of X alpha calculations agree well with expectations from cl assical ligand-field theory, but both are far from being in agreement with the results given by ab initio calculations. The HOMO in the ab i nitio Hartree-Fock molecular orbital diagrams of all these complexes i s a chalcogen p(pi) lone pair orbital rather than the metal nonbonding d(xy) orbital previously proposed. Electronic transition energies wer e calculated by CASSCF and CI methods. The results suggest that in the cases when Q = S, Se, and Te the lowest energy transitions should be those from the p(pi) lone pair orbitals to the metal-chalcogen pi orb itals. The calculated and observed electronic spectra of the oxo compl ex are in good agreement and very different from the spectra of the ot her complexes, and the lowest absorptions were accordingly assigned to transitions of different origins.