AB-INITIO MOLECULAR-ORBITAL CHARACTERIZATION OF THE [OS(PR3)3H5]+ COMPLEX

The problem of the structural characterization of [Os(PR3)3''H-5'']+co mplexes, which is experimentally unsolved, is considered from the theo retical point of view with a systematic study of the model system [Os( PH3)3H5]+ with the ab initio MO methodology. A fair selection of more than 20 possible isomers, including [Os(PH3)3H(H2)2]+, [Os(PH3)3H3(H2) ]+, and [Os(PH3)3H5]+ Species, is considered, and geometry optimizatio n at the correlated MP2 level is extensively used. The most stable iso mer is an eight-coordinate pentahydride complex [Os(PR3)3H5]+, its coo rdination polyhedron being a dodecahedron with the three phosphine lig ands in B sites. Results are compared with available experimental data on other similar species and current general ideas on high coordinati on number complexes.