AN AB-INITIO MOLECULAR-ORBITAL STUDY OF THE [OS(PR3)3H4] SYSTEM - PEEKING INTO THE PECULIARITIES OF 7-COORDINATION

General features of seven-coordination are investigated through an ab initio study of the [Os(PR3)3H4] complex. After an assessment of the v alidity of different methods for the description of the generic [Os(PR 3)3 ''H-4''] system, the characteristics of the two different sites of the preferred coordination polyhedron, a pentagonal bipyramid, are ex amined. It is found that formally neutral ligands prefer axial sites, while formally negative ligands prefer equatorial sites. The energy de composition analysis (EDA) explains this result as a consequence of th e anisotropy in electron distribution associated with the metal nonbon ding d orbitals.