PUB - Publikationen an der Universität Bielefeld

The structures of the metaphosphates (metaphosphites) PX(3)((-)))(PX(2)((-))), (X = CH2, SiH2, NH, PH, O, S) are investigated by means of ab initio calculations of double-zeta quality. For the cases X = CH2 and SiH2 a second-order Jahn-Teller distortion on the planar (D-3h) geometries is operative in the metaphosphate anions. It causes pyramidalization of the terminal groups (for X = CH2, SiH2) and the central phosphorus atom (for X = SiH2). The various types of symmetry distortions are accompanied by a weak alpha+pi bonding of the terminal ligands towards the central phosphorus atom. This matter is evidenced by an analysis of corresponding group transfer reactions. In contrast, for the cases X = NH, PH, O, S, planar structures (C-3h, D-3h) are adopted with formation of strong alpha+pi bonds of the central phosphorus towards the ligand atoms, again in accord with corresponding group transfer reactions. The vibrational analysis within the harmonic approximation reveals a decreasing tendency for pyramidalization at the central phosphorus atom, with increasing electronegativity of the ligand X. This is due to depletion of pi-electron density at the central atom. A silylene unit is more strongly bound in the tris(silylene))metaphosphate anion than in the silylene-phosphorane, according to corresponding group transfer reactions.