ON THE ELECTRONIC-STRUCTURES OF THE METAPHOSPHATE (METAPHOSPHITE) ANIONS PX(3)((-))(PX(2)((-))), X=CH2, SIH2, NH, PH, O, S

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) geome tries is operative in the metaphosphate anions. It causes pyramidaliza tion of the terminal groups (for X = CH2, SiH2) and the central phosph orus atom (for X = SiH2). The various types of symmetry distortions ar e accompanied by a weak alpha+pi bonding of the terminal ligands towar ds the central phosphorus atom. This matter is evidenced by an analysi s of corresponding group transfer reactions. In contrast, for the case s X = NH, PH, O, S, planar structures (C-3h, D-3h) are adopted with fo rmation of strong alpha+pi bonds of the central phosphorus towards the ligand atoms, again in accord with corresponding group transfer react ions. The vibrational analysis within the harmonic approximation revea ls a decreasing tendency for pyramidalization at the central phosphoru s atom, with increasing electronegativity of the ligand X. This is due to depletion of pi-electron density at the central atom. A silylene u nit is more strongly bound in the tris(silylene))metaphosphate anion t han in the silylene-phosphorane, according to corresponding group tran sfer reactions.