Structure and Bonding in the Trichalcogenometaphosphate-Pyridine Anions [PX3-Py]- (X = O, S, Se, Te)

The structures, bonding, NMR chemical shifts, and Lewis acid/base behavior of the trichalcogenometaphosphate-pyridine anions [PX3 - Py]- (X = O, S, Se, Te) have been evaluated theoretically on the basis of B3LYP/ECP(Se,Te) + cc-pVTZ//PM3 calculations using quasirelativistic pseudopotentials for Se and Te. The molecular and electronic structures of the trichalcogenometaphosphate-pyridine adducts [PX3 - Py]- (X = O, S, Se, Te) were calculated, and the bonding was analyzed and discussed on the basis of natural bond orbital (NBO) analyses and qualitative valence bond (VB) considerations. The (sigma(px)-LP(N)) (sigma(px)-LP*(P)) donor-acceptor interaction increases, and, consequently, the P-N bond length decreases from PO3 - Py- (weak interaction, long P-N bond) to PTe3 - Py- (strong interaction, short P-N bond). This theoretical finding is in good accord with the measured P-N bond lengths in [PSe3 - Py]- and [PTe3 - Py]-. In the gas phase, all four [PX3 - Py]- (X = O, S, Se, Te) adducts are unstable with respect to dissociation into free PX3- and pyridine.