Bonding and33S NMR chemical shielding in the thiophosphoryl group

B3LYP and MP2 calculations at the 6-311 + G(nd,p) level (with n = 2 for second-row elements and n = 1 otherwise) were carried out using the atoms-in-molecules (AIM) approach to characterize the thiophosphoryl bond. A series of R(3)PS molecules were studied and compared with the corresponding R(3)PO systems. As with the phosphoryl bond, one cannot distinguish the thiophosphoryl bond from a standard P=S double bond by comparing bond distances. On the basis of the P=S bond in HP=S having a reference bond order of 2.0, the thiophosphoryl bond has a bond order of about 1.6. Examination of localized orbitals show that this bond is less polar than the corresponding PO bond. As with the phosphoryl bond, what sets the thiophosphoryl bond apart is the high degree of back-bonding that contributes to the delocalization index (and covalent bond order) and is the basis for its stronger than single bond character and short bond distance. (33)S NMR shielding calculations were also carried out and, in a few instances, compared directly with newly determined experimental shieldings.