Properties of complexes H2C═(X)P:PXH2, for X = F, Cl, OH, CN, NC, CCH, H, CH3, and BH2: P···P pnicogen bonding at σ-holes and π-holes.

Ab initio MP2/aug'-cc-pVTZ calculations have been carried out on complexes H2C═(X)P:PXH2, for X = F, Cl, OH, CN, NC, CCH, H, CH3, and BH2. Three sets of complexes have been found on the potential surfaces. Conformation A complexes have A-P···P-A approaching linearity, with A the atom of X directly bonded to P. Conformation B complexes have A-P···P linear, but the P···P═C orientation of H2C═PX may differ significantly from linearity. Conformation C complexes are unique, since the pnicogen bond involves π-electron donation and acceptance by H2C═PX. The order of binding energies of the three conformations of H2C═(X)P:PXH2 is C > A > B, with two exceptions. Although the binding energies of conformation C complexes tend to be greater than the corresponding conformation A complexes, intermolecular distances in conformation C tend to be longer than those in conformation A. Charge transfer stabilizes H2C═(X)P:PXH2 complexes. The preferred direction of charge transfer is from H2C═PX to PXH2. In conformations A and B, charge transfer occurs from a P lone pair on one molecule to an antibonding σ* orbital on the other. However, in conformation C, charge transfer occurs from the π orbital of H2C═PX to the σ*P-A orbital of PXH2, and from the lone pair on P of PXH2 through the π-hole to the π*P═C orbital of H2C═PX. Changes in charges on P upon complexation do not correlate with changes in (31)P chemical shieldings. Computed EOM-CCSD spin-spin coupling constants correlate with P-P distances. At each distance, the ordering of (1p)J(P-P) is A > B > C. Binding energies and spin-spin coupling constants of conformation A complexes of (PH2X)2, H2C═(X)P:PXH2, and (H2C═PX)2 with A-P···P-A approaching linearity have been compared. For complexes with the more electronegative substituents, binding energies are ordered (PH2X)2 > H2C═(X)P:PXH2 > (H2C═PX)2, while the order is reversed for complexes formed from the more electropositive substituents. A plot of ΔE(PH2X)2/ΔE(H2C═PX)2 versus ΔE[H2C═(X)P:PXH2]/ΔE(H2C═PX)2 indicates that there is a systematic relationship among the stabilities of these complexes. Complexes (PH2X)2 tend to have larger spin-spin coupling constants and shorter P-P distances than H2C═(X)P:PXH2, which in turn have larger coupling constants and shorter P-P distances than (H2C═PX)2, although there is some overlap. Complexes having similar P-P distances have similar values of (1p)J(P-P).