Characterizing Complexes with Pnicogen Bonds Involving sp2 Hybridized Phosphorus Atoms: (H2C=PX)2 with X = F, Cl, OH, CN, NC, CCH, H, CH3, and BH2

Ab initio MP2/aug'-cc-pVTZ searches of the potential surfaces of (H2C=PX)(2) complexes, with X = F, Cl, OH, CN, NC, CCH, H, CH3, and BH2, have been carried out to identify and characterize the properties of complexes with P center dot center dot center dot P pnicogen bonds. All (H2C=PX)(2) form equilibrium conformation A dimers with C-2h symmetry in which A-P center dot center dot center dot P-A approaches a linear alignment, with A the atom of X directly bonded to P. Conformation A dimers containing the more electronegative substituents are stabilized by a P center dot center dot center dot P pnicogen bond, have shorter P-P distances, and have binding energies which correlate with the P-P distance. Dimers stabilized by a P center dot center dot center dot P pnicogen bond and two P center dot center dot center dot H-b interactions consist of those with the more electropositive substituents, have shorter P-H-b distances, and have binding energies which are too high for their P-P distances. Conformation A complexes with P center dot center dot center dot H-b interactions in addition to the P center dot center dot center dot P bond are more stable than the corresponding (PH2X)(2) complexes, while with only one exception, complexes stabilized by only a P center dot center dot center dot P bond are less stable than the corresponding (PH2X)(2) complexes. In the region of the potential surfaces with C-P center dot center dot center dot P-C approaching linearity (conformation B), the only planar equilibrium complex is (H2C=POH)(2), which is stabilized primarily by two O-H center dot center dot center dot P hydrogen bonds. The remaining (H2C=PX)(2) complexes are not stabilized by pnicogen bonds, but by p interactions between the two H2C=PX monomers which are in parallel planes. When A-P center dot center dot center dot PC approaches linearity, two types of equilibrium structures with P center dot center dot center dot P bonds exist. Of the conformation C dimers, (H2C=POH)(2) is planar and the most stable, with a P center dot center dot center dot P pnicogen bond and an O-H center dot center dot center dot P hydrogen bond. (H2C=PH)(2) and (H2C=PCH3)(2) are also planar, and stabilized by a P center dot center dot center dot P pnicogen bond and a P center dot center dot center dot Hb interaction. The absence of a P center dot center dot center dot H-b interaction results in nonplanar C' conformations with structures in which the monomers essentially retain their symmetry plane, but the plane of one molecule is rotated about the P center dot center dot center dot P bond relative to the other. C and C' dimers are less stable than the corresponding A dimers, except for (H2C=PCH3)(2). P-31 chemical shielding patterns are consistent with the changing nature of the interactions which stabilize (H2C=PX)(2) complexes. EOM-CCSD P-31-P-31 spin-spin coupling constants increase quadratically as the PP distance decreases.