Can HN=NH, FN=NH, or HN=CHOH bridge the σ-hole and the lone pair at P in binary complexes with H2XP, for X = F, Cl, NC, OH, CN, CCH, CH3, and H?
Ab initio MP2/aug′-cc-pVTZ calculations have been carried out to investigate the properties of complexes formed between H2XP, for X = F, Cl, NC, OH, CN, CCH, CH3, and H, and the possible bridging molecules HNNH, FNNH, and HNCHOH. H2XP:HNNH and H2XP:FNNH complexes are stabilized by P⋯N pnicogen bonds, except for H2(CH3)P:FNNH and H3P:FNNH which are stabilized by N–H⋯P hydrogen bonds. H2XP:HNCHOH complexes are stabilized by P⋯N pnicogen bonds and nonlinear O–H⋯P hydrogen bonds. For a fixed H2XP molecule, binding energies decrease in the order HNCHOH > HNNH > FNNH, except for the binding energies of H2(CH3)P and H3P with HNNH and FNNH. Binding energies of complexes with HNCHOH and HNNH increase as the P–N1 distance decreases, but binding energies of complexes with FNNH show little dependence on this distance. The large binding energies of H2XP:HNCHOH complexes arise from a cooperative effect involving electron-pair acceptance by P to form a pnicogen bond, and electron-pair donation by P to form a hydrogen bond. The dominant charge-transfer interaction in these complexes involves electron-pair donation by N across the pnicogen bond, except for complexes in which X is one of the more electropositive substituents, CCH, CH3, and H. For these, lone-pair donation by P across the hydrogen bond dominates. AIM and NBO data for these complexes are consistent with their bonding characteristics, showing molecular graphs with bond critical points and charge-transfer interactions associated with hydrogen and pnicogen bonds. EOM-CCSD spin–spin coupling constants 1pJ(P–N) across the pnicogen bond for each series of complexes correlate with the P–N distance. In contrast, 2hJ(O–P) values for complexes H2XP:HNCHOH do not correlate with the O–P distance, a consequence of the nonlinearity of these hydrogen bonds.