Using (FH)2 and (FH)3 to Bridge the σ-Hole and the Lone Pair at P in Complexes with H2XP, for X=CH3, OH, H, CCH, F, Cl, NC, and CN
Ab initio MP2/aug′-cc-pVTZ calculations are used to investigate the binary complexes H2XP:HF, the ternary complexes H2XP:(FH)2, and the quaternary complexes H2XP:(FH)3, for X=CH3, OH, H, CCH, F, Cl, NC, and CN. Hydrogen-bonded (HB) binary complexes are formed between all H2XP molecules and FH, but only H2FP, H2ClP, and H2(NC)P form pnicogen-bonded (ZB) complexes with FH. Ternary complexes with (FH)2 are stabilized by F−H⋅⋅⋅P and F−H⋅⋅⋅F hydrogen bonds and F⋅⋅⋅P pnicogen bonds, except for H2(CH3)P:(FH)2 and H3P:(FH)2, which do not have pnicogen bonds. All quaternary complexes H2XP:(FH)3 are stabilized by both F−H⋅⋅⋅P and F−H⋅⋅⋅F hydrogen bonds and P⋅⋅⋅F pnicogen bonds. Thus, (FH)2 with two exceptions, and (FH)3 can bridge the σ-hole and the lone pair at P in these complexes. The binding energies of H2XP:(FH)3complexes are significantly greater than the binding energies of H2XP:(FH)2 complexes, and nonadditivities are synergistic in both series. Charge transfer occurs across all intermolecular bonds from the lone-pair donor atom to an antibonding σ* orbital of the acceptor molecule, and stabilizes these complexes. Charge-transfer energies across the pnicogen bond correlate with the intermolecular P−F distance, while charge-transfer energies across F−H⋅⋅⋅P and F−H⋅⋅⋅F hydrogen bonds correlate with the distance between the lone-pair donor atom and the hydrogen-bonded H atom. In binary and quaternary complexes, charge transfer energies also correlate with the distance between the electron-donor atom and the hydrogen-bonded F atom. EOM-CCSD spin-spin coupling constants 2hJ(F–P) across F−H⋅⋅⋅P hydrogen bonds, and 1pJ(P–F) across pnicogen bonds in binary, ternary, and quaternary complexes exhibit strong correlations with the corresponding intermolecular distances. Hydrogen bonds are better transmitters of F–P coupling data than pnicogen bonds, despite the longer F⋅⋅⋅P distances in F−H⋅⋅⋅P hydrogen bonds compared to P⋅⋅⋅F pnicogen bonds. There is a correlation between the two bond coupling constants 2hJ(F–F) in the quaternary complexes and the corresponding intermolecular distances, but not in the ternary complexes, a reflection of the distorted geometries of the bridging dimers in ternary complexes.