Carbenes as Electron-Pair Donors for P⋅⋅⋅C Pnicogen Bonds.

Ab initio MP2/aug'-cc-pVTZ calculations were performed on the P⋅⋅⋅C pnicogen-bonded complexes of the singlet carbene molecules C(NH ) , C(OH) , and cyclic C(OCH) [OHC] with H XP molecules, with X=F, Cl, NC, OH, CH , CN, CCH, and H. The H XP:C(NH ) and H XP:C(OH) complexes have C symmetry and two different structures: one in which the symmetry plane of the complex and the local symmetry plane of the carbene are non-coplanar, and the other in which they are coplanar. The non-coplanar H XP:C(NH ) and H XP:C(OH) complexes arise only when X is one of the more electronegative substituents. Coplanar H XP:C(NH ) complexes form when X is one of the more electropositive substituents, whereas coplanar H XP:C(OH) complexes exist for all X. H XP:C(NH ) and H XP:C(OH) are stabilized by covalent P-C bonds or P⋅⋅⋅C pnicogen bonds, but co-planar H (CH )P:C(OH) and H P:C(OH) are stabilized by O-H⋅⋅⋅P hydrogen bonds. The H XP:OHC complexes have non-coplanar structures that are also stabilized by P-C covalent bonds or pnicogen bonds. The H (CH )P:OHC and H P:OHC complexes in which the symmetry plane of the complex and the local symmetry plane of the carbene are perpendicular are stabilized by P⋅⋅⋅π bonds with P acting as the electron-pair donor to the OHC π system. The H XP:C(NH ) , H XP:C(OH) , and H XP:OHC complexes are described in terms of their binding energies, charge-transfer energies, electron density properties, and equation-of-motion coupled cluster singles and doubles spin-spin coupling constants.