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(NH2)(2), C(OH)(2), and cyclic C(OCH)(2) [OHC] with H2XP molecules, with X=F, Cl, NC, OH, CH3, CN, CCH, and H. The H2XP:C(NH2)(2) and H2XP:C(OH)(2) complexes have C-s 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 H2XP:C(NH2)(2) and H2XP:C(OH)(2) complexes arise only when X is one of the more electronegative substituents. Coplanar H2XP:C(NH2)(2) complexes form when X is one of the more electropositive substituents, whereas coplanar H2XP:C(OH)(2) complexes exist for all X. H2XP:C(NH2)(2) and H2XP:C(OH)(2) are stabilized by covalent P-C bonds or P...C pnicogen bonds, but co-planar H-2(CH3)P:C(OH)(2) and H3P:C(OH)(2) are stabilized by O-H...P hydrogen bonds. The H2XP:OHC complexes have non-coplanar structures that are also stabilized by P-C covalent bonds or pnicogen bonds. The H-2(CH3)P:OHC and H3P:OHC complexes in which the symmetry plane of the complex and the local symmetry plane of the carbene are perpendicular are stabilized by P...pi bonds with P acting as the electron-pair donor to the OHC system. The H2XP:C(NH2)(2), H2XP:C(OH)(2), and H2XP: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.