An ab initio study on the nature of σ-hole interactions in pnicogen-bonded complexes with carbene as an electron donor

A theoretical study of the complexes formed between ZH2X (Z = P, As, Sb, Bi; X = F, Cl, Br, CN, NC, OH, NH2) and an N-heterocyclic carbene (imidazol-2-ylidene) is carried out by means of ab initio calculations. According to molecular electrostatic potential analysis, it is inferred that the divalent C atom of the carbene can act as a Lewis base with the pnicogen atom Z of ZH2X. The pnicogen bond distances (Z–C) are in the range of 2.050–2.911 for these complexes. While the Z−X bonds are longer than the corresponding Z−C bonds in the X = Cl and Br complexes, most of the Z−X bonds are short enough to suggest that they should be considered as covalent bonds which have lost some degree of covalency. For a given Z, the ZH2Br forms the strongest complex, followed by ZH2Cl and ZH2F. On the other hand, the binding energy in the halogenated ZH2X complexes follows the reverse ranking expected based on the values of the σ-hole of the isolated ZH2X monomers. The nature of the pnicogen bond interaction in these complexes is analysed by quantum theory of atoms in molecules (QTAIM) and natural bond orbital methods. According to QTAIM analysis, a partially covalent character can be attributed to the pnicogen bonds studied here.