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

<p>A theoretical study of the complexes formed between ZH<sub>2</sub>X (Z = P, As, Sb, Bi; X = F, Cl, Br, CN, NC, OH, NH<sub>2</sub>) and an <i>N</i>-heterocyclic carbene (imidazol-2-ylidene) is carried out by means of <i>ab initio</i> 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 ZH<sub>2</sub>X. 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 ZH<sub>2</sub>Br forms the strongest complex, followed by ZH<sub>2</sub>Cl and ZH<sub>2</sub>F. On the other hand, the binding energy in the halogenated ZH<sub>2</sub>X complexes follows the reverse ranking expected based on the values of the σ-hole of the isolated ZH<sub>2</sub>X 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.</p> <p></p>