Comparison of halogen bonds in M-X⋯N contacts (M = C, Si, Ge and X = Cl, Br)

The halogen bonds (XB) formed between some Si-X- and Ge-X- (X is Cl and Br) containing molecules and NCH (as a Lewis base) have been investigated and compared with C-X⋯N halogen bond. Although, in all cases, the existence of a positive electrostatic potential (σ-hole) along the extension of M-X was responsible for halogen bond formation, multipole expansion of electrostatic potential reveals that these positive potentials originate from different atomic multipole moments. Indeed, in addition to the monopole moment of M atoms, the quadrupole moment of X in C-X molecules, the dipole moment of the halogen in Si-X molecules and both the dipole and quadrupole moments of X in Ge-X molecules are mainly responsible for the existence of the σ-hole along the extension of the M-X bond. From a different point of view, the distribution of the Laplacian of electron density shows that all the studied M-X⋯N halogen bonds can be regarded as "lump-hole" interactions; a region of charge depletion (hole) in the valence shell charge concentration (VSCC) of the halogen atom interacts with a region of charge concentration (lump) in the VSCC of nitrogen and forms a halogen bond. On the other hand, interacting quantum atoms (IQA) analysis of atomic energies indicates that, in contrast to C-X⋯N contacts, in which the interaction between halogen and nitrogen is attractive, there is a net repulsive interaction between X and N in Si-X⋯N and Ge-X⋯N complexes. Indeed, the attraction between Si/Ge and nitrogen is mainly responsible for the formation of these halogen bonds.