Structures and Chemical Bonding in Antimony(III) Bromide Complexes with Pyridine.

Weakly or "partially" bonded molecules are an important link between the chemical and van der Waals interactions. Molecular structures of six new SbBr -Py complexes in the solid state have been determined by single-crystal X-ray diffraction analysis. In all complexes all Sb atoms adopt a pseudo-octahedral coordination geometry which is completed by additional Sb⋅⋅⋅Br contacts shorter than the sum of the van der Waals radii, with Br-Sb⋅⋅⋅Br angles close to 180°. To reveal the nature of Sb-Br and Sb-N interactions, the DFT calculations were performed followed by the analysis of the electrostatic potentials, the orbital interactions and the topological analysis. Based on Natural Bond Orbital (NBO) analysis, the Sb-Br interactions range from the covalent bonds to the pnictogen bonds. A simple structural parameter, non-covalence criterion (NCC) is defined as a ratio of the atom-atom distance to the linear combination of sums of covalent and van der Waals radii. NCC correlates with E(2) values for Sb-N, Sb-Cl and Sb-Br bonds, and appears to be useful criterion for a preliminary evaluation of the bonding situation.