Cation–anion chalcogen bonds in ion pairs: A combined crystallographic survey and computational investigation

Cation–anion chalcogen bonds (ChBs) have been frequently observed in the X-ray crystals of the salts formed by chalcogenated cations and a diversity of anions. In this work, we first collected a huge number of crystal structures consisting of cation–anion ChBs from the Cambridge Structural Database (CSD). Then, a set of ion-pairing complexes of chalcogenated cations with four anions (Cl⁻, Br⁻, I⁻ and NO₃⁻) were selected to study such interactions found in crystal structures. Owing to the much short bonding distances and the considerably large absolute binding energies, cation–anion ChBs in the complexes are very strong, leading to some degree of covalency. These interactions are directional both in solid state and in gas phase, and heavier ChBs tend to have a higher covalent content. As revealed by energy decomposition analysis, the orbital term has an important contribution to the attraction of ChBs between cations and anions.