Halogen bond and its counterparts: Bent's rule explains the formation of nonbonding interactions.

Ab Initio calculations were carried out on complexes of F(3)CCl. The Cl center may act as the Lewis acid as well as the Lewis base. Hence various interactions are analyzed for several complexes of F(3)CCl: halogen bond, dihalogen bond, halogen-hydride bond, hydrogen bond, and others. It was found that Lewis acid-Lewis base interactions lead to the electron charge redistribution being in agreement with Bent's rule. This rule explains the formation of nonbonding interactions. For example, if the C-Cl bond acts as the Lewis acid thus the increase of the s-character in C-orbital of C-Cl is observed as a result of complexation. However, if the C-Cl bond acts as the Lewis base thus the mentioned above s-character decreases. Numerous analogies between the halogen bond and the hydrogen bond are analyzed and various relationships between energetic, geometrical and the natural bond orbitals method (NBO) parameters are shown.