Competition between sigma and Pi holes on the same atom

Abstract

Quantum chemical calculations compare the ability of σ and π-holes on the same atom to engage in a noncovalent bond. The first series of Lewis acids are the hypervalent XR₃ series where X refers to a central halogen atom. These molecules adopt a T-shape with a π-hole above the molecular plane and a σ-hole along the extension of the vertical of the T. A similar T-shape is characteristic of the AeX₂Y series where Ae is an aerogen/noble gas atom and Y is a chalcogen O or S. In all of these cases the σ-hole is deeper and forms a stronger bond with a NH₃ base. Also studied is a set of MX₂ and MX₃ units where M refers to a transition metal atom. Despite a variety of molecular shapes, encompassing planar trigonal, T-shape, bent, and linear, there is only one sort of hole present on each M atom, either σ or π. Other sorts of molecules also contain only a π-hole although the shape allows the possibility in principle of a σ-hole as well.