Expansion of the sigma-hole concept.

The term "sigma-hole" originally referred to the electron-deficient outer lobe of a half-filled p (or nearly p) orbital involved in forming a covalent bond. If the electron deficiency is sufficient, there can result a region of positive electrostatic potential which can interact attractively (noncovalently) with negative sites on other molecules (sigma-hole bonding). The interaction is highly directional, along the extension of the covalent bond giving rise to the sigma-hole. Sigma-hole bonding has been observed, experimentally and computationally, for many covalently-bonded atoms of Groups V-VII. The positive character of the sigma-hole increases in going from the lighter to the heavier (more polarizable) atoms within a Group, and as the remainder of the molecule becomes more electron-withdrawing. In this paper, we show computationally that significantly positive sigma-holes, and subsequent noncovalent interactions, can also occur for atoms of Group IV. This observation, together with analogous ones for the molecules (H3C)2SO, (H3C)2SO2 and Cl3PO, demonstrates a need to expand the interpretation of the origins of sigma-holes: (1) While the bonding orbital does require considerable p character, in view of the well-established highly directional nature of sigma-hole bonding, a sizeable s contribution is not precluded. (2) It is possible for the bonding orbital to be doubly-occupied and forming a coordinate covalent bond.