σ-Hole Bond vs π-Hole Bond: A Comparison Based on Halogen Bond

The sigma-hole and pi-hole are the regions with positive surface electrostatic potential on the molecule entity; the former specifically refers to the positive region of a molecular entity along extension of the Y-Ge/P/Se/X covalent sigma-bond (Y = electron-rich group; Ge/P/Se/X = Groups IV-VII), while the latter refers to the positive region in the direction perpendicular to the sigma-framework of the molecular entity. The directional noncovalent interactions between the sigma-hole or pi-hole and the negative or electron-rich sites are named sigma-hole bond or pi-hole bond, respectively. The contributions from electrostatic, charge transfer, and other terms or Coulombic interaction to the pi-hole bond and sigma-hole bond were reviewed first followed by a brief discussion on the interplay between the sigma-hole bond and the pi-hole bond as well as application of the two types of noncovalent interactions in the field of anion recognition. It is expected that this review could stimulate further development of the sigma-hole bond and pi-hole bond in theoretical exploration and practical application in the future.