Halogen bonding and other σ-hole interactions: a perspective

A sigma-hole bond is a noncovalent interaction between a covalently-bonded atom of Groups IV-VII and a negative site, e.g. a lone pair of a Lewis base or an anion. It involves a region of positive electrostatic potential, labeled a sigma-hole, on the extension of one of the covalent bonds to the atom. The sigma-hole is due to the anisotropy of the atom's charge distribution. Halogen bonding is a subset of sigma-hole interactions. Their features and properties can be fully explained in terms of electrostatics and polarization plus dispersion. The strengths of the interactions generally correlate well with the magnitudes of the positive and negative electrostatic potentials of the sigma-hole and the negative site. In certain instances, however, polarizabilities must be taken into account explicitly, as the polarization of the negative site reaches a level that can be viewed as a degree of dative sharing (coordinate covalence). In the gas phase, sigma-hole interactions with neutral bases are often thermodynamically unfavorable due to the relatively large entropy loss upon complex formation.