Polar flattening

There are two general conclusions of importance. First, the distance r(Z- X), where Z is the electron donor atom/centre in the complex B- XY, is smaller than the sum of the van der Waals radii ax and ax of these atoms. This result has been shown [179] to be consistent with the conclusion that the van der Waals radius of the atom X in the dihalogen molecule X is shorter along the XY internuclear axis than it is perpendicular to it, i.e. there is a polar flattening of the atom X in the molecule XY of the type suggested by Stone et al. [180]. This result has been shown to hold for the cases XY = CI2 [174], BrCl [175], C1F [176] and IC1 [178], but not for F2, in which the F atom in the molecule appears (admittedly on the basis of only a few examples) to be more nearly spherical [177]. 

The Lewis dot formalism shows any halogen in a molecule surrounded by three electron lone pairs. An unfortunate consequence of this perspective is that it is natural to assume that these electrons are equivalent and symmetrically distributed (i.e., that the iodine is sp3 hybridized). Even simple quantum mechanical calculations, however, show that this is not the case [148]. Consider the diiodine molecule in the gas phase (Fig. 3). There is a region directly opposite the I-I sigma bond where the nucleus is poorly shielded by the atoms electron cloud. Allen described this as polar flattening , where the effective atomic radius is shorter at this point than it is perpendicular to the I-I bond [149]. Politzer and coworkers simply call it a sigma hole [150,151]. This area of positive electrostatic potential also coincides with the LUMO of the molecule (Fig. 4). 

Nyburg SC (1979) Polar flattening non-spherical effective shapes of atoms in crystals. Acta CrystA35 641-645... 

Fig. 1 Left Schematic representation of the anisotropic distribution of the electron density in monovalent halogen derivatives. The polar flattening and the different geometry of interactions involving halogen atoms are reported. Right Localization of regions of higher and lower electron density in divalent chalcogen derivatives and directionality of interactions involving the different regions...

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