Dissection of the Origin of π-Holes and the Noncovalent Bonds in Which They Engage

Abstract

Accompanying the rapidly growing list of σ-hole bonds has come the acknowledgment of parallel sorts of noncovalent bonds which owe their stability in large part to a deficiency of electron density in the area above the molecular plane, known as a π-hole. The origins of these π-holes are probed for a wide series of molecules, comprising halogen, chalcogen, pnicogen, tetrel, aerogen, and spodium bonds. Much like in the case of their σ-hole counterparts, formation of the internal covalent π-bond in the Lewis acid molecule pulls density toward the bond midpoint and away from its extremities. This depletion of density above the central atom is amplified by an electron-withdrawing substituent. At the same time, the amplitude of the π*-orbital is enhanced in the region of the density-depleted π-hole, facilitating a better overlap with the nucleophile’s lone pair orbital and a stabilizing n → π* charge transfer. The presence of lone pairs on the central atom acts to attenuate the π-hole and shift its position somewhat, resulting in an overall weakening of the π-hole bond. There is a tendency for π-hole bonds to include a higher fraction of induction energy than σ-bonds with proportionately smaller electrostatic and dispersion components, but this distinction is less a product of the σ- or π-character and more a function of the overall bond strength.

Strongly Bound π‐Hole Tetrel Bonded Complexes between H2SiO and Substituted Pyridines. Influence of Substituents

“…This can be understood from the study of Scheiner as well. [2] As indicated before, the complex formation results in a significant deformation of the X 2 CO and X 2 SiO units and both of them loose their planar structure. It prompted us to examine the extent of deformation energy (DFE) required for the monomers to attain the structures in the complex.…”

“…It should be noted here that 1(r c ) values in the similar range have been predicted for other TB, chalcogen bond and pnicogen bond as well. [2] Interestingly, these 1(r c ) values are comparatively lower than the values (ranging between 0.1421 to 0.1265 a. u.)…”

“…26 [26,39,45] Moreover, the V s,max value for H 2 SiO is much higher than the reported V s,max values of SO 3 (288.8 kJ/mol), SO 2 (180.2 kJ/mol), SF 2 (185.5 kJ/mol), CS 2 (23.1 kJ/mol on C atom) and OCS (86.1 kJ/mol on C atom). [40][41][42] Since the presence and the strength of π-hole are often found to be associated with the depletion of electron density in the region, [2] it is expected that there is an electron deficient zone around the Si-atom of H 2 SiO and perpendicular to the ChemPhysChem molecular plane. The large difference in electronegativity between the Si and O atoms would polarize the SiÀ O bond, with accumulation of electron density towards the O-atom.…”

“…Although most of the works dealt with hydrogen bonds, the past decade has seen a shift, and we find a renaissance in the studies on σ-hole and π-hole based non-covalent interactions. The σ-hole [1] is a region of positive electrostatic potential generated on an atom along the extension of the bond, whereas π-hole [2,3] is also an electron deficient region existing above a planar molecule or planar groups (of a molecule), such as NO 2 , phenyl and carbonyl. These σ/π-hole based bonds (or interactions) are named according to their appearance in various groups of the periodic table i. e. halogen bond for group 17 elements, [4] chalcogen bond for group 16, [5] tetrel bond (TB) for group 14 elements, [6] etc.…”

The Bifurcated σ-Hole···σ-Hole Stacking Interactions

“…The case is similar for the interactions between two negative σ-holes; for example, in a recent study, Frontera and coworkers reported the σ-hole•••σ-hole stacking interaction between two I 3 anions in the crystal structure of a hetero-trinuclear cobalt(III)/potassium complex [9]. Very recently the term of σ-hole•••σ-hole stacking interaction has also been discussed in other papers [10,11]. The word "stacking" in σ-hole•••σ-hole stacking interaction has three-fold implications: (i) the net attractive interaction occurs between two nucleophilic regions or between two electrophilic regions; (ii) the two σ bonds associated with two σ-holes are linear or nearly linear; (iii) the σ-hole•••σ-hole stacking interaction is dispersion-dominated as it is for the π•••π stacking interaction [7,8].…”