On the Importance of σ-Hole Interactions in Crystal Structures

Elements from groups 14-18 and periods 3-6 commonly behave as Lewis acids, which are involved in directional noncovalent interactions (NCI) with electron-rich species (lone pair donors), pi systems (aromatic rings, triple and double bonds) as well as nonnucleophilic anions (BF4-, PF6-, ClO4-, etc.). Moreover, elements of groups 15 to 17 are also able to act as Lewis bases (from one to three available lone pairs, respectively), thus presenting a dual character. These emerging NCIs where the main group element behaves as Lewis base, belong to the sigma-hole family of interactions. Particularly (i) tetrel bonding for elements belonging to group 14, (ii) pnictogen bonding for group 15, (iii) chalcogen bonding for group 16, (iv) halogen bonding for group 17, and (v) noble gas bondings for group 18. In general, sigma-hole interactions exhibit different features when moving along the same group (offering larger and more positive sigma-holes) or the same row (presenting a different number of available sigma-holes and directionality) of the periodic table. This is illustrated in this review by using several examples retrieved from the Cambridge Structural Database (CSD), especially focused on sigma-hole interactions, complemented with molecular electrostatic potential surfaces of model systems.

Automated summary

Main group elements can act as both Lewis acids and bases in noncovalent interactions, presenting a dual character. These emerging NCIs involving sigma-hole interactions show different features when moving along the same group or row of the periodic table.