Boron and other Triel Lewis Acid Centers: From Hypovalency to Hypervalency

Abstract

MP2/aug-cc-pVTZ calculations were performed on triel trifluorides, ZF3 (Z=B, Al, Ga, In, Tl), and their complexes with N2 and HCN species, which are acting as Lewis bases. Interaction of the ZF3 trigonal structure with a single HCN or N2 ligand may lead to a tetrahedral structure with the tetravalent triel atom, whereas interaction of ZF3 with two ligands results in a trigonal bipyramidal structure with a pentacoordinate Z atom. Consequently, the Z atom in ZF3 compounds suffers from electron deficiency (hypovalency), it obeys the octet rule in ZF3-NCH and ZF3-N2 moieties, and it may be considered as a hypervalent center in complexes of ZF3 with two ligands. Much weaker interactions are observed for the boron complexes than for the other triel systems. This is because of the lower acidity of the B center relative to that of the other triel centers, and it may be the result of a stronger backbonding effect for BF3 than for the other triel trifluorides. Interactions in aluminum complexes are characterized by meaningful electrostatic contributions, whereas for gallium complexes, the most important electron charge shift is observed as a result of complexation. Analysis of the geometry of the triel complexes and of the Z⋅⋅⋅N interactions (triel bonds) in these complexes is based on ab initio calculations as well as on the quantum theory of atoms in molecules and the natural bond orbitals method.

Abnormal synergistic effects between Lewis acid–base interaction and halogen bond in F3B···NCX···NCM

“…Very recently, Grabowski performed a detailed analysis for the complexes of triel trifluorides with hydrogen cyanide and named these interactions as triel bonds [24,25]. Complexes F 3 B···NCX···NCM have C 3v symmetry, and BF 3 molecule exhibits a significant change in geometry from a planar structure in the monomer to an umbrella one in the complex ( Figure 1).…”

Two faces of triel bonds in boron trihalide complexes

“…Hence, boron often interacts strongly with Lewis bases complementing the octet structure by their electrons . These are triel bonds that are defined as interactions where the element of the 13th group plays a role of the Lewis acid center …”

“…π‐holes are also regions of the depletion of the electron density that results in the positive EP; they are situated perpendicularly to the plane of molecule or molecular fragment . The aforementioned triel bonds are classified as the π‐hole bonds . The triel bonds between boron and amines are often a preliminary stage of the reaction of releasing of molecular hydrogen .…”

π‐Hole Bonds: Boron and Aluminum Lewis Acid Centers

“…The triel trifluorides were analyzed recently and the nucleophilic attack of the Lewis base in the direction perpendicular to the plane of the triel molecule was considered 30. The nitrogen atoms of HCN and N 2 were taken into account as the Lewis base centers, and in the case of these linear molecules the ZF 3 ‐NCH and ZF 3 ‐N 2 complexes characterized by a C 3 v symmetry were formed.…”