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

MP2/aug-cc-pVTZ calculations were performed on triel trifluorides, ZF(3) (Z=B, Al, Ga, In, Tl), and their complexes with N-2 and HCN species, which are acting as Lewis bases. Interaction of the ZF(3) trigonal structure with a single HCN or N-2 ligand may lead to a tetrahedral structure with the tetravalent triel atom, whereas interaction of ZF(3) with two ligands results in a trigonal bipyramidal structure with a pentacoordinate Z atom. Consequently, the Z atom in ZF(3) compounds suffers from electron deficiency (hypovalency), it obeys the octet rule in ZF(3) -NCH and ZF(3) -N-2 moieties, and it may be considered as a hypervalent center in complexes of ZF(3) 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 center dot center dot center dot 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.