π-Hole Bonds: Boron and Aluminum Lewis Acid Centers

MP2/aug-cc-pVTZ calculations were performed on complexes of boron and aluminum trihydrides and trihalides with hydrogen cyanide (ZH(3)-NCH and ZX(3)-NCH; Z=B, Al; X=F, Cl). The complexes are linked through the BN and AlN interactions, which are named as triel bonds and which are classified as -hole bonds. It was found that they possess numerous characteristics of typical covalent bonds, since they are ruled mainly by processes of the electron charge shift from the Lewis base to the Lewis acid unit. Other configurations of the ZH(3)-NCH and ZX(3)-NCH complexes linked by the dihydrogen, hydrogen, and halogen bonds were found. However, these interactions are much weaker than the corresponding -hole bonds. The quantum theory of atoms in molecules and the natural bond orbital approaches were applied to characterize the complexes and interactions analyzed. The crystal structures of triel trihydrides and triel trihalides were also analyzed for comparison with the results of calculations.