The Nature of Triel Bonds, a Case of B and Al Centres Bonded with Electron Rich Sites

The second-order MOller-Plesset perturbation theory calculations with the aug-cc-pVTZ basis set were performed on complexes of triel species: BCl3, BH3, AlCl3, and AlH3 acting as Lewis acids through the B or Al centre with Lewis base units: NCH, N-2, NH3, and Cl- anion. These complexes are linked by triel bonds: B/Al...N or B/Al...Cl. The Quantum Theory of ' Atoms in Molecules ' approach, Natural Bond Orbital method, and the decomposition of energy of interaction were applied to characterise the latter links. The majority of complexes are connected through strong interactions possessing features of covalent bonds and characterised by short intermolecular distances, often below 2 angstrom. The BCl3 center dot center dot center dot N-2 complex is linked by a weak interaction corresponding to the B...N distance of similar to 3 angstrom. For the BCl3 center dot center dot center dot NCH complex, two configurations corresponding to local energetic minima are observed, one characterised by a short B...N distance and a strong interaction and another one characterised by a longer B center dot center dot center dot N distance and a weak triel bond. The tetrahedral triel structure is observed for complexes linked by strong triel bonds, while, for complexes connected by weak interactions, the structure is close to the trigonal pyramid, particularly observed for the BCl3...N-2 complex.