Two faces of triel bonds in boron trihalide complexes.

The N⋅⋅⋅B triel bonds in complexes of boron trihalides, BX3 (X = F, Cl, Br, and I), with species acting as Lewis bases through the nitrogen center, NH3 , N2 , and HCN, are analyzed theoretically (MP2/aug-cc-pVTZ calculations). It is confirmed that stronger Lewis acid properties of the boron center are observed for the BCl3 moiety than for the BF3 one in complexes with the strong Lewis base (NH3 ); while the opposite order is observed for complexes with the weak Lewis base (N2 ). The BX3 NCH complexes (for X = Cl, Br, and I) are characterized by two tautomeric forms and by two corresponding N⋅⋅⋅B distances, the shorter one possesses characteristics of the covalent bond. In a case of the BF3 NCH complex one energetic minimum is observed. Ab initio calculations are supported by an analysis of molecular electrostatic potentials (EPs) and electron density distributions. The quantum theory of 'atoms in molecules' and the decomposition of the energy of interaction are applied. The aforementioned acidity orders as well as the existence of two tautomers for some of complexes result partly from the electrostatic interactions' balance; the EP distribution is different for the BF3 species than for the other BX3 species where X = Cl, Br, and I.