5 years ago

Two faces of triel bonds in boron trihalide complexes

Two faces of triel bonds in boron trihalide complexes
Sławomir J. Grabowski
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 BX3NCH 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 BF3NCH 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. © 2017 Wiley Periodicals, Inc. Hydrogen cyanide complexes with BX3 (X = Cl, Br, and I) are characterized by two tautomers, for one of them a strong and partially covalent N⋅⋅⋅B interaction occurs, for the second one a weak interaction. The existence of two tautomers results from the balance between electrostatic interactions in these complexes. For covalent in nature interactions, a large deformation energy is observed, the boron center becomes to accept the tetrahedral configuration (tetravalent boron) while for weak interactions, with negligible deformation energies, the trigonal boron occurs (trivalent boron).

Publisher URL: http://onlinelibrary.wiley.com/resolve/doi

DOI: 10.1002/jcc.25056

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