Boron Triel Bonds: A Quantum Chemical Topology Perspective

Abstract

Different quantum chemical topology (QCT) methods were used to study the boron triel bonded complexes between BX3 (X=H, F, Cl, and Br) and some Lewis bases (NH3, CO and NCH). The Laplacian of electron density distribution indicates that the weak boron‐Lewis base complexes can be classified as lump‐hole interactions. On the other hand, in the covalent‐type complexes, the lump in the valence shell of the Lewis base is polarized toward the boron atom, leading to tetravalent borons. Natural atom in molecule (NAIM) analysis demonstrates that a bonding NBO (natural bond orbital) is the primary source of electron density at the triel bond critical points in the covalent‐type complexes. The interacting quantum atom (IQA) approach shows that in both closed‐shell and covalent‐type complexes, the exchange‐correlation terms are the dominant inter‐fragment interactions, indicating the importance of electron sharing in the formation of complexes. Natural adaptive orbital (NAdO) analysis reveals that donation and back‐donation channels contribute almost equally to inter‐fragment electron delocalization. Interestingly, in the closed‐shell systems, the electrostatic inter‐fragment interactions are negligibly small, and the exchange‐correlation terms are large enough to compensate for the unstabilizing deformation energies. In contrast, in the covalent‐type complexes both exchange‐correlation and electrostatic terms are essential to overcome deformations.