Comparison of Chemical and Interpretative Methods: The Carbon-boron π-Bond as a Test Case

Quantum chemical calculations using DFT and NBO, ETS-NOCV, QTAIM and ELF interpretative approaches have been carried out on X-BH₂⁺ borenium complexes for 39 divalent C-donor ligands X including various N-heterocyclic carbenes and carbones. The C-B bond length and the barrier of rotation around the C-B bond were calculated and compared with various descriptors of the C-B pi-bond strength obtained from the orbital localization, energy partitioning or topological methods. Two families of descriptors emerged: intrinsic indicators, which measure the intensity of the pi-bond in the investigated molecule, and relative indicators, among them the rotational barrier, which compare the studied molecule with its conformer in which the pi-interaction is prevented. Relative indicators are influenced by other interactions in addition to purely pi-interactions. For both families of descriptors, excellent correlations are obtained, showing that the interpretative methods, despite their conceptual differences, describe the same chemical properties. These results also reveal noticeable shortcomings in these methods, and some precautions that need to be taken to interpret their results adequately.