ETS-NOCV Analysis of σ-Donation and π-Backdonation in Complexes of Boron Based Lewis Acids with N2, CO and NH3

Abstract

Lewis acids B(SiR₃)₃ and B(GeR₃)₃ form anomalously strong complexes with Lewis bases N₂ and CO. Intramolecular B−N/C bonds are generally in the range 1.45–1.50 Å and shorter than the sum of B and N/C covalent radii. Bonding analyses have shown that the strong bonds are a consequence of a novel σ-donation and π-backdonation mechanism, where electrons are donated into an empty sp³-type orbital on B (LUMO) from the σ-orbitals of N₂/CO and electrons are backdonated from the B−Si/Ge σ-bonds into the π-type orbitals of N₂/CO. Here we have analyzed the complexes between Lewis acids B(SiH₃)₃ and B(CF₃)₃ and Lewis bases N₂, CO and NH₃ using the extended transition state – natural orbitals for chemical valence (ETS-NOCV) method. Both σ-donation and π-backdonation are present in all complexes, and deformation densities due to the two mechanisms, i. e. NOCV pair densities, are surprisingly similar in character. Energy stabilization due to π-backdonation is much larger for the complexes of B(SiH₃)₃ with N₂ and CO, and σ-donation stabilization is also enhanced compared to the corresponding complexes of B(CF₃)₃. Differential electrostatic potential indicate that the enhanced stabilization of the B(SiH₃)₃ complexes is largely an effect of reduced charge separation due to the balance between σ-donation and π-backdonation.