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(SiR 3 ) 3 and B(GeR 3 ) 3 form anomalously strong complexes with Lewis bases N 2 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 sp 3 ‐type orbital on B (LUMO) from the σ‐orbitals of N 2 /CO and electrons are backdonated from the B−Si/Ge σ‐bonds into the π‐type orbitals of N 2 /CO. Here we have analyzed the complexes between Lewis acids B(SiH 3 ) 3 and B(CF 3 ) 3 and Lewis bases N 2 , CO and NH 3 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(SiH 3 ) 3 with N 2 and CO, and σ‐donation stabilization is also enhanced compared to the corresponding complexes of B(CF 3 ) 3 . Differential electrostatic potential indicate that the enhanced stabilization of the B(SiH 3 ) 3 complexes is largely an effect of reduced charge separation due to the balance between σ‐donation and π‐backdonation.