LAUSR

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 showed 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 s‐orbitals of N2/CO and electrons are backdonated from the B‐Si/Ge s‐bonds into the p‐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.