A challenge to chemical intuition: Donor-acceptor interactions in H ₃B-L and H₂B⁺ -L (L = CO; EC₅H ₅, e = N-Bi)

The equilibrium geometries and bond energies of the complexes H ₃B-L and H₂B⁺-L (L = CO; EC₅H ₅: E = N, P, As, Sb, Bi) have been calculated at the BP86/TZ2P level of theory. The nature of the donor-acceptor bonds was investigated by energy decomposition analysis (EDA). The bond strengths of H₃B-L have the order CO>N>P>As>Sb>Bi. The calculated values are between D _e = 37.1 kcal mol^-1 for H₃B-CO and D _e = 6.9 kcal mol^-1 for H₃B-BiC ₅H₅. The bond dissociation energies of the cations H ₂B⁺-CO and H₂B⁺-EC₅H ₅ are larger than for H₃B-L, particularly for complexes of the heterobenzene ligands. The calculated values are between D_e = 51.9 kcal mol^-1 for H₂B⁺-CO and D_e = 122.1 kcalmor^-1 for H₂B⁺ -NC₅H ₅. The trend of the BDE of H₂B⁺-CO and H ₂B⁺-EC₅H₅ is N> P > As > Sb > Bi > CO. A surprising result is found for H₂B ⁺-CO, which has a significantly stronger and yet substantially longer bond than H₃B-CO. The reason for the longer but stronger bond in H₂B⁺-CO compared with that in H₃B-CO comes mainly from the change in electrostatic attraction and p bonding at shorter distances, which increases more in the neutral system than in the cation, and to a lesser extent from the deformation energy of the fragments. The H ₂B⁺?NC₅H₅ p? donation plays an important role for the stronger interactions at shorter distances compared with those in H₃B-NC₅H₅. The attractive interaction in H₂B⁺-CO further increases at bond lengths that are shorter than the equilibrium value, but this is compensated by the energy which is necessary to deform BH₂⁺ from its linear equilibrium geometry to the bent form in the complex. The EDA shows that the contributions of the orbital interactions to the donor-acceptor bonds are always larger than the classical electrostatic contributions, but the latter term plays an important role for the trend in bond strength. The largest contributions to the orbital interactions come from the s orbitals. The EDA calculations suggest that heterobenzene ligands may become moderately strong p donors in complexes with strong Lewis acids, while CO is only a weak p donor. The much stronger interaction energies in H ₂B⁺-EC₅H₅ compared with those in H₃B-EC₅H₅ are caused by the significantly larger contribution of the p? orbitals in H₂B ⁺-EC₅H₅ and by the increase of the binding interactions of the s + p_||orbitals. © 2006 Wiley-VCH Verlag GmbH & Co. KGaA.