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 De = 51.9 kcal mol-1 for H2B+-CO and De = 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.
- Bond energy
- Bond theory
- Density functional calculations
- Donor-acceptor systems
- Energy decomposition analysis