Theoretical studies of inorganic compounds. 19 Quantum chemical investigations of the phosphane complexes X ₃B-PY ₃ and X ₃Al-PY ₃ (X = H, F, Cl; Y = F, Cl, Me, CN)

We report about quantum chemical ab initio calculations at the MP2/6-311+G(2d)//MP2/6-31G(d) level and DFT calculations at BP86/TZP of the geometries and bond dissociation energies of the borane-phosphane complexes X ₃B-PY ₃ and the alane-phosphane complexes X ₃Al-PY ₃ (X = H, F, Cl; Y = F, Cl, Me, CN). The nature of the B-P and Al-P bonds is analyzed with a bond energy partitioning method. The calculated bond dissociation energies D _e of the borane adducts X ₃B-PY ₃ show for the phosphane ligands the trend PMe ₃ > PCl ₃ ∼ PF ₃ > P(CN) ₃. A similar trend PMe ₃ > PCl ₃ > PF ₃ > P(CN) ₃ is predicted for the alane complexes X ₃Al-PY ₃. The order of the Lewis acid strength of the boranes depends on the phosphane Lewis base. The boranes show with PMe ₃ and PCl ₃ the trend BH ₃ > BCl ₃ > BF ₃ but with PF ₃ and P(CN) ₃ the order is BH ₃ > BF ₃ > BCl ₃. The bond energies of the alane complexes show always the trend AlCl ₃ ≥ AlF ₃ > AlH ₃. The bonding analysis shows that it is generally not possible to correlate the trend of the bond energies with one single factor which determines the bond strength. The preparation energy which is necessary to deform the Lewis acid and Lewis base from the equilibrium form to the geometry in the complex may have a strong influence on the bond energies. The intrinsic interaction energies may have a different order than the bond dissociation energies. The trend of the interaction energies are sometimes determined by a single factor (Pauli repulsion, electrostatic attraction or covalent bonding) but sometimes all components are important. The higher Lewis acid strength of BCl ₃ compared with BF ₃ in strongly bonded complexes is not caused by the deformation energy of the fragments but it is rather caused by the intrinsic interaction energy. P(CN) ₃ is a weaker Lewis base than PF ₃, PCl ₃ and PMe ₃ mainly because of its weaker electrostatic attraction. The complex H ₃B-P(CN) ₃ is predicted to have a bond dissociation energy D _o = 14.8 kcal/mol which should be sufficient to synthesize the compound as the first adduct with the ligand P(CN) ₃. The calculated bond energies at the BP86 level are in most cases very similar to the MP2 results. In a few cases significantly different absolute values have been found which are caused by the method and not by the quality of the basis set.