Structure, bonding, and energetic properties of nitrile-borane complexes: RCN-BH3.

The structural and energetic properties of CH(3)CN-BH(3), HCN-BH(3), FCH(2)CN-BH(3), and F(3)CCN-BH(3) have been examined via density functional theory and post-Hartree-Fock calculations. The B-N distances in these systems are notably short, less than 1.6 Å, and the binding energies are substantial, about 20 kcal/mol. The properties of these systems do vary as a result of the nitrile substituent, but surprisingly, more electronegative substituents result in shorter B-N distances. For example, the B-N distance for F(3)CCN-BH(3) is 1.576 Å via MP2/aug-cc-pVTZ, while that for CH(3)CN-BH(3) is 1.584 Å. However, the binding energies vary as expected, from 17.4 kcal/mol in the case of F(3)CCN-BH(3) to 22.6 kcal/mol for CH(3)CN-BH(3) (via MP2/aug-cc-pVTZ). The extent of charge transfer and the degree of covalent character in the B-N bonds were explored by a natural bond orbital analysis, and the atoms in molecules formalism, respectively, and do provide some rationale for the substituent effects. Frequency calculations indicate that BH(3)-localized vibrational modes do shift appreciably upon complex formation, especially the BH(3) asymmetric stretch. For CH(3)CN-BH(3), experimental and calculated frequency shifts compare well for the asymmetric BH(3) bending mode, but the observed shift for the BH(3) asymmetric stretch, the most structurally sensitive mode, is about 40 cm(-1) larger than the predictions. While this may suggest a very slight contraction of the B-N bond upon formation of solid CH(3)CN-BH(3) (for which experimental data are available) the balance of evidence indicates that no significant medium effects occur in these complexes. We also discuss the distinct differences between these complexes and their BF(3) analogs. The underlying reasons for the markedly different structural properties are illustrated through an energy decomposition analysis applied to HCN-BH(3) and HCN-BF(3). These data indicate that less Pauli repulsion of the electrons on each respective subunit is the most significant factor that favors the overall stability of the BH(3) complex.