A Short Yet Very Weak Dative Bond: Structure, Bonding, and Energetic Properties of N₂-BH₃

Smith, E. L.; Sadowsky, D.; Phillips, J. A.; Cramer, C. J.; Giesen, D. J.
J. Phys. Chem. A 2010, 114, 2628. (doi:10.1021/jp909059n).

The structure, bonding, and energetic properties of the N₂-BH₃ complex are reported as characterized by density functional theory and post-Hartree Fock calculations. The equilibrium structure of the complex exhibits a short B-N distance near 1.6 ang, comparable to that of a strong acid-base complex like H₃N-BH₃. However, the binding energy is only 5.7 kcal/mol at the CCSD(T)/6-311+G(2df,2dp) level of theory, which is reminiscent of a weak, non-bonded complex. Natural Bond Orbital (NBO) and Atoms in Molecules (AIM) analyses of the electron density from both DFT and post-HF calculations do indicate that the extent of charge transfer and covalent character in the B-N dative bond is only somewhat less than in comparable systems with fairly large binding energies (e.g. H₃N-BH₃ and OC-BH₃). Energy decomposition analysis indicates key differences between the N₂, CO, and NH₃ complexes, primarily associated with the natures of the lone pairs involved (sp vs sp³) and the donor/acceptor characteristics of the relevant occupied and virtual orbitals, both sigma and pi. Also, CCSD/6-311+G(2df,2dp) calculations indicate that the B-N distance potential is rather anharmonic, and exhibits a flat, shelf-like region ranging from 2.1 to 2.5 ang that lies about 1.5 kcal/mol above the minimum at 1.67 ang. However, this region is more sloped, and lies about 2.5 kcal/mol above the equilibrium region according to the CCSD(T)/6-311+G(2df,2dp)//CCSD/6-311+G(2df,2dp) potential. A 1-D analysis of the vibrational motion along the B-N stretching coordinate in the CCSD/6-311+G(2df,2dp) potential indicates that the average B-N distance in the ground vibrational state is 1.71 ang, about 0.04 ang longer than the equilibrium distance. Furthemore, the vibrationally-averaged distance obtained via an analysis of the CCSD(T)/6-311+G(2df,2dp)//CCSD/6-311+G(2df,2dp) potential was found to be 0.03 ang longer than the CCSD(T)/6-311+G(2df,2dp) minimum.