A short yet very weak dative bond: Structure, bonding, and energetic properties of N₂-BH₃

The structure, bonding, and energetic properties of the N ₂-BH₃ complex are reported as characterized by density functional theory (DFT) and post-Hartree-Fock (HF) calculations. The equilibrium structure of the complex exhibits a short B-N distance near 1.6 Å, comparable to that of a strong acid-base complex like H₃N-BH ₃. 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, nonbonded 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 orbitais, both sigma and pi. Also, CCSD/6-311+G(2df,2dp) calculations indicate that the B-N distance potential is rather anharmonic and exhibit a flat, shelf-like region ranging from 2.1 to 2.5 Å that lies about 1.5 kcal/mol above the minimum at 1.67 Å. 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 ID analysis of the vibrational motion along the B-N stretching coordinate in the CCSD/6311+G(2df,2dp) potential indicates that the average B-N distance in the ground vibrational state is 1.71 Å, about 0.04 Å longer than the equilibrium distance. Furthermore, the vibrationally averaged distance obtained via an analysis of the CCSD(T)/6-31 l+G(2df,2dp)//CCSD/6- 311+G(2df,2dp) potential was found to be 0.03 À longer than the CCSD(T)/6-311+G(2df,2dp) minimum.