van der Waals vs Covalent Bonding: Microwave Characterization of a Structurally Intermediate Case

The gas-phase adduct CH₃CN-BF₃ has been observed by rotational spectroscopy. The structure is that of a symmetric top with the nitrogen end of the CH₃CN toward the boron. The B-N bond length is 2.011 ± 0.007 Å, and the NBF angle is 95.6 ± 0.6°. The structure is reminiscent of the classical dative bond chemistry known for adducts of BF₃ with nitrogen-containing donors, and of related weakly bound complexes of BF₃, but is extremely unusual in that the bond length and bond angle are intermediate between the limits normally observed for van der Waals and covalently bonded systems. Moreover, the B-N bond length is 0.381 ± 0.011 Å longer and the N-B-F angle 10 ± 1° smaller than those determined in the solid state by X-ray crystallography, indicating a significant influence of neighboring molecules in the crystal on the structure of a single dimeric unit. We draw analogy with the well-known crystallographic work of Bürgi and Dunitz and co-workers and interpret the structure of this system as a gas-phase snapshot along the reaction path for the formation of the boron-nitrogen dative bond.