How to photograph a chemical reaction

We report the structural characterization of the gas phase adducts HCN and CH₃CN with BF₃. Both have symmetric top structures with the nitrogen end of the R-CN toward the boron, reminiscent of the well known dative bond chemistry of BF₃ with nitrogen donors. The B-N bond lengths and N-B-F angles, however, are intermediate between those expected for van der Waals or covalent interactions. Moreover, in CH₃CN-BF₃, where comparison with x-ray crystallographic studies is possible, the gas phase adduct shows a markedly longer bond length and smaller N-B-F angle. We show that in a series of related BF₃ and BH₃ adducts, the bond length and bond angle can, in fact, be tuned almost continuously between the covalent and van der Waals limits. By analogy with classic crystallographic work by Buergi and Dunitz and coworkers, we discuss how members of such a series can be interpreted as snapshots along a generalized reaction path for the formation of the dative bond. Finally, in the context of such a path, we examine the evolution of other (non-structural) properties of the BF₃ adducts as the donor-acceptor bond formation proceeds.