We report the structural characterization of the gas phase adducts HCN and CH$-3$/CN with BF$-3$/. 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 BF$-3$/ 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 CH$-3$/CN-BF$-3$/, 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 BF$-3$/ and BH$-3$/ 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 BF$-3$/ adducts as the donor- acceptor bond formation proceeds.
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