An X-ray diffraction study of crystals of HsCCN-BF a leads to an orthorhombic unit of structure with a= 7.76, b = 7-20, c = 8.34 A., and containing four stoichiometric molecules. All of the X-ray data are consistent with the space group Pnma. Approximate positions of the atoms (other than hydrogen) were determined by Patterson and Fourier syntheses. The parameters so obtained were refined by the method of Booth.Bond distances, B-F= 1.32 A., B-N= 1.64 A., and bond angles, F-B-N= 103 °, F-B-F= 114 ° were observed. It is of interest to compare these results with those of Geller & Hoard for HaCH~N-BF a who found B-F= 1.37 A., B-N= 1.58 A., /F-B-N= 108 °, /F-B-F= 111% HaCCN-BF s is completely dissociated in the vapor phase at 50 ° C. whereas monomethylamine-boron trifluoride is stable at this temperature. The weaker bond in the former is characterized by a longer B-N distance and smaller changes in the original planar configuration of BF a accompanying the formation of the donor-acceptor bond. We conclude that the nature of this type of bond and the resulting configuration of the addition compound depends on the character of the substituent groups attached to the 'donor' atom and that simple bond theory, which predicts complete hybridization of bond orbitals and the 'equivalent' tetrahedral configuration of bonds about the 'acceptor' atom (i.e. boron), is not adequate.The C-C distance of 1.44 + 0.02 A. agrees well within limits of combined error with that observed in methyl cyanide by Pauling and co-workers.
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