The Electron Diffraction Investigation of Difluorodibromosilane

The electron diffraction investigations of the halides of the fourth, fifth, and sixth group elements (1) have shown many interatomic distances to be shorter than those given by Pauling and Huggins’ table of empirical covalent bond radii (2), representing the distances between atoms in pure single bonds in the elements and in other bonds that have considerable ionic character. In the silicon fluorides, which are usually written formally as containing four t-wo-electron bonds, large shortenings occur, and in SiF4, in which the interatomic distance is 5% less than that corresponding to a pure double bond, the explanation of anomalous shortenings applied by Brockway to the fluorochloromethanes (3) breaks down. This laboratory has undertaken careful electron diffraction studies of substances showing these unexpected shortenings in the hope that improved explanations may result from a larger accumulation of data. At present, for example, some explanations require postulating sets of resonance structures which altogether give to the atoms reversed orders of electronegativities which disagree with values obtained from electric moments and ionic resonance energies.