Structure of Boron Hydrides

Abstract

THE chief contention put forward in our earlier paper was that in diborane and similar molecules there were pairs of hydrogen atoms which occupied a special position in linking together different parts of the molecule. We agree with Dr. Burawoy that the formulation of these links as resonance hybrids represents only one way of describing this state of affairs: for example, a molecular orbital treatment of the system would not involve the use of the resonance concept. On the other hand, we doubt whether this linkage can be regarded as an electrostatic 'hydrogen bond' of the type met with in HF, H₂O and NH₃. The tendency to this type of dipole association decreases rapidly along the series FH, —OH, $$ NH, C$$H, and it would be remarkable if it appeared again to a high degree in $$BH. It is, of course, conceivable that the BH link has a considerable dipole with the negative end on the hydrogen atom, but it is not likely that this would lead to the formation of a stable dimer showing no tendency to further polymerization. A much more likely result would be the formation of an indefinite series of polymers, as is the case with H₂O, HF and NH₃. It is improbable that the small size of the boron atom is a determining factor, since the hydrogen compounds of aluminium and gallium dimerize as well as those of boron.