Boron bond dissociation energies

The cubic crystalline form of boron nitride has a diamond-like structure, each boron atom is bonded to four nitrogen atoms at a bond distance of 156 pm. The mean NB bond energy calculated from the energy of atomization is 321 kJ mol i.e. more than 2.5 times larger than the NB bond dissociation energy of amine borane. We conclude that the NB bond in amine borane is not only unusually long, but also unexpectedly weak. 

The thermochemistry of boron alkoxides was studied by Chamley et al., which led to an estimate of the average B-O bond dissociation energies for boron alkoxides in the range of 110 5kcalmol these results are consistent with the order B-F > B-N > B-O > B-Cl, reported earlier by Sidgwick." In another study " the reported B-O bond dissociation energy D(B-OR) of B(OMe)3, B(OEt>3, B(OPr">3, were 118.0, 117.7, 119.0 2kcalmor, respectively. 

All the other elements except boron, which has a very complicated structure, are metallic, and the only kind of molecular formation known is the occurrence of small concentrations of A2 molecules in the vapours of the alkali metals. Such molecules are analogous to those of hydrogen, but their dissociation energy is small and their existence in the crystalline state is unknown. In the metallic state we have to assume a third kind of bond, which will be dealt with at a later stage. 

Rather different, and higher, values of the boron-boron bond energies were obtained by Dibeler and his co-workers from photoionization mass spectrometric studies of B2F4 and B2CI4 and the corresponding trihalides. Essentially, this work combines known heats of formation of the subhalides (106) with threshold energies for dissociative ionization processes such as... 

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