Reaction of Compounds of the Nonmetallic Elements

Abstract

The pyrolysis of the boranes is a topic of continuing interest. Lipscomb et al. ⁽¹⁾ have published calculations that cast doubt on the long-held view that in the pyrolysis of diborane the decomposition of B₃H₉ an adduct of BH₃ and B₂H₆ is a rate-determining stage. The same group have also published calculations that are consistent with the idea that a BH₅ species is an intermediate in the aqueous hydrolysis of the borohydride anion.⁽²⁾ Greenwood et al. ⁽³⁾ suggest reactions (1)-(3)

$$\begin{array}{*{20}{c}} {{{B}_{6}}{{H}_{{12}}} \to {{B}_{5}}{{H}_{9}} + B{{H}_{3}}} & {slow} \\ \end{array}$$

(1)

$$2B{{H}_{3}} \rightleftharpoons {{B}_{2}}{{H}_{6}}$$

(2)

$${{B}_{5}}{{H}_{{11}}} \to {{B}_{4}}{{H}_{8}} + B{{H}_{3}}$$

(3)

for the pyrolysis of B₆H₁₂ and B₅H₁₁. Calculations on the diamond-squarediamond isomerization mechanism in C₂B₅H₇ have been reported.⁽⁴⁾ However, for the thermal rearrangements of the icosahedral carboranes the extended triangle rotation mechanism is held to rationalize the data better.⁽⁵⁾ Barrier heights for bridge hydrogen scrambling in B₆H₁₀ and CB₅H₇ have been calculated.⁽⁶⁾ Slow deuterium-protium exchange for B₂H₆ in FSO₃D/ SbF₅ at low temperature probably involves a B₂H; species, [H₂B(µH)₂BH₂D]⁺.⁽⁷⁾ The reaction of nido-2,3-R₂C₂B₄H5 with organic halides R’X gives B(4)-substituted products together with B(6)-substituted enantiomers, probably⁽⁸⁾ via a bridged intermediate R₂C₂B₄H₅-µ (4, 5)-R’.