= Solution
React pentaborane with an alkyne under relatively mild conditions to obtain a \b[<nido> $\mathrm{C_2B_4H_8}$ framework], conventionally nido-2,3-dicarbahexaborane(8) for acetylene. The two entering <carbon> atoms initially retain their adjacency. A specific mild example uses 2-butyne with triethylamine-assisted $\mathrm{B_5H_9}$ at or below room <temperature> to give nido-$\mathrm{Me_2C_2B_4H_6}$. This is a practical route to an explicitly <nido carborane>, rather than an assumption that an alkyne supplies a <closo> cage under every condition.<Improved Synthetic Routes to Small Carboranes: Bench-Scale Preparation of nido-2,3-C2B4H8 Derivatives from B5H9 and from B3H8- Salts by Narayan S. Hosmane and Russell N. Grimes>[1979, abstract and preparation discussion: amine-assisted substituted-alkyne reaction under mild conditions]{ref}
More energetic gas-phase reaction or pyrolysis promotes <hydrogen> loss, boron-fragment loss and cage rearrangement, giving \b[<closo> <carboranes>], for example $1,5$-$\mathrm{C_2B_3H_5}$ and $1,6$-$\mathrm{C_2B_4H_6}$, with other cage sizes possible in the mixture. Thermolysis of a previously formed <nido carborane> is an alternative route to this stage. The point is not a clean same-composition nido-to-closo <isomerization>: the change of <hydrogen> content and sometimes cage size must be retained in the formulas.
Milder conditions favour kinetically trapped adjacent-carbon products; higher temperatures permit migration to carbon-separated arrangements, often increasing favourable B–C contacts relative to adjacent C–C/B–B contacts. Conditions and Lewis bases also influence which separated-carbon cage is isolated; <temperature> alone does not prescribe a single quantitative isomer distribution. Primary thermolysis studies and the small-carborane literature document both cage rearrangement and reagent-dependent product mixtures.<Synthesis of Carboranes from Dihydrocarboranes by Thomas P. Onak and others>[1963, printed p. 3378, introduction and product summary]{ref}
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