= Solution
The usual qualitative prediction is the \b[carbon-separated $\mathrm{CBCBB}$ isomer]. It has four B–C ring edges and one B–B edge, whereas the adjacent-carbon isomer has two B–C, two B–B and one C–C edge. Separating the more electron-rich <carbon> vertices distributes skeletal <electron> density more evenly and avoids concentrating both <carbon> vertices on one edge, a useful <carborane> stability heuristic.
This is a thermodynamic suggestion, not a consequence of the peak count. <hydrogen> placement, metal–ring interactions and entropy can affect the ordering. Establishing the preferred isomer experimentally would require reversible interconversion and an equilibrium composition, rather than simply identifying the product formed fastest.
Back to article page
