carboranes

Take a borane cage and ask: what if we swap one or two of the boron vertices for carbon? Carbon brings one more valence electron than boron, so this is like quietly topping up the electron budget of an electron-deficient cluster. The result is the carboranes, mixed boron-carbon-hydrogen cages that turned out to be far more stable and far more useful than many pure boranes.

Carboranes are cluster compounds whose vertices are a mixture of carbon (as C-H units) and boron (as B-H units). The most famous is the icosahedral closo carborane C2B10H12, a near-perfect twenty-faced cage of two carbons and ten borons. Because a C-H vertex contributes one more skeletal electron than a B-H vertex, you can think of replacing a vertex BH by CH (or by CH plus removing a charge) as an isoelectronic substitution: B6H6^2- and the hypothetical neutral carborane analogue share the same skeletal electron count and the same shape. That is the deep reason Wade's rules treat boranes and carboranes with one and the same counting scheme. The two-carbon icosahedral carborane even comes in three isomers (ortho, meta, para) depending on how far apart the carbons sit on the cage.

Carboranes matter because they are remarkably robust: ortho-carborane C2B10H12 shrugs off heat and many reagents, so it is used in heat-resistant polymers and as a rigid scaffold in catalysis and drug design. Cooler still, the carborane CB11H12- and its relatives are among the least nucleophilic, most weakly coordinating anions known, which makes them prized for stabilizing extremely reactive cations and even for building some of the strongest known acids. And boron-rich carboranes are studied for boron neutron capture therapy, a targeted cancer treatment that exploits boron-10's appetite for neutrons.