[9-92-6-19]

I was reading about the superacid carborane acid; the structure is amazing to me, but there is one peculiar thing that I don't quite understand: the carbon atoms seem to have five bonds, rather than the usual four.

I don't quite understand how this is so. Even the non-acid variant (o-carborane) exhibits two pentavalent carbon atoms, but that seem incredibly counter-intuitive.

It would be greatly appreciated if one could explain my folly on this subject.

[Corribus]

There's no folly - carbon almost always is tetravalent but on occasion it can form five bonds.  As you might imagine, they are typically incredibly unstable and are most prevalent as reaction intermediates or in outer space, where pressures (and therefore collision frequencies) are low.

[9-92-6-19]

That makes sense, cosmic rays and such can do some pretty neat things; however, in carborane acid, the anion CHB₁₁Cl₁₁^- is "[...] incredibly stable ... so it is non-corrosive like other strong acids."

Could that be why carborane acid is a such a strong acid? The extra proton attached to the carbon atom is so weakly bonded to the rest of the structure owing to near-complete ionization.

That makes a lot of sense, but it does not explain other carborane structures that are not acids. Could it be that the B—C bond is a "three-center two electron" system?

[AWK]

http://www.ch.imperial.ac.uk/rzepa/blog/?p=4479

[9-92-6-19]

Thank you so much; that really has cleared things up.