Dicarbollide ligand ($[{\mathrm{C}}_2{\mathrm{B}}_9{\mathrm{H}}_{11}{]}^{2-}$)

The open-face dianion $[{\mathrm{C}}_2{\mathrm{B}}_9{\mathrm{H}}_{11}{]}^{2-}$ can bind through its five-membered open face as an ${\eta }^5$, six-electron donor. Capping this face with a metal fragment completes a 12-vertex metallacarborane; two faces can also support a bis(dicarbollide) metal donor.