Undecahydrohexaborate anion ($[{\mathrm{B}}_6{\mathrm{H}}_{11}{]}^{-}$)

$[{\mathrm{B}}_6{\mathrm{H}}_{11}{]}^{-}$, an arachno borane anion with nine skeletal pairs. A localized BH3-adduct drawing does not describe its lowest-energy fluxional cage adequately.