Pentaborane(9) (${\mathrm{B}}_5{\mathrm{H}}_9$)

${\mathrm{B}}_5{\mathrm{H}}_9$, a nido borane with a square-pyramidal boron skeleton. A bridging proton can be removed to form $[{\mathrm{B}}_5{\mathrm{H}}_8{]}^{-}$.