Pentaborane(11)

Pentaborane(11) is inorganic compound with the chemical formula B5H11. It is an obscure boron hydride cluster, especially relative to the heavily studied pentaborane(9) (B5H9). With two more hydrogen atoms than nido-pentaborane(9), pentaborane(11) is classified as an arachno- cluster.

Synthesis
Like many boron hydride clusters, pentaborane(11) was originally obtained from the pyrolysis of diborane. A more systematic synthesis entails treatment of [B4H9]− with boron tribromide. The Lewis acid abstracts hydride to give unstable B4H8, the precursor to B5H11:
 * [B4H9]− + BBr3   →   B4H8  +  HBBr3−
 * 2 B4H8  →   B5H11  +  "B3H5"