Pentaborane(11) is an inorganic compound with the chemical formula B<sub>5</sub>H<sub>11</sub>. It is an obscure boron hydride cluster, especially relative to the heavily studied pentaborane(9) (B<sub>5</sub>H<sub>9</sub>). With two more hydrogen atoms than nido-pentaborane(9), pentaborane(11) is classified as an arachno- cluster.
Like many boron hydride clusters, pentaborane(11) was originally obtained from the pyrolysis of diborane. A more systematic synthesis entails treatment of [B<sub>4</sub>H<sub>9</sub>]<sup>âÂÂ</sup> with boron tribromide. The Lewis acid abstracts hydride to give unstable B<sub>4</sub>H<sub>8</sub>, the precursor to B<sub>5</sub>H<sub>11</sub>: