Octahydrotriborate is the boron hydride anion B<sub>3</sub>H<sub>8</sub><sup>âÂÂ</sup>. It forms a variety of salts that are colorless and air-stable. The tetrabutylammonium salt is soluble in organic solvents such as acetonitrile and methylene chloride. The anion is an intermediate is the synthesis of various higher boron hydrides, such as pentaborane(9). B<sub>3</sub>H<sub>8</sub><sup>âÂÂ</sup> can be viewed as the conjugate base of triborane B<sub>3</sub>H<sub>9</sub>.
Octahydrotriborate is prepared by partial oxidation of borohydride with iodine or boron trifluoride:
As shown by X-ray crystallography of various salts, B<sub>3</sub>H<sub>8</sub><sup>âÂÂ</sup> consists of a distorted triangle of three BH<sub>2</sub> vertices. Two edges of the triangle are occupied by bridging hydrides.
It is converted to the bromide B<sub>3</sub>H<sub>7</sub>Br<sup>âÂÂ</sup> using HBr (illustrating its hydridic character):
Pyrolysis of this bromide gives pentaborane(9).
Also consistent with its basicity, B<sub>3</sub>H<sub>8</sub><sup>_</sup> functions as a bidentate ligand in a variety of coordination complexes, e.g. Cr(B<sub>3</sub>H<sub>8</sub>)<sub>2</sub>.