Octahydrotriborate
# Octahydrotriborate

Octahydrotriborate is the boron hydride anion $B_3H_8^−.$ 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). $B_3H_8^−$ can be viewed as the conjugate base of triborane $B_3H_9.$

## Preparation

Octahydrotriborate is prepared by partial oxidation of borohydride with iodine or boron trifluoride:

    3 $BH_4^−$ + $I_2$ → $B_3H_8^−$ + 2 $H_2$ + 2 $I^−$
    5 $BH_4^−$ + 4 $BF_3O(C_2H_5)_2$ → 2 $B_3H_8^−$ + 2 $H_2$ + 4 $O(C_2H_5)_2$ + 3 $BF_4^−$

## Structure and reactions

As shown by X-ray crystallography of various salts, $B_3H_8^−$ consists of a distorted triangle of three $BH_2$ vertices. Two edges of the triangle are occupied by bridging hydrides.

It is converted to the bromide $B_3H_7Br^−$ using HBr (illustrating its hydridic character):

    $B_3H_8^−$ + HBr → $B_3H_7Br^−$ + $H_2$

Pyrolysis of this bromide gives pentaborane(9).

    5 $B_3H_7Br^−$ → 3 $B_5H_9$ + 5 $Br^−$ + 4 $H_2$

Also consistent with its basicity, $B_3H_8^_$ functions as a bidentate ligand in a variety of coordination complexes, e.g. $Cr(B_3H_8)_2.$

