Structure
The borohydride anion has a tetrahedral structure with boron in the center and the hydrogens located at the four vertices of the tetrahedron. In metal complexes, the borohydride ion is bound to the metal by means of bridging hydrogen atoms, usually in one of three different ways: monodentate (one hydrogen bridge), bidentate (two hydrogen bridges), and tridentate (three hydrogen bridges) (η1, η2, or η3). The preferred coordination mode is strongly affected by the nature of the metal and its oxidation state.
| Hydride CAS registry number | Mol.Wt. | Hydrogen Density | Density g/cm3 | m.p. (°C) | solubility in water (g/100 mL, 25 °C) | solubility in CH3OH (g/100 mL, 25 °C) | solubility in ether (g/100 mL, 25 °C) | solubility in THF (g/100 mL, 25 °C) |
|---|---|---|---|---|---|---|---|---|
| LiBH4 | 21.78 | 18.5 | 0.66 | 280 | 20.9 | decomp. (44 in EtOH) | 4.3 | 22.5 |
| NaBH4 | 37.83 | 10.6 | 1.07 | 505 | 55 | 16.4 (20 °C) | insol. | 0.1 (20 °C) |
| NaBH3CN | 62.84 | 6.4 | 1.20 | 240 with deccomp. | deccomp. | 217 | insol. | 36 |
| KBH4 | 53.94 | 7.4 | 1.17 | 585 (under H2) | 19 | insol. | insol. | insol. |
| LiBH(C2H5)3 | 105.94 | 0.95 | unknown | unknown | decomp. | decomp. | na | high (supplied commercially) |
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