Uranium borohydride

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Uranium borohydride
ChemSpider 15385433 YesY
Jmol-3D images Image
Molar mass 297.27 g/mol
Solubility in other solvents Decomposes
Except where noted otherwise, data is given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
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Infobox references

Uranium borohydride U(BH4)4 is a volatile uranium complex with borohydride. This green-coloured compound is polymeric in the solid state but evaporates to a tetrahedral monomer.


This compound was first prepared by reacting uranium tetrafluoride with aluminium borohydride:[1]

UF4 + 2 Al(BH4)3 → U(BH4)4 + 2 Al(BH4)F2

It may also be prepared by the solid-state reaction of uranium tetrachloride with lithium borohydride in vacuo:[1]

UCl4 + 4 LiBH4 → U(BH4)4 + 4 LiCl

Although U(BH4)4 is polymeric in the solid state, U(BH3CH3)4 is monomeric and hence more volatile.


During the Manhattan Project the need arose to find volatile compounds of uranium suitable for use in the diffusion separation of uranium isotopes. Uranium borohydride is, after uranium hexafluoride, the most volatile compound of uranium known with a vapor pressure of 4 mmHg (530 Pa) at 60 °C. Uranium borohydride was discovered by Hermann Irving Schlesinger and Herbert C. Brown, who also discovered sodium borohydride.

Uranium hexafluoride is very corrosive, which presented serious handling difficulties, leading to serious consideration of the borohydride. However by the time the synthesis method was finalized, the problems related to uranium hexafluoride were already solved. Borohydrides are nonideal ligands for isotope separations, since boron occurs naturally with two abundant isotopes, 10B (20%) and 11B (80%).

Enrico Fermi's purported comment when he observed the neutron cross section for boron, "My God! It's as big as the side of a barn!", not only gave a name to the unit of cross section (the barn), but also put an end to using uranium borohydride in the diffusion process.


  1. ^ a b Ephritikhine, M. (1997). "Synthesis, Structure, and Reactions of Hydride, Borohydride, and Aluminohydride Compounds of the f-Elements". Chemical Reviews 97 (6): 2193–2242. doi:10.1021/cr960366n. PMID 11848899.