Uranium borohydride

Uranium borohydride

Identifiers
ChemSpider	15385433
Jmol interactive 3D	Image
InChI InChI=1S/2BH5.U/h2*1H5;/q2*-1;+2 Key: KUMAXXHQRVBPEC-UHFFFAOYSA-N InChI=1/2BH5.U/h2*1H5;/q2*-1;+2 Key: KUMAXXHQRVBPEC-UHFFFAOYAW
SMILES [BH5-].[BH5-].[U+2]
Properties
Chemical formula	U(BH4)4
Molar mass	297.27 g/mol
Solubility in other solvents	Decomposes
Except where otherwise noted, data are 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.

Preparation

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.

History

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.

References

1. 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.