Formula U(BH4)4 | Molar mass 297.27 g/mol | |
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.
Contents
Preparation
This compound was first prepared by reacting uranium tetrafluoride with aluminium borohydride:
UF4 + 2 Al(BH4)3 → U(BH4)4 + 2 Al(BH4)F2It may also be prepared by the solid-state reaction of uranium tetrachloride with lithium borohydride in vacuo:
UCl4 + 4 LiBH4 → U(BH4)4 + 4 LiClAlthough 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.