Uranium borohydride

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Uranium borohydride
Zirconium-borohydride-3D-balls-A.png
Identifiers
3D model (JSmol)
PubChem CID
  • InChI=1S/2BH5.U/h2*1H5;/q2*-1;+2 Yes check.svgY
    Key: KUMAXXHQRVBPEC-UHFFFAOYSA-N Yes check.svgY
  • InChI=1/2BH5.U/h2*1H5;/q2*-1;+2
    Key: KUMAXXHQRVBPEC-UHFFFAOYAW
  • [BH4-].[BH4-].[U+4].[BH4-].[BH4-]
Properties
U(BH4)4
Molar mass 297.27 g/mol
Solubility in other solventsDecomposes
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Yes check.svgY  verify  (what is  Yes check.svgYX mark.svgN ?)

Uranium borohydride is the inorganic compound with the empirical formula U(BH4)4. Two polymeric forms are known, as well as a monomeric derivative that exists in the gas phase. Because the polymers convert to the gaseous form at mild temperatures, uranium borohydride once attracted much attention. It is solid green. [1]

Contents

Structure

It is a homoleptic coordination complex with borohydride (also called tetrahydroborate). These anions can serve as bidentate2-BH4) bridges between two uranium atoms or as tridentate ligands (κ3-BH4) on single uranium atoms. In the solid state, a polymeric form exists that has a 14-coordinate structure with two tridentate terminal groups and four bidentate bridging groups. [2] Gaseous features a monomeric 12-coordinate uranium, with four κ3-BH4 ligands, which envelop the metal, conferring volatility. [3]

Ball-and-stick model of one of two forms of polymeric U(BH4)4. EntryWithCollCode63132.png
Ball-and-stick model of one of two forms of polymeric U(BH4)4.

Preparation

This compound was first prepared by treating 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: [1]

UCl4 + 4 LiBH4 → U(BH4)4 + 4 LiCl

Although solid U(BH4)4 is a polymer, it undergoes cracking, converting to the monomer. The related methylborohydride complex U(BH3CH3)4 is monomeric as a solid 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. [1]

Uranium hexafluoride is corrosive, which led to serious consideration of the borohydride. However, by the time the synthesis method was finalized, the problems related to uranium hexafluoride were solved. Borohydrides are nonideal ligands for isotope separations, since there are isotopes of boron that occur naturally in high abundance: 10B (20%) and 11B (80%), while fluorine-19 is the only isotope of fluorine that occurs in nature in more than trace quantities.

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Boranes is the name given to compounds with the formula BxHy and related anions. Many such boranes are known. Most common are those with 1 to 12 boron atoms. Although they have few practical applications, the boranes exhibit structures and bonding that differs strongly from the patterns seen in hydrocarbons. Hybrids of boranes and hydrocarbons, the carboranes are also well developed.

<span class="mw-page-title-main">Uranium hexafluoride</span> Chemical compound

Uranium hexafluoride (UF6), (sometimes called "hex") is an inorganic compound with the formula UF6. Uranium hexafluoride is a volatile white solid that reacts with water, releasing corrosive hydrofluoric acid. The compound reacts mildly with aluminium, forming a thin surface layer of AlF3 that resists any further reaction from the compound. UF6 is used in the process of enriching uranium, which produces fuel for nuclear reactors and nuclear weapons.

<span class="mw-page-title-main">Gaseous diffusion</span> Old method of enriching uranium

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<span class="mw-page-title-main">Scorpionate ligand</span> Tridentate ligand which "pinches" the central metal atom

The term scorpionate ligand refers to a tridentate (three-donor-site) ligand which would bind to a metal in a fac manner. The most popular class of scorpionates are the hydrotris(pyrazolyl)borates or Tp ligands. These were also the first to become popular. These ligands first appeared in journals in 1966 from the then little-known DuPont chemist of Ukrainian descent, Swiatoslaw Trofimenko. Trofimenko called this discovery "a new and fertile field of remarkable scope".

<span class="mw-page-title-main">Octahedral molecular geometry</span> Molecular geometry

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References

  1. 1 2 3 4 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.
  2. Charpin, P.; Nierlich, M.; Vigner, D.; Lance, M.; Baudry, D. (1987). "Structure of the Second Crystalline Form of Uranium(IV) Tetrahydroborate". Acta Crystallographica Section C. 43 (8): 1465–p1467. doi:10.1107/S0108270187091431.
  3. Haaland, Arne; Shorokhov, Dmitry J.; Tutukin, Andrey V.; Volden, Hans Vidar; Swang, Ole; McGrady, G. Sean; Kaltsoyannis, Nikolas; Downs, Anthony J.; Tang, Christina Y.; Turner, John F. C. (2002). "Molecular Structures of Two Metal Tetrakis(tetrahydroborates), Zr(BH4)4 and U(BH4)4: Equilibrium Conformations and Barriers to Internal Rotation of the Triply Bridging BH4 Groups". Inorganic Chemistry. 41 (25): 6646–6655. doi:10.1021/ic020357z. PMID   12470059.