In 2013, researchers at the University of California, Irvine reported the first isolable molecular uranium complex with uranium in the formal +2 oxidation state. They made it by reducing a uranium(III) compound and isolated it as a crystal-containing salt. This was not the discovery of a new element, but a significant result in fundamental inorganic chemistry.
What does uranium’s +2 oxidation state mean?
An oxidation state is a formal accounting device chemists use to describe electron distribution in a compound; it is not, by itself, a complete picture of where every electron resides. In this report, “uranium(II)” refers to the formal oxidation state assigned to uranium in the molecular complex. The compound’s electronic structure requires a more detailed description than the oxidation-state label alone.
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The result mattered because it established that a molecular uranium complex in formal oxidation state +2 could be isolated as a crystalline material and studied. It did not establish an industrial or consumer use.
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How did the researchers make uranium(II)?
In their 2013 paper, Matthew R. MacDonald, Megan E. Fieser, Jefferson E. Bates, Joseph W. Ziller, Filipp Furche and William J. Evans reported reducing tris(cyclopentadienyl)uranium, Cp′3U, where Cp′ is C5H4SiMe3. They flash-reduced the uranium(III) precursor in a column of potassium graphite in the presence of 2.2.2-cryptand. The product crystallized as [K(2.2.2-cryptand)][(C5H4SiMe3)3U]. The cryptand holds the potassium counterion; the uranium-containing component is the [Cp′3U]− anion.
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The authors described this as the first isolable molecular U2+ complex in their 2013 Journal of the American Chemical Society paper. It was published online August 28, 2013, and appeared in the September 11 issue, volume 135, issue 36, pages 13310–13313.
How did they distinguish it from a uranium hydride?
A uranium(III) hydride could potentially have a similar crystal structure, so the authors tested that alternative rather than relying on the structure alone. They prepared the hydride by adding KH to Cp′3U and also formed it by reducing hydrogen with the uranium(II) complex. They report that this hydride is a different compound from the isolated uranium(II) salt.
What does the electronic-structure analysis show?
For the [Cp′3U]− anion, the researchers’ density functional theory calculations assigned a 5f3 6d1 quintet ground state. They reported that this computational assignment matched strong transitions observed in the optical spectrum. The 5f3 6d1 description is the authors’ interpretation of the anion’s electronic structure, not a replacement for the formal +2 oxidation-state label.
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The work made a previously inaccessible low-valent uranium molecular complex experimentally available for chemical study. The novelty was its isolation and characterization as a molecular uranium(II) complex, not the existence of a new element. The paper and contemporaneous reporting present it as a fundamental chemistry result rather than a material with an established practical application.
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Chemistry World reported that the compound remained stable at room temperature for several days as a solid and for about an hour and a half when dissolved in THF. Those are reported observations under the conditions described in that coverage, not a general stability guarantee for uranium(II) compounds.
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