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In 2013, researchers 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 characterized the resulting crystalline salt using structural, spectroscopic, and computational evidence. This was a new molecular form of uranium chemistry—not the discovery of a new element.
What does uranium’s +2 oxidation state mean?
An oxidation state is a formal way chemists account for electron distribution in a compound. Uranium(II), often written U(II) or U2+, means uranium is assigned a formal oxidation state of +2 in the compound. It does not mean the isolated material consists of bare uranium ions: the reported substance is a molecular anion surrounded by cyclopentadienyl ligands, paired with a potassium counterion held by a cryptand.
The report by Matthew R. MacDonald, Megan E. Fieser, Jefferson E. Bates, Joseph W. Ziller, Filipp Furche, and William J. Evans appeared online on August 28, 2013, and in the September 11, 2013 issue of the Journal of the American Chemical Society. The authors described it as the first isolable molecular U2+ complex. Read the paper (DOI: 10.1021/ja406791t).
How did the researchers make uranium(II)?
The team started with tris(cyclopentadienyl)uranium, written Cp′3U, where Cp′ is C5H4SiMe3. They flash-reduced this uranium(III) precursor in a column of potassium graphite in the presence of 2.2.2-cryptand. The product was a crystalline salt containing the [Cp′3U]− anion and a potassium counterion held by the cryptand.
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The cryptand is part of the isolated salt’s environment: it binds the potassium counterion. The chemically significant uranium-containing component is the [Cp′3U]− anion, assigned formal oxidation state +2.
How did the authors distinguish the product from a uranium(III) 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 synthesized the hydride by adding potassium hydride (KH) to Cp′3U and also formed it by reducing hydrogen with the uranium(II) complex. They reported that the hydride was a different compound from the isolated uranium(II) salt.
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What is known about the complex’s electronic structure?
The formal oxidation state is not a complete description of how electrons occupy the uranium-containing anion. The authors’ density functional theory calculations assigned [Cp′3U]− a 5f36d1 quintet ground state. They reported that this computational assignment matched strong transitions observed in the optical spectrum. The 5f36d1 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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Why was the 2013 result unusual?
Uranium(II) compounds are challenging to isolate because uranium’s chemistry commonly involves higher oxidation states. The 2013 result established an experimentally accessible molecular uranium complex at formal +2, giving chemists a low-valent uranium platform to study. Its importance is fundamental inorganic chemistry; the cited sources do not establish an industrial or consumer application for this particular compound.
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Contemporaneous Chemistry World reporting said the solid remained stable at room temperature for several days, while the compound in tetrahydrofuran (THF) remained stable for about an hour and a half. Those are reported observations for this compound under the conditions described in that coverage, not general stability guarantees for uranium(II) compounds. Chemistry World’s 2013 report.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What the discovery does—and does not—establish
- It establishes: the isolation of a crystalline molecular uranium complex assigned formal oxidation state +2.
- It supports: the product’s identification through the distinction from a uranium(III) hydride, together with optical observations and the authors’ computational analysis.
- It does not establish: a consumer use, an industrial process, or a general rule that uranium(II) compounds have the reported stability.
The publisher identifies supporting information with additional experimental and crystallographic details. The paper’s indexed abstract and the cited contemporaneous report do not establish a yield, independent replication, or downstream application, so those should not be inferred from the discovery claim.
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