Uranocenium isolated: uranocene’s missing cation, 58 years on

Chemists in Manchester pulled one electron off uranocene, a uranium “sandwich” molecule first made in 1968, and got crystals of uranocenium that are stable at room temperature, yet its uranium–carbon bonds shortened by only about 0.02 Å.

Peer reviewed · Laboratory synthesis and measurements · One research group, two related compounds

Uranocene, a uranium atom wedged between two flat rings of eight carbon atoms, has finally given up an electron in a form chemists can isolate as crystals. Cameron Deakin, Conrad Goodwin and colleagues at the University of Manchester, working with Andrew Kerridge at Lancaster University, report crystals of the cation, uranocenium, in Nature Communications. It completes a 58-year-old comparison with ferrocene, the iron sandwich chemists use as a voltage reference. The catch: the cation survives only with carefully chosen solvents and salts, and the +5 oxidation state assigned to uranium is a formal label.

Evidence at a glance

Is this a big deal?

For chemists, a solid and overdue step; for everyone else, a curiosity. Isolating the cation fills a gap open since the 1970s, and the evidence that it exists is strong. Why its shape hardly changes is a calculated explanation that other groups will now test.

What scientists found

A sandwich compound holds a metal atom between two flat rings of carbon. Ferrocene is the famous one: iron between two five-carbon rings, whose discovery fed into the 1973 Nobel Prize in Chemistry. Uranocene uses uranium and bigger rings. Andrew Streitwieser and Ulrich Müller-Westerhoff reported it in 1968, and their paper’s title presented it as a new class of sandwich built on atomic f orbitals.

Those f orbitals are why chemists care. Electrons occupy regions called orbitals, labelled s, p, d and f by shape (our guide to the parts of an atom covers the basics). Bonding in f-block elements is often highly ionic, the paper notes: an electrical attraction between charged pieces. Uranocene was one of the first molecules to show uranium’s 5f orbitals joining in chemical bonding.

Ferrocene loses an electron easily to become ferrocenium, a positive ion or cation. Labs use that clean switch as a voltage reference. Uranocene’s matching cation had appeared only in theory or as a minor gas-phase product. The team now has it as crystals that the paper says are stable indefinitely at room temperature in suitable solvents.

Its switch is as clean as ferrocene’s. At a scan rate of 1,000 mV per second, the gap between oxidation and reduction peaks was 100 mV for uranocenium and 280 mV for ferrocene; a smaller gap means a cleaner switch.

How they found it

The first hint was an accident. In 2025, Goodwin writes in a blog post, postdoctoral researcher Liam Griffin was testing a trityl salt on rare-earth compounds, with a silyl-substituted uranocene in the same reaction. The rare-earth compound was untouched. The uranocene was oxidised.

Attempts in the late 1970s and early 1980s had used a salt, [nBu₄N][PF₆], to carry current through the solution. The team found that uranocenium pulls fluoride out of the PF₆⁻ ion and falls apart, and detected the phosphorus fluoride by-products by NMR spectroscopy. A borate salt gave clean, reversible electrochemistry instead.

EPR spectroscopy at 5 K, which detects unpaired electrons, and magnetometry both fit a single unpaired 5f electron, as expected for uranium(V). The work was done in argon gloveboxes in a dedicated laboratory with contamination monitoring; the paper’s safety note describes natural uranium as a weak alpha emitter with radiotoxic and heavy-metal hazards.

The surprise was the size. Uranium’s ion shrinks by 0.13 Å from +4 to +5, using the six-coordinate values the paper cites. The average uranium–carbon distance shrank by just 0.018 Å in the parent cation and 0.023 Å in the silyl version, a seventh to a sixth as much. That is about two trillionths of a metre.

Calculations offer the reason. Picture a cushion: the more highly charged uranium presses the rings harder, but extra shared electrons pile into the space between uranium and carbon and push back, because electrons repel each other. Uranium’s calculated 5f share of the two key bonding orbitals rose from 28.1% and 33.7% in uranocene to 42.7% and 43.0% in uranocenium. The cushion picture has a limit: the balance is an energy result from quantum chemistry, not a measured force.

Why it matters to you

This won’t change anything in your life, and neither the paper nor the release claims a practical use. Uranium is best known for splitting its nucleus; this study is about its electrons and how they bond. The payoff is a cleaner test bed for theories of heavy-element bonding.

What the evidence does not show

The +5 label is formal. Some chemists, dividing bonding electrons differently, describe a related uranium sandwich with seven-carbon rings as uranium(III), the authors note. They report that their uranium(V) and uranium(III) compounds behave entirely differently, and leave the deeper question to further spectroscopy.

The parent cation’s bond change, 0.018(16) Å, is barely larger than its uncertainty; the paper says the changes “either do not, or barely, reach statistical significance.” Calculated orbital energies also shift with the method used. And stability is conditional: in tetrahydrofuran, both cations start polymerising the solvent and revert to uranocene.

How this fits what we already knew

Eight-carbon-ring sandwiches were known for thorium, uranium, neptunium, plutonium and berkelium, but only with the metal at +3 or +4. The name “uranocenium” was used in 2019 by F.-S. Guo, Richard Layfield and colleagues for a different cation: uranium(III) between two five-carbon rings. The new compound is the oxidised partner of uranocene itself.

Independent perspective

SciQuest found no on-record comment from a chemist outside the author team as of 25 October 2026, 11:14 UTC. The journal names Dumitru-Claudiu Sergentu as one peer reviewer; SciQuest has not assessed the published peer review file.

How other headlines framed it

• The University of Manchester’s release led on “more than 50 years” and the idea that electron sharing holds the molecule’s shape.
• Phys.org carried the same release, calling it a “sandwich molecule” rather than a cation.
• Goodwin’s blog post told the accidental-discovery story and stressed completing the ferrocene analogy.

The release says the bonds shrank “several times less” than ion size predicts; SciQuest gives the paper’s values and uncertainty instead.

What happens next

The team plans to explore what uranocenium reacts with and to test whether heavier elements such as neptunium form similar cations. For another way actinides are making news, see the thorium nuclear clock.

In one sentence

Uranocene can lose an electron after all, and the resulting uranium(V) sandwich keeps almost the same size because uranium shares more of its electrons with the carbon rings.

Read next

• What are the parts of an atom? The electrons behind this chemistry.
• What happens if you split an atom? Uranium’s nuclear side.
• Thorium nuclear clock runs for a day: another actinide at the frontier.

Sources and disclosure

1. Deakin CN, Turcas I, Réant BLL, Griffin LP, Seed JA, Brookfield A, Collison D, McInnes EJL, Kerridge A, Goodwin CAP. “Isolation of uranocenium, the bis-[8]annulene uranium cation.” Nature Communications 17: 10914, published 23 October 2026. doi:10.1038/s41467-026-77066-2.
2. Streitwieser A Jr, Müller-Westerhoff U. “Bis(cyclooctatetraenyl)uranium (uranocene). A new class of sandwich complexes that utilize atomic f orbitals.” Journal of the American Chemical Society 90: 7364 (1968). doi:10.1021/ja01028a044.
3. Deakin CN et al. “Uranocenium – The bis-[8]annulene Uranium Cation.” ChemRxiv preprint, 11 February 2026. doi:10.26434/chemrxiv.15000008/v1.
4. University of Manchester news release, “Elusive uranium ‘sandwich’ cation isolated after more than 50 years”, 23 October 2026.
5. Goodwin C. “A Charged Uranium Sandwich”, Springer Nature Research Communities, 23 October 2026.
6. Guo F-S, Chen Y-C, Tong M-L, Mansikkamäki A, Layfield RA. “Uranocenium: Synthesis, Structure, and Chemical Bonding.” Angewandte Chemie International Edition 58: 10163 (2019). doi:10.1002/anie.201903681.

Disclosure: This article was drafted with AI assistance and checked against the sources above, including the full open-access paper. The authors declare no competing interests; funders include the Royal Society, the European Research Council and EPSRC. Calculations by SciQuest: 0.018 ÷ 0.13 ≈ 0.14 and 0.023 ÷ 0.13 ≈ 0.18 (a seventh to a sixth); 58 years is 2026 minus 1968. The graphic is a SciQuest original built from the paper’s reported values. Spotted an error? See our corrections policy.