One of the heaviest rings yet joins the ranks of aromatic molecules

Aromaticity—where electrons circulate around a ring to stabilize a molecule—is a central idea in chemistry, best known from organic compounds such as benzene. Chemists have also reported aromatic behavior in some all-metal clusters. But whether that picture holds for very heavy elements remains unclear.

Now researchers report a rare three-atom ring of bismuth that displays aromatic behavior in one of the heaviest such systems yet studied (Nat. Chem. 2026, DOI: 10.1038/s41557-026-02123-8).

For bismuth, forming simple rings is difficult. Instead, the atoms tend to assemble into larger clusters, making three-atom rings, which serve as simple models for studying aromaticity, hard to isolate and study.

To Stephen Liddle, an inorganic chemist at the University of Manchester and a corresponding author of the study, these systems are particularly interesting because they resemble heavier versions of well-characterized three-atom organic aromatic molecules such as cyclopropenium.

Even when such rings can form, he says, “their electronic behavior with respect to aromaticity has remained open to different interpretations.”

To address this uncertainty, the researchers synthesized complexes in which three bismuth atoms form a triangular ring held between uranium or thorium centers that stabilize the otherwise hard-to-isolate structure. X-ray measurements confirm a nearly perfect triangular arrangement, revealing a highly symmetric ring.

Calculations of the molecule’s magnetic properties show that electrons circulate continuously around the bismuth ring—a key signature of aromaticity. The resulting ring current was comparable to, and in some cases greater than, that of aromatic molecules such as benzene.

But more surprising than the aromaticity is its unusual nature, Liddle says. In systems such as benzene, electrons are locked into a stable circulating pattern. But in the bismuth ring, according to the team’s computational analysis, the electrons are less tightly bound and can be more easily rearranged, pointing to fundamental differences between light and heavy elements.

In addition, unlike classic cases, where the circulating electrons (typically π electrons) are separate from those that hold atoms together, in this case, the same electrons that form the bonds between the bismuth atoms (σ electrons) also contribute to the circulating current.

That distinction may not be entirely unexpected, says E. D. Jemmis, a theoretical chemist at the Indian Institute of Science who was not involved in the study, noting that similar behavior has been seen in other three-atom rings. He nonetheless says the work showcases “a good example of all-metal aromaticity.”

“The ingenious way of getting the right ligand-metal combination to stabilize the system stands out here,” he adds, a feat that has been notoriously hard to achieve.

Liddle says there is still considerable debate over how well traditional ideas of aromaticity apply to elements beyond the second row of the periodic table.

“We believe uncovering new systems like this will continue to develop our understanding of all-metal aromaticity,” helping to connect ideas from organic and inorganic chemistry and build a clearer picture of how aromaticity works.