Seaborgium Carbonyl Reveals Relativistic Bond Weakening in the Heaviest Elements

Chemists probing the far edge of the periodic table have, for the first time, measured the strength of a chemical bond in a compound of a superheavy element and found exactly what Einstein’s relativity predicts: the bond is weaker than it should be. In a landmark experiment at the GSI Helmholtzzentrum für Schwerionenforschung in Darmstadt, Germany, an international team synthesized seaborgium hexacarbonyl, Sg(CO)6, one atom at a time, and determined the energy needed to tear a single carbon monoxide ligand away from the metal. The result, published in Nature, confirms a decades-old theoretical prediction that relativistic effects destabilize the valence orbitals of seaborgium and thereby reverse a smooth chemical trend that holds for every lighter member of its group.

Seaborgium, element 106, sits deep in the seventh row of the periodic table among the superheavy elements, those with atomic numbers of 104 or greater. These elements do not exist in nature; they must be forged in accelerator experiments by slamming heavy ions into thin targets, and they decay within seconds or less. Yet their chemistry matters. According to the abstract of the study, the chemical properties of superheavy elements are unique because strong relativistic effects reshape their valence electron shells, and the stability of metal–ligand bonding in carbonyl complexes was predicted to be particularly sensitive to such effects. Measuring that stability directly has been one of the most demanding goals in modern nuclear chemistry.

The physics behind the prediction is subtle. In very heavy atoms, electrons in the innermost shells move at speeds approaching a substantial fraction of the speed of light. Relativistic effects contract and stabilize the 7s and 7p1/2 orbitals, which is why elements such as copernicium and flerovium behave as surprisingly volatile, nearly inert metals. But there is an indirect consequence as well: the contracted inner shells screen the nucleus more effectively, destabilizing and expanding the 6d orbitals that dominate the bonding of the 6d transition series. In seaborgium, those destabilized 6d orbitals participate in bonding with carbon monoxide ligands, and theory predicted this would weaken the metal–CO bond.

Chemical bonding in hexacarbonyls is a textbook story of two competing electron flows. Density functional studies of the lighter homologues chromium, molybdenum and tungsten showed that the metal–CO bond forms through σ-donation, in which the carbon monoxide ligands push electron density toward the metal’s d orbitals, and π-back donation, in which the metal pushes electron density back into the ligands’ π orbitals. Earlier calculations by Nash and Bursten in 1999 suggested the first bond dissociation energy, the energy required to detach one CO ligand, should actually increase slightly from tungsten to seaborgium, because stronger back donation would overcompensate weaker σ-donation. More recent relativistic calculations by Iliaš and Pershina disagreed, predicting a decrease of roughly 10 kilojoules per mole driven by the relativistic destabilization of the seaborgium 6d orbitals. The experiment was designed to settle the dispute.

The detection scheme exploited the dramatic difference in volatility between complete and incomplete carbonyl complexes. Gas flushed the reaction products into two detector arrays connected in series. The first, miniCOMPACT, was kept at room temperature and covered with silicon dioxide; highly reactive, non-volatile fragments such as bare atoms and unsaturated carbonyl species M(CO)n with one to five ligands stick there immediately on first contact. Only the chemically inert, volatile hexacarbonyls survive the journey and pass through a capillary into the second array, COMPACT, whose gold-covered detector surfaces were chilled along a temperature gradient down to about 110 kelvin. Each detector panel consists of silicon photodiodes that register the alpha particles and fission fragments emitted when a radioactive atom decays, allowing the researchers to identify individual atoms of seaborgium and its decay daughter rutherfordium-255 by their characteristic decay chains.

The statistics were, by any ordinary chemical standard, vanishingly small. In the first array the team registered 126 decay chains, of which 72 began with the decay of seaborgium-259 and 54 with its rutherfordium daughter, a pattern that also allowed them to estimate a gas flush-out time of about 250 milliseconds. In the second, colder array, only three decay chains starting from seaborgium appeared, and these were assigned to Sg(CO)6 molecules that had adsorbed on the gold surface. Accounting for Poisson statistics, the three events correspond to a confidence range of roughly 1.4 to 6.3 events. Tungsten-161 and tungsten-162, produced in a shorter companion run using a tin-112 target, served as the chemical reference: about 20 percent of the tungsten events appeared in the cold array as volatile W(CO)6, compared with roughly 4 percent for seaborgium.

That ratio is the key to the headline result. Because both hexacarbonyls formed under identical conditions of reaction time and carbon monoxide concentration, the ratio of their volatile fractions equals the ratio of the rate constants for the final complex-formation step, and via the Arrhenius equation this ratio yields the difference in first bond dissociation energies. The analysis showed the seaborgium value to be 4(2) kilojoules per mole lower than that of tungsten. Taking the established experimental value of 192(8) kilojoules per mole for W(CO)6, the team arrived at a first bond dissociation energy of 188(9) kilojoules per mole for Sg(CO)6. The steady increase in bond strength from chromium through molybdenum to tungsten, seen across the whole group, is thus reversed at seaborgium, precisely as the most advanced relativistic calculations predicted. The measured decrease is somewhat smaller than the roughly 10 to 15 kilojoules per mole calculated by Iliaš and Pershina, but the direction of the trend, the essential signature of the relativistic effect, is fully confirmed.

The experiment also delivered a second thermochemical quantity. By comparing the positions where the hexacarbonyl molecules deposited along the chilled gold surface with Monte Carlo simulations based on a mobile adsorption model, the researchers determined an adsorption enthalpy of Sg(CO)6 on gold of 45 (+4/−6) kilojoules per mole, alongside 40(5) kilojoules per mole for W(CO)6, consistent with earlier gold-surface measurements. Both values are several kilojoules per mole lower than the corresponding adsorption enthalpies on silicon dioxide reported in the pioneering 2014 study that first synthesized a seaborgium carbonyl complex at RIKEN, and they indicate that deposition on both surfaces occurs through weak physisorption rather than chemical attachment.

Beyond settling a theoretical controversy, the work establishes a sensitive new method for single-atom chemistry. Because the technique compares fractional yields of two complexes formed simultaneously under identical conditions, it extracts bond energies even from a handful of detected atoms, something earlier breakthrough-curve approaches could not achieve with the available seaborgium statistics. The authors note that the same principle, applied to mass-resolved carbonyl fragments, could in future experiments determine individual bond dissociation energies for specific fragments, including species of odd-numbered elements in which only mononuclear complexes can exist. As superheavy element research pushes toward copernicium, nihonium and beyond, the ability to measure a real physical observable, a bond energy, atom by atom, opens a pathway for testing how Einstein’s relativity rewrites the rules of chemistry at the bottom of the periodic table.

Subject of Research: Relativistic effects on the metal–carbon monoxide bond strength of seaborgium hexacarbonyl measured by single-atom gas chromatography

Article Title: Observation of relativistic bond weakening in seaborgium hexacarbonyl

Article References: Yakushev, A., Khuyagbaatar, J., Düllmann, C. E., Pershina, V., Schädel, M., Ballof, J., Bartl, P., Block, M., Cantemir, R.-A., Dietzel, D., Giacoppo, F., Hermainski, K., Herzberg, R.-D., John, J., Krier, J., Kurz, N., Löchner, S., Maiti, M., Mošať, P., … Wieczorek, P. (2026). Observation of relativistic bond weakening in seaborgium hexacarbonyl. Nature, 658(8135), 337-341. https://doi.org/10.1038/s41586-026-11105-2

Image Credits: AI Generated

DOI: 10.1038/s41586-026-11105-2

Keywords: seaborgium, superheavy elements, relativistic effects, carbonyl complexes, bond dissociation energy, gas chromatography, nuclear chemistry, heavy ion fusion, GSI, actinide chemistry, quantum chemistry, periodic table