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1Clear out junk files and repair common Windows errors2Scan for outdated or missing drivers - takes under a minute3Repair Windows errors before they cause bigger problemsSeaborgium’s bond to carbon monoxide is about 4 kJ/mol weaker than the corresponding bond in tungsten hexacarbonyl. That is the opposite of the trend seen from chromium through molybdenum to tungsten, and the 2026 result is the first direct bond-strength measurement in a superheavy-element compound. Relativistic changes to seaborgium’s electron shells explain why the periodic trend reverses.
What was measured in Sg(CO)6?
Seaborgium is element 106. In the compound seaborgium hexacarbonyl, written Sg(CO)6, one seaborgium atom is surrounded by six carbon-monoxide (CO) ligands. The key quantity is the strength of the bond to the sixth ligand: forming that sixth bond determines whether a newly created atom becomes a volatile hexacarbonyl molecule that can leave the production target.
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An international collaboration led by GSI/FAIR, with Alexander Yakushev as spokesperson, reported the measurement in the 2026 Nature paper “Observation of relativistic bond weakening in seaborgium hexacarbonyl” (DOI: 10.1038/s41586-026-11105-2). Relative to tungsten hexacarbonyl, the seaborgium–CO bond was inferred to be 4 kJ/mol weaker. The published reporting gives that difference but does not state a formal uncertainty interval in its accessible summary; the paper contains the detailed statistical and computational treatment.
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Why the result contradicts the usual group-6 trend
Chromium, molybdenum and tungsten occupy the same periodic-table group. Their hexacarbonyl bond strengths increase down that sequence, so a simple extrapolation would predict an even stronger bond for seaborgium, the next group-6 element.
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Seaborgium’s very high nuclear charge changes that extrapolation. Electrons close to the nucleus move at speeds where relativistic effects are significant. Their energies and spatial distribution change, which also changes the shielding experienced by outer electrons. The resulting shell structure is different from what a nonrelativistic extension of the periodic table suggests.
Fully relativistic quantum-chemical calculations reproduce the observed reversal: they predict a weaker seaborgium–CO bond than tungsten’s. Calculations that leave out relativity instead continue the normal strengthening trend. Relativistic and spin–orbit effects therefore are not a small correction here; they determine the direction of the trend.
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How scientists measured a bond with only a few atoms
The experiment had to combine chemical separation with nuclear-decay identification because seaborgium can be made only atom by atom and its useful isotopes disappear almost immediately. Seaborgium-259 has two nuclear states, each with a lifetime of less than half a second.
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One free scan finds every outdated or missing driver and matches the right update for your exact hardware.Free scan · exact hardware match- Produce the isotope. Researchers synthesized seaborgium-259 and short-lived tungsten isotopes for the comparison.
- Stop the products in gas. In the TASCA separator, recoiling atoms were slowed in an atmosphere containing carbon monoxide, allowing some to react and form carbonyl molecules.
- Flush products through detector channels. The gas stream carried reaction products toward two detector arrays.
- Separate by volatility. Nonvolatile products stuck to the first detector array. Volatile hexacarbonyl molecules continued to a second, cooled array, where they were adsorbed.
- Identify atoms by decay. Radioactive decay chains revealed where each atom had landed. The fraction arriving at the second array measured how often the sixth CO ligand formed.
Only 75 seaborgium atoms were registered, and just three reached the hexacarbonyl detector. In the tungsten comparison, 17% of the atoms formed the analogous hexacarbonyl. Those tiny counts make rapid transport and unambiguous decay tagging essential: an atom that decays before reaching the cooled array cannot be counted as a transported molecule.
What the numbers mean chemically
- Bond-strength comparison: Sg–CO is inferred to be 4 kJ/mol weaker than W–CO in the corresponding hexacarbonyls.
- Seaborgium yield: Three of 75 registered seaborgium atoms reached the detector used to identify volatile hexacarbonyls.
- Tungsten reference: 17% of tungsten atoms formed the analogous hexacarbonyl under the comparison conditions.
- Trend: Cr(CO)6, Mo(CO)6 and W(CO)6 show increasing bond strength down the group, whereas seaborgium reverses that progression.
The 4 kJ/mol value is an inferred relative weakening from formation and transport behavior, not a conventional bulk calorimetry measurement. There is no weighable sample of Sg(CO)6; the experiment extracts chemical information from where individual radioactive atoms are detected.
Why this matters for superheavy-element chemistry
Superheavy elements are often discussed as if their chemistry can be predicted by extending lighter-element patterns. This result supplies a direct test showing where that approach fails. Once relativistic shell changes become large enough, familiar periodic trends can bend or reverse.
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The work also demonstrates that chemistry can be measured in a regime of extreme scarcity and sub-second lifetimes. Volatility is not merely a convenient property in this field: it is the transport mechanism that separates a molecule from nonvolatile reaction products before the atom decays.
For future compounds, researchers can compare five linked features: the sixth-ligand bond strength, the fraction of atoms forming a hexacarbonyl, volatility and transport through the gas system, relativistic and spin–orbit contributions in calculations, and the practical losses imposed by atom scarcity and radioactive decay. Together these measurements can test whether other superheavy elements show similar departures from lighter homologues.
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What remains to be learned
The Nature article provides the full statistical analysis and computational details behind the reported comparison. The headline result establishes the direction and approximate size of the weakening, but further experiments with more atoms or related ligands would narrow the uncertainty and show how broadly the reversal applies. For now, seaborgium hexacarbonyl is the clearest experimental demonstration that relativistic electronic structure can overturn a periodic-table prediction.
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