Relativity Weakens the Chemical Bond in a Superheavy Element Compound

In a laboratory deep beneath the hills of Darmstadt, Germany, chemists have accomplished something that sounds almost impossible: they measured the strength of a single chemical bond in a compound built around atoms of seaborgium, element 106, one of the rarest and most short-lived substances humans have ever made. An international collaboration led by a research group at GSI/FAIR, the Helmholtz research center for heavy ion research, has reported the first measurement of the chemical stability of a superheavy element compound. The molecule in question is seaborgium hexacarbonyl, written Sg(CO)6, in which a single seaborgium atom is clasped by six carbon monoxide ligands. What the team found overturns a trend that chemists had every reason to trust: the bond between the seaborgium atom and its carbon monoxide ligand is weaker than expected from the element’s position in the periodic table, and the culprit is relativity itself. The results are published in the journal Nature.

To appreciate why this measurement is such a feat, it helps to consider what superheavy elements actually are. The elements with atomic numbers from 104 to 118 do not exist in nature in any usable quantity; they are manufactured atom by atom in nuclear fusion reactions at particle accelerators, at rates of at most a few atoms per day. Their lifetimes are measured in seconds at most, which means any chemistry performed on them must be breathtakingly fast and exquisitely sensitive. A single wrong design choice in the apparatus, and the handful of atoms produced vanish before anything can be learned. Yet the effort is worth it, because superheavy elements offer a unique window onto one of the strangest predictions of modern physics: that in the shadow of an enormous nuclear charge, the familiar rules of chemistry begin to bend.

The bending comes from relativistic effects. Because superheavy nuclei carry enormous positive charge, the electrons closest to the nucleus are accelerated to velocities approaching the speed of light. According to Einstein’s special theory of relativity, objects moving near that limiting speed become heavier. In the picture offered by Bohr’s model of the atom, this relativistic mass increase pulls the innermost electrons closer to the nucleus and binds them more tightly. Meanwhile, the outer electrons do not experience the same relativistic mass increase, but they are affected indirectly: the more tightly bound inner electrons shield the positive nuclear charge more effectively, so the outer electrons are actually pushed farther away. Both effects reshape the electronic structure of superheavy atoms, and because an element’s chemistry is dictated by its electronic structure, the chemical properties of superheavy elements can deviate substantially from those of their lighter homologs in the same periodic table group.

This is precisely what makes comparative studies so powerful. By measuring the same chemical property in a superheavy element and in its lighter homologs, scientists can test the fundamental underpinnings of the periodic table itself — the assumption that chemical behavior repeats in predictable patterns down each column. For the group-6 elements, that pattern seemed rock solid. In the hexacarbonyl complexes of chromium, molybdenum and tungsten, the bond between the central metal atom and the carbon monoxide ligands grows steadily stronger as the atomic number increases. Extrapolating that trend, one would expect seaborgium, sitting directly below tungsten, to form the strongest metal-carbonyl bond of the family. The new measurements show the opposite.

Dr. Alexander Yakushev of GSI/FAIR, spokesperson of the collaboration, explained the finding: For the first time, the team succeeded in measuring the strength of the chemical bond in a compound of a superheavy element, namely that between the central seaborgium atom and a carbon monoxide ligand, and then compared it to the strength of the same bond in the hexacarbonyl of the lighter homolog, tungsten. From chromium over molybdenum to tungsten, the bond strength in the hexacarbonyls increases, suggesting it should be even stronger in seaborgium. But the measurements show that the high atomic number of seaborgium alters its electronic shell structure, overturning the trend and leading to a bond weakening. Recent quantum-chemical calculations that fully account for the influence of relativity reproduce this effect, while simpler calculations that neglect relativity predict a continuation of increasing bond strength — a clean experimental verdict on which theoretical framework describes reality.

The compound itself has a short but notable history. Seaborgium hexacarbonyl was synthesized for the first time about ten years ago at the RIKEN Nishina Center in Japan, in an experiment led by scientists from Johannes Gutenberg University Mainz and GSI/FAIR. It was the first compound of the carbonyl class ever created for a superheavy element, and its well-defined bonding properties made it immediately attractive as a target for attempts to measure the strength of the bond between the central seaborgium atom and a carbon monoxide ligand. Those first attempts, however, remained unsuccessful. The gap between making a molecule once and quantifying its bond strength proved to be a decade wide, bridged only by refinements in synthesis, detection and data analysis.

The key to the experimental success was the combination of fast chemical synthesis with high yields and elaborate methods for registering the rapid radioactive decay that identifies each individual atom. In the experiment at GSI/FAIR, the researchers produced the isotope seaborgium-259, for which two nuclear states with different but very short lifetimes — both below half a second — are known. For comparison, the similarly short-lived tungsten isotopes 161 and 162 were produced under matching conditions. After production and isolation in the experimental setup TASCA, the TransActinide Separator and Chemistry Apparatus, the atoms came to rest in a gas atmosphere containing carbon monoxide. That gas served a double duty: it drove the formation of the carbonyl compounds and flushed all the atoms into a narrow detector channel equipped with two consecutive arrays that register each atom through its nuclear decay.

The detection scheme is an elegant piece of chemical logic. The products of the reaction of seaborgium and tungsten with carbon monoxide that have not formed the complete hexacarbonyl remain chemically reactive and deposit on the first detector array. Only the volatile hexacarbonyl complexes survive the journey past this first channel, traveling onward to a second detector array cooled to temperatures low enough to adsorb them. The efficiency with which atoms are transformed into the hexacarbonyl complex depends critically on the bond strength between the sixth carbon monoxide ligand and the central atom, so measuring the fraction of all atoms that reach the cold trap allows the bond strength to be deduced. In total, 75 seaborgium atoms were registered, of which just three arrived at the hexacarbonyl detector. By contrast, 17 percent of all tungsten atoms formed the analogous compound. From the much lower yield of Sg(CO)6 compared to W(CO)6, the team deduced a bond strength lower by 4 kilojoules per mole in the seaborgium compound — a small number in everyday chemistry, but a decisive one for a trend that had held across three elements.

With the trend from chromium to tungsten thus inverted at seaborgium, the experiment delivers one of the most direct demonstrations yet that relativistic effects are not a theoretical curiosity but a measurable force that rewrites chemical behavior at the bottom of the periodic table. Professor Christoph E. Düllmann, head of the superheavy element chemistry departments at GSI/FAIR and at the Helmholtz Institute Mainz and professor at Mainz University, emphasized that this breakthrough experiment would not have been possible without interdisciplinary and international collaboration between research institutions from around the world, and that it complements the international scientific programs at GSI and FAIR, where the origin of the elements in the universe and their properties are being investigated. The measurement also validates the entire methodology: single-atom chemistry, gas-phase synthesis, decay-tagged detection and quantum-chemical prediction all converged on a consistent picture.

The collaboration is not stopping at element 106. Having completed the first detailed study of chemical bonding in a molecule of a superheavy element, the researchers are already preparing experiments with yet heavier elements beyond seaborgium, where relativistic effects are expected to grow even stronger and the deviations from periodic-table expectations correspondingly more dramatic. Each such measurement constrains the models that describe how electrons behave in the most extreme atomic environments, connecting accelerator physics, quantum chemistry and the century-old logic of the periodic table. For now, the message from Darmstadt is strikingly simple: even a chemical bond, that most fundamental unit of chemistry, cannot escape the consequences of Einstein’s relativity — and in the realm of the superheavy elements, relativity wins.

Subject of Research: Relativistic effects on the chemical bond strength of seaborgium hexacarbonyl

Article Title: Chemical bond of a superheavy element compound probed at GSI/FAIR

Article References: Chemical bond of a superheavy element compound probed at GSI/FAIR. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: seaborgium, superheavy elements, relativistic effects, chemical bond, hexacarbonyl, GSI/FAIR, TASCA, carbonyl complex, quantum chemistry, periodic table, Nature, single-atom chemistry