Relativity, Not Textbook Chemistry, Governs Triple Bonds in Heavy Atoms

Brown University chemists have produced direct experimental evidence that the standard textbook model of triple chemical bonds breaks down for heavy elements. Conventional theory describes a triple bond as one strong head-on sigma bond plus two weaker side-by-side pi bonds. That clean separation holds for light elements, but near the bottom of the periodic table the heavier nuclei force orbiting electrons to move at a substantial fraction of the speed of light, bringing relativistic effects into play. In this regime, an electron’s spin and orbit become coupled, smearing the boundary between sigma and pi bonds.

To test this, the team led by Lai-Sheng Wang built molecules from carbon and bismuth — a heavy element next to lead — cooled them to near absolute zero, and probed them with photoelectron spectroscopy, using a laser to eject electrons and measuring how tightly each was bound. The carbon-bismuth spectra did not match the expected one-sigma, two-pi pattern. Instead the bonding looks more like one pi bond and two hybrid sigma-pi bonds, the predicted signature of relativistic bonding. The idea that relativity matters in heavy-element chemistry dates to the 1970s, but this is direct spectroscopic confirmation rather than theory.

The finding, published in Science and funded by the NSF and DOE, could push a rewrite of how bonding is taught as heavy elements gain practical importance. Bismuth in particular is being explored as a non-toxic replacement for lead in next-generation solar cells and in quantum materials and quantum computing research, where understanding its actual bonding structure matters.