Lanthanide orbitals spring a reaction surprise

The lanthanide elements’ 4f electron orbitals are notoriously shy and retiring, and they were long thought to be unwilling partners in chemical bonding. Now chemists have brought out a surprising extrovert streak in the orbitals, in what they believe is the first clear example of 4f bonding actually steering the course of a reaction (Nat. Chem. 2025, DOI: 10.1038/s41557-025-01791-2).

“As the evidence began to mount, I got really excited because I've been looking for something like this for basically my whole independent career,” says Eric J. Schelter of the University of Pennsylvania, part of the team behind the work. In principle, the discovery could help to develop lanthanide catalysts or reveal better ways to separate these critical elements.

Found at the heavier end of the periodic table, the lanthanides are the first elements to carry electrons in their 4f orbitals. Chemical dogma suggests that these orbitals are not keen on forming bonds because they lie relatively close to the atomic nucleus and don’t tend to overlap much with the orbitals of other atoms. But in recent years, researchers have created various coordination complexes that encourage 4f orbitals to make a modest contribution to covalent bonds.

In the latest work, Schelter and his colleagues have gone one step further by showing that 4f orbitals can play an essential role in guiding the reaction of a molecule attached to a lanthanide.

The researchers made a series of complexes from titanium, zirconium, hafnium, cerium, and thorium, all with the same basic structure. In each complex, the metal atom is cradled by a nitroxide-based ligand and binds to another molecule containing a cyclopropene ring. But only in the cerium complex could the cyclopropene undergo a ring-opening reaction to form an allene.

Unusually, the researchers studied this reaction inside a single crystal of the complex. “That was essential here because the product is actually unstable in solution,” Schelter says. The team used X-ray crystallography to take structural snapshots every 4 h for 14 days and saw a gradual, irreversible shift from cyclopropene to allene in the cerium complex.

Theoretical calculations suggest that the cerium’s 4f orbital has just the right energy and spatial position to stabilize an intermediate in this reaction, which features a cerium-carbon double bond.

Thomas Albrecht of the Colorado School of Mines, who was not involved in the research, agrees that the results suggest that the 4f orbital is playing a key role in the reaction. “There have been hints of this for decades, but this is the only definitive example,” he says. “It's outstanding.”

Lanthanides are widely used in high-tech applications, from smartphones to electric vehicles, but the elements are famously difficult to separate from one another. Conventional separations often use specially designed ligands and copious amounts of organic solvents to extract the metal ions from water. A better understanding of 4f bonding could help chemists to accentuate differences between lanthanides, Albrecht suggests, enabling more efficient separation of the elements from ores and waste.

Meanwhile, Schelter points out that metal-carbon multiple bonds are important intermediates in a range of catalytic reactions. The team now hopes to further stabilize and study the cerium-carbon double bond, to explore whether it could be exploited in a catalyst.

Albrecht adds that 4f orbitals may already be secretly influencing many other reactions involving lanthanides, particularly those in the earlier part of the series. “I think this opens the door for other people to look if it’s involved,” he says. “This is one of those important papers that I think will lead the way to a lot of other discoveries.”