New study discovers unexpected role of 4f-orbital covalency in driving chemical reactivity

The willingness of the 4f orbitals of lanthanide metals to participate in chemical reactions is as rare as their presence in Earth’s crust. A recent study, however, witnessed the 4f orbital in a cerium-based compound actively participate in bond formation, triggering a unique chemical reaction.

The researchers observed that a cerium-containing cyclic complex formed a 4f-covalent interaction, leading to a ring-opening isomerization from cyclopropene to allene. The findings are published in Nature Chemistry.

Lanthanides are heavy, rare-earth metallic elements, occupying positions 57 through 71 in the periodic table—from lanthanum to lutetium—and are widely used in modern technologies ranging from electronics to clean energy. In nature, these elements are usually found together in their ore form and separating them using current methods is extremely challenging and energy-intensive. Understanding how these elements bond or interact with other atoms at an electronic level could help us to distinguish between lanthanides and design effective separation strategies.

Years of research have established that the 4f orbitals of lanthanides are generally reluctant to participate in chemical reactions. Deep-seated and shielded by the outer 5s and 5p orbitals, the 4f orbitals lie close to the atomic nucleus, making it difficult for them to overlap with orbitals of other atoms and form chemical bonds.

Recent studies have shown the involvement of 4f and 5f orbitals in coordination chemistry, where a central metal atom bonds with a ligand—an ion or molecule that donates electrons to the metal atom—to form coordination complexes. However, scientists haven’t been able to find evidence that could clearly indicate how 4f-orbital covalency influences the reactivity of these compounds.

To investigate the same, the researchers synthesized a series of tetravalent metal (M⁴⁺)–cyclopropenyl complexes, where M is either titanium (Ti), zirconium (Zr), cerium (Ce), hafnium (Hf), or thorium (Th). Each complex consisted of a common cyclopropene-based ligand framework, and three nitroxide ligands that formed a scaffold around the metal centers.

They found that among all the complexes, only the cerium (Ce⁴⁺) complex underwent a single-crystal-to-single-crystal ring-opening isomerization reaction to form a cerium–allenyl complex.

The isomerization process by which the original cerium complex molecule transformed into another molecule of the same chemical composition, but a different structure or configuration, was captured by the team using single-crystal X-ray diffraction. These observations, alongside theoretical calculations, suggest that the cerium’s 4f orbital participates significantly in the bonding of the reactive intermediate and stabilizes it to facilitate the ring-opening process.

This comparative study of chemical reactivity among a series of isostructural and isoelectronic d- and f-block complexes demonstrates that 4f-orbital covalency can lead to distinct chemical reactivity. The researchers note that these findings can open the door for further exploration of orbital covalency effects in molecular compounds, especially in solid-state chemical transformations.

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