Praseodymium powers up to +5

Pushing the known limits of lanthanide oxidation, chemists have isolated praseodymium (Pr) in the +5 oxidation state. While Pr⁵⁺ compounds have been observed in the gas phase and in noble gas matrix isolation conditions before, this is the first time chemists have been able to characterize one using X-ray crystallography. The finding suggests that chemists should shift how they situate lanthanides in the periodic table.

In some current periodic table arrangements, cerium sits beneath the group 5 metals. The existence of Pr⁵⁺ suggests that instead, its lanthanide neighbor praseodymium should be aligned underneath the group 5 metals, says Henry S. La Pierre of the Georgia Institute of Technology, who led the research effort with Ivan A. Popov of Washington State University and Bess Vlaisavljevich of the University of Iowa. “There might be other high valent lanthanides besides praseodymium, but our current structure of the periodic table is not necessarily pointing to which one is the next most logical step,” La Pierre says.

The researchers discovered the new Pr⁵⁺ compound while electrochemically analyzing a Pr⁴⁺ compound they’d made using imidophosphorane ligands (Nat. Chem. 2025, DOI: 10.1038/s41557-025-01797-w). “We were characterizing that material and also trying to understand why our yields were not that good, and we found we were essentially overoxidizing it,” La Pierre says.

Results from phosphorus nuclear magnetic resonance (NMR) spectroscopy showed coupling that could be explained only if they’d made Pr⁵⁺. “It was that one piece of NMR evidence that really allowed this project to come together,” La Pierre says.

Next, the researchers plan to use the same imidophosphorane ligands to attempt to access higher oxidation states in neptunium and plutonium compounds.

Tsinghua University’s Jun Li, who was part of a team that identified Pr⁵⁺ in the gas phase in 2016, says in an email that “the recent synthesis of Pr⁵⁺ compounds is a remarkable achievement in rare-earth chemistry, which fulfills the dream of chemists for pushing the limit of oxidation state since 1950s. I believe this landmark discovery will open a new avenue for high oxidation state chemistry of f-block heavy elements.”