The Highest Oxidation States of the Rare-Earth Elements and the Challenge of Praseodymium(+5) Compounds

Oxidation-State is one of the most important heuristic concepts in modern practical and theoretical chemistry. Its exploration deepens our understanding of both this concept as well as some fundamental properties of the element families. However, the well-defined oxidation state of the IUPAC leads to discussions in borderline cases, where conceptual clarity and detailed knowledge of the electronic structure are required. Our review focuses on the high oxidation states of the intriguing case of the 17 rare-earth elements (REEs: Sc, Y, La─Lu), with particular attention to the + 5 oxidation state in some new praseodymium compounds. We provide a comprehensive overview from early speculation to the first identification of Pr(+5) in the gas phase to its recent molecular and solid compounds. We explain why high-valent REE compounds are so difficult to synthesize, from the perspective of atomic 4f valence orbital radii and energies. The predicted and synthesized Pr(+5) compounds with F, O, or N ligands pave the way for the development of new complexes and extended periodic networks in the field of highly-oxidized rare-earth chemistry. Our aim is to summarize current knowledge, distill emerging insights, and offer a roadmap for future investigations into the higher oxidation states of these elements.