Praseodymium in the Formal +5 Oxidation State

The pentavalent oxidation state of praseodymium – a long-sought connection between lanthanide, early-transition, and actinide metal redox chemistries – has only been identified in gas-phase or matrix isolation environments. We report the low-temperature characterization of a molecular praseodymium complex in the formal +5 oxidation state, [Pr5+(NP(tBu3)4][X–] (where tBu = tert-butyl and X– = tetrakis(pentafluorophenyl)borate or hexafluorophosphate). Single-crystal X-ray diffraction, solution-state spectroscopic, solution magnetometric, density functional theory, and multireference wavefunction-based methods indicate a highly multiconfigurational, singlet ground state. An inverted ligand field drives this unique electronic structure, which establishes a critical link in understanding the bonding of high-valent metal complexes across the periodic table.