Lanthanide Complexes Containing a Terminal Ln═O Oxo Bond: Revealing Higher Stability of Tetravalent Praseodymium versus Terbium

We report on the reactivity of gas-phase lanthanide-oxide nitrate complexes, [Ln(O)(NO3)3]- (denoted LnO2+), produced via elimination of NO2• from trivalent [LnIII(NO3)4]- (Ln = Ce, Pr, Nd, Sm, Tb, Dy). These complexes feature a LnIII-O• oxyl, a LnIV═O oxo, or an intermediate LnIII/IV oxyl/oxo bond, depending on the accessibility of the tetravalent LnIV state. Hydrogen atom abstraction reactivity of the LnO2+ complexes to form unambiguously trivalent [LnIII(OH)(NO3)3]- reveals the nature of the oxide bond. The result of slower reactivity of PrO2+ versus TbO2+ is considered to indicate higher stability of the tetravalent praseodymium-oxo, PrIV═O, versus TbIV═O. This is the first report of PrIV as more stable than TbIV, which is discussed with respect to ionization potentials, standard electrode potentials, atomic promotion energies, and oxo bond covalency via 4f- and/or 5d-orbital participation.