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)(NO)] (denoted LnO), produced via elimination of NO from trivalent [Ln(NO)] (Ln = Ce, Pr, Nd, Sm, Tb, Dy). These complexes feature a Ln-O oxyl, a Ln═O oxo, or an intermediate Ln oxyl/oxo bond, depending on the accessibility of the tetravalent Ln state. Hydrogen atom abstraction reactivity of the LnO complexes to form unambiguously trivalent [Ln(OH)(NO)] reveals the nature of the oxide bond. The result of slower reactivity of PrO versus TbO is considered to indicate higher stability of the tetravalent praseodymium-oxo, Pr═O, versus Tb═O. This is the first report of Pr as more stable than Tb, 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.