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 3 ) 3 ] - (denoted LnO 2+ ), produced via elimination of NO 2 • from trivalent [Ln III (NO 3 ) 4 ] - (Ln = Ce, Pr, Nd, Sm, Tb, Dy). These complexes feature a Ln III -O • oxyl, a Ln IV ═O oxo, or an intermediate Ln III/IV oxyl/oxo bond, depending on the accessibility of the tetravalent Ln IV state. Hydrogen atom abstraction reactivity of the LnO 2+ complexes to form unambiguously trivalent [Ln III (OH)(NO 3 ) 3 ] - reveals the nature of the oxide bond. The result of slower reactivity of PrO 2+ versus TbO 2+ is considered to indicate higher stability of the tetravalent praseodymium-oxo, Pr IV ═O, versus Tb IV ═O. This is the first report of Pr IV as more stable than Tb IV , 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.