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.