Isolation of uranocenium, the bis-[8]annulene uranium cation

Uranocene, [U^IV(η⁸-C₈H₈)₂], is the actinide equivalent to ferrocene. While ferrocene is readily oxidised to the ferrocenium cation, higher oxidation states of uranocene are unknown. Here we report the elusive uranocenium cation, [U^V(η⁸-C₈H₈)₂]⁺, and demonstrate using electron paramagnetic resonance spectroscopy and magnetometry that this formal U(V) complex has a 5 f ¹ ground state. Enhanced uranium 5 f and 6 d mixing into filled δ-symmetry bonding orbitals in uranocenium, compared to uranocene, amplifies interorbital electron-electron repulsion, and counteracts bond length contraction, which minimises structural change. This manifests in solution where the uranocene/uranocenium redox couple exhibits near-ideal electrochemical reversibility, and in the solid-state where X-ray diffraction studies show minimal U-C bond length change upon oxidation. This demonstrates a case where actinide covalency governs molecular structure and physical properties across both solid and solution phases.