Uranocenium – The bis-[8]annulene Uranium Cation

Uranocene, [U IV (η 8 -C 8 H 8 ) 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 (η 8 -C 8 H 8 ) 2 ] + , and demonstrate using electron paramagnetic resonance spectroscopy and magnetometry that this formal U(V) complex has a 5 f 1 ground state. The uranocene/uranocenium redox couple exhibits near-ideal electrochemical reversibility, and X-ray diffraction studies show minimal U-C bond length changes on oxidation. 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 offsets bond length contraction. This demonstrates a case where actinide electronic structure directs molecular structure and physical properties across both solid and solution phases.