EPR and Angle-Selected ENDOR Study of 5f-Ligand Interactions in the [U(η⁷-C₇H₇)₂]⁻ Anion, an f¹ Analogue of Uranocene

A frozen solution of [U(η 7 -C 7 H 7 ) 2 ] - is studied by EPR and angle-selected ENDOR spectroscopy. EPR confirms the anticipated f 1 configuration of this compound. Analysis of the g tensor indicates that the ground state molecular orbital is principally made of 5f π (51%) and 5f σ (39%) uranium orbitals. This composition results from the combined effect of the spin−orbit interaction and the strong f δ -ligand interactions. The small admixture of 5f φ (9%) uranium orbitals could result from a low-symmetry effect imposed by the solid-state packing forces at low temperature. Starting from the ground-state orbital, the f-level ordering consistent with the g tensor is found to be (f π,f σ ) < f φ < f π < f φ, (f σ,f π ) < f δ (antibonding). Proton ENDOR spectroscopy shows that there is a positive spin density ρ π ≥ 4 × 10 - 2 in 2p π carbon orbitals, which manifests the covalent character of the ground-state orbital.