Pressure-induced spin transition and emergence of ϕ bonds in actinide sandwich complexes AnIII(COT)2−/AnIV(COT)2 (An = U, Np, Pu)

Abstract

Pressure can modulate the bonding properties of actinide compounds, enabling the formation of ϕ-type bonds through pressure-enhanced metal–ligand orbital interactions. Using relativistic density functional theory (DFT) calculations combined with bonding analysis, we investigated the effect of pressure (0–200 GPa) on An^III(COT)₂⁻/An^IV(COT)₂ (An = U, Np, Pu, COT = cyclooctatetraene) systems. Compression shortens An–C and C–C distances, strengthens electrostatic attraction (ΔE_elec) and enhances the covalent orbital interactions (ΔE_orb) between An^3+/4+ and ligands, evidenced by energy decomposition analysis (EDA). Quantum theory of atoms in molecules (QTAIM) analysis indicates significant increases in electron density ρ(r) and |V(r)/G(r)| of An–C bonds under pressure, evidencing enhanced covalency. Notably, the studied systems undergo spin transition under pressure due to one electron flipping spin, enabling the formation of previously inaccessible ϕ bonds. U^IV(COT)₂ even exhibits spin quenching at 80–120 GPa. Across all systems, ϕ orbitals show substantial 5f–2p mixing (26–79% An 5f), demonstrating strong covalent contributions. Np complexes exhibit the largest number of 5f-ϕ type orbital interactions, highlighting the interplay between actinide contraction and f-electron population, while trivalent species generally display higher 5f participation due to more diffuse orbitals. These findings offer a new avenue to design and modulate bonding properties in actinide chemistry.