Pressure-Induced Spin Transition and Emergence of φ Bond 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 AnIII(COT)2-/AnIV(COT)2 (An = U, Np, Pu, COT = cyclooctatetraene) systems. Compression shortens An-C and C-C distances, strengthens electrostatic attraction (∆Eelec) and enhanced the covalent orbital interactions (∆Eorb) between An3+/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 pressures due to one electron flipping spin, enabling formation of previously inaccessible ϕ bonds. UIV(COT)2 even exhibit 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, highlight 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.