Electronic Structure, Covalency, and Magnetic Anisotropy in [AnCp3] (where An = Th-Cf) Complexes: Insights from First Principle Calculations.

Understanding the nature of actinide-ligand bonding, covalency, and magnetic properties is a burgeoning research topic in the field of actinide chemistry. In the present manuscript, we have thoroughly investigated the electronic structure, magnetic anisotropy of nine [AnCp₃] complexes (An = Th(III)-Cf(III)) using scalar relativistic density functional theory (SR-DFT) and complete active space self-consistent field (CASSCF) method to shed light on the nature of actinide-ligand covalency, with particular emphasis on the role of 5f versus 6d covalency in the bonding. DFT and CASSCF calculations predict 6d¹(Th), 5f¹6d¹ (Pa), and 5fⁿ configurations for U-Cf analogues. A range of computational methods, including molecular orbital (MO), natural population analysis (NPA), natural localized molecular orbital analysis (NLMO) analysis, and ab initio ligand field theory (AILFT) were used to elucidate the orbital-driven and energy-driven component in describing the 5f-ligand covalency in [AnCp₃] complexes. DFT calculations highlight dominant 6d-covalency for earlier actinides, while dominant 5f-covalency in heavier actinides and, importantly, underscores the emergence of energy-driven covalency in delineating trends in the 5f-ligand covalency in [AnCp₃] complexes. CASSCF calculations with ligand orbitals in active space nicely reproduce the experimental g-shifts and magnetic susceptibility, thereby highlighting the importance of 5f-ligand covalency in describing the magnetic properties of [AnCp₃] complexes.