High-energy resolution X-ray spectroscopy reveals bonding characteristics of La3+ homologues of actinium radiopharmaceuticals

This is particularly relevant for f-elements, where bonding interactions play crucial role, particularly in radiopharmaceutical developments. We present advanced spectroscopic analysis, including core-to-core resonant inelastic X-ray scattering (CC-RIXS) and high-energy resolution X-ray absorption near-edge structure (HR-XANES), to investigate metal-ligand interactions using lanthanum (La) as a non-radioactive homologue of actinium (Ac) applied in emerging and highly potent therapeutic radiopharmaceuticals.

By analyzing the interplay between La 4f and 5d orbitals in various environments, we extract key information about ligand-field effects and bond covalency. Our findings demonstrate that spectroscopic features of the La L$_2$-edge CC-RIXS map reflect the nephelauxetic effect, which arises from central-field 4\textit{f} orbital-specific bond covalency. The energy separation between the pre-edge and main absorption edge of the La L$_2$-edge HR-XANES spectra also serves as direct probe of electron density for both 4f and 5d orbitals.