High-energy resolution X-ray spectroscopy reveals bonding characteristics of La$^{3+}$ homologues of actinium radiopharmaceuticals

Spectroscopic techniques are essential for accurately probing the electronic structures of coordination compounds and revealing the nature of their chemical bonding. 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 4f 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. ... mehrQuantum chemical modeling, including ligand-field density-functional theory (LFDFT) and ab initio bond analysis, complements our experiments. This allows us to establish a direct correlation between spectroscopic observables and theoretical metrics for bonding properties, offering a framework to understand the coordination chemistry of f-elements. Beyond advancing fundamental chemistry, our findings will also inform future studies on Ac$^{3+}$-radiopharmaceutical agents, where precise knowledge of bonding interactions is essential for their development.