High-energy resolution x-ray spectroscopy reveals long-sought bonding characteristics of La$^{3+}$ homologues for 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 \textit{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 4\textit{f} and 5\textit{d} 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 4\textit{f} and 5\textit{d} orbitals. ... mehrQuantum chemical modeling, including ligand-field density-functional theory (LFDFT) and \textit{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 \textit{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.