Establishing XANES signatures of rhenium oxidation states: a combined theoretical and experimental reference study for analytical applications

Abstract

X-ray absorption near edge structure (XANES) spectroscopy provides unique insights into the oxidation state and local coordination environment of transition metals, yet systematic reference datasets remain scarce. Here, we report a comprehensive experimental and theoretical study of Re L₃ and L₁ edge XANES spectra across oxidation states 0 to +7 (excluding +2), including oxides, halides, carbonyls, and polyoxometalates. The results demonstrate that the L₃ edge white line position is strongly affected not only by the formal oxidation state but also by ligand donor–acceptor properties, while the L₁ edge exhibits reduced ligand sensitivity with informative pre-edge features for Re(VII) species. Analysis of aqueous solutions reveals that polyoxorhenates persist in dilute aqueous solutions, underscoring the utility of XANES for solution speciation studies. Correlation of edge energies with coordination charge yields a more robust descriptor than formal oxidation state, enabling improved interpretation of spectral shifts. Density of states calculations further clarify the role of 5d-ligand orbital hybridization in shaping edge and post-edge features. Based on a substantial reference library of rhenium XANES data, this study establishes a new approach to interpreting L-edge XANES shifts, emphasizing the role of the ligand field, and provides generalizable strategies that can be applied to other polyvalent transition metals.