A Combined Spectroscopic and Multivariate Analysis Approach for the Structural Characterization of Metal-Based Deep Eutectic Solvents: Choline Chloride and Cobalt Chloride Hexahydrate in Water Mixtures

Ultraviolet-visible (UV-vis) spectroscopy, Co K-edge X-ray absorption spectroscopy (XAS), and multivariate curve resolution (MCR) were combined to investigate cobalt speciation in the choline chloride/CoCl2·6H2O (1:2) metal-based deep eutectic solvent and its aqueous mixtures (1:2:W, W = 0-50). UV-vis spectroscopy provided information on coordination geometry, while XAS yielded quantitative structural information on the short-range environment of the metal center. MCR analysis of the XAS data set resolved overlapping spectral contributions and enabled identification of distinct cobalt species. Three complexes were identified: tetrahedral [CoCl4]2- (Co-Cl = 2.30(1) Å), mixed-ligand octahedral [CoCl(H2O)5]+ (Co-Cl = 2.34(1) Å; Co-O = 2.09(1) Å), and fully hydrated [Co(H2O)6]2+ (Co-O = 2.09(1) Å). Chloride-rich species dominate at low hydration, whereas increasing water content shifts the equilibrium toward hydrated octahedral complexes through progressive ligand substitution. These results provide a quantitative description of hydration-driven changes in cobalt coordination and demonstrate that the combined XAS-MCR approach is effective for resolving metal speciation in complex eutectic and multicomponent liquid systems.