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

High Resolution Image Download MS PowerPoint Slide 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/CoCl 2 ·6H 2 O (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 [CoCl 4 ] 2– (Co–Cl = 2.30(1) Å), mixed-ligand octahedral [CoCl(H 2 O) 5 ] + (Co–Cl = 2.34(1) Å; Co–O = 2.09(1) Å), and fully hydrated [Co(H 2 O) 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.