Cobalt(II) complex formation with nitrate and chloride anions in the [C4mim][Tf2N] ionic liquid

This study investigates the coordination chemistry and thermodynamics of cobalt(II) complex formation with nitrate (NO₃ − ) and chloride (Cl − ) anions in the dry ionic liquid [C 4 mim][Tf 2 N]. Using UV–visible spectrophotometry, isothermal titration calorimetry (ITC), density functional theory (DFT) calculations, and molecular dynamics (MD) simulations, we elucidate the stability, structure, and energetics of the formed complexes. Spectroscopic and computational results reveal that cobalt(II) maintains octahedral coordination with nitrate, forming stepwise 1:1, 1:2, and 1:3 complexes, with a progressive displacement of [Tf 2 N] − anions. In contrast, chloride coordination induces a transition from octahedral (1:1) to a mixed tetrahedral/penta-coordinated (1:2) geometry, ultimately stabilizing tetrahedral 1:3 and 1:4 species. Thermodynamic analysis highlights the central role of solvation effects and ionic liquid restructuring in determining complex stability. These findings enhance the understanding of cobalt coordination in Tf 2 N-based ionic liquids and constitute a contribution to the understanding of fundamental phenomena with implications for metal recovery. • Co(II) forms highly stable complexes with NO₃ − and Cl − in [C₄mim][Tf₂N]. • Co(II) forms up to three octahedral species with NO₃ − . • When the 1:2 Co:Cl species a switch from octahedral to tetrahedral geometry occurs. • DFT calculations provided the structures of the possible species present. • Simulations reveal a remarkable restructuring of the ionic liquid upon complexation.