Investigation of the Chemical Basis of the Temperature- and Environment-Dependent Color Change of Cobalt(II) Ions

Abstract

The temperature- and environment-dependent color change of cobalt(II) ions is a chemical process closely related to the structural characteristics and electronic behavior of the complexes they form. In aqueous media, the Co²⁺ ion predominantly forms the octahedral, pink-colored [Co(H₂O)₆]²⁺ complex. In this complex, the d-electrons of the cobalt ion split into specific energy levels under the field effect created by the water ligands, and the wavelength of the absorbed light results in the appearance of the pink color. As the temperature increases, the detachment of water molecules from the complex becomes easier, and the chloride ions present in the system form the tetrahedral, blue-colored [CoCl₄]²⁻ complex with Co²⁺. Since the ligand field splitting in this complex differs from that in the octahedral one, the absorption spectrum also changes, and consequently the solution turns blue. The endothermic nature of the equilibrium causes the reaction to shift to the right with increasing temperature, favoring the formation of the blue complex. The ionic strength of the medium, particularly the concentration of Cl⁻, significantly affects the rate and stability of this transition. In certain cases, pH may also influence the stability of the complexes. This color change is one of the remarkable processes that illustrates the fundamental principles of coordination chemistry and holds particular importance in the study of cobalt(II) ions.