Thermochromic and Chromotropic Phenomena of Cobalt(II) Chloride Solutions and Related Systems

The pink color of an aqueous solution of cobalt(II) chloride is known to be due to that of the 6-coordinate octahedral hexaaqua cation, [Co(H2O)6]2+. However, anhydrous CoCl2 is blue and its solutions in various organic solvents, such as alcohols or acetone, are also blue. In general, the blue color is ascribed to the formation of tetrahedral 4- coordinate complexes. These are often formed when the Co-L (L: ligand) bond is rather weak and ionic, and the ligand is bulky (hence, interhgand repulsion favors a tetrahedral geometry). For example, the spectral and conductometric studies of a number of investigators (e.g. Katzin, Libuś, Osugi, and their collaborators [14,17,31]) have indicated that the blue neutral species of the composition [Co(Solv)2Cl2] predominates in ethanol and higher alcohols (Solv = a solvent molecule; cf. Chap. B.II).