Colour differences in structural isomers. Three samples of hydrated chromium(III) chloride, CrCl3. 6H2O, showing marked colour differences between the three isomeric forms. In each case, a chloride molecule is associated with six water molecules. The violet form has the hexahydrate [Cr(H2O)6]3+ ion. The olive green sample contains the pentahydrate [Cr(H2O)5Cl]2+ ion, and the dark green sample contains the tetrahydrate [Cr(H2O)4Cl2]+ ion. These three isomeric forms have different electronic configurations, so absorb light of different wavelengths.