Crystal field theory CFT is a model that describes the electronic structure of transition metal complexes and helps explain their color. The color of a coordination complex arises from the absorption of visible light, which causes electronic transitions between different energy levels d-orbitals of the central metal ion. The energy difference between these levels determines the wavelength of light absorbed, and thus the color observed.In the case of the three coordination complexes mentioned, the central metal ion is iron Fe in different oxidation states and ligands:1. [Fe H2O 6]2+: Iron II hexaaqua complex2. [Fe CN 6]4-: Iron II hexacyanide complex3. [FeF6]3-: Iron III hexafluoride complexThe difference in colors observed for these complexes can be explained by the strength of the ligand field created by the ligands surrounding the central iron ion. The ligand field strength depends on the nature of the ligands, and it affects the energy gap between the d-orbitals of the central metal ion.In general, the ligand field strength increases in the following order: H2O < F- < CN-. This means that the hexacyanide complex has the strongest ligand field, followed by the hexafluoride complex, and the hexaaqua complex has the weakest ligand field.As the ligand field strength increases, the energy gap between the d-orbitals also increases. This results in the absorption of light with shorter wavelengths higher energy and the transmission of light with longer wavelengths lower energy . Consequently, the color observed for each complex will be the complementary color of the absorbed light.For the [Fe H2O 6]2+ complex, the weak ligand field created by the water molecules results in a small energy gap, causing the absorption of lower energy light red region and the transmission of higher energy light blue-green region . Therefore, this complex appears pale green or blue-green in color.For the [Fe CN 6]4- complex, the strong ligand field created by the cyanide ligands results in a large energy gap, causing the absorption of higher energy light violet-blue region and the transmission of lower energy light yellow-orange region . Therefore, this complex appears yellow or orange in color.For the [FeF6]3- complex, the intermediate ligand field created by the fluoride ligands results in a moderate energy gap, causing the absorption of light in the green region and the transmission of light in the red region. Therefore, this complex appears red or reddish-brown in color.In summary, the difference in colors observed for the coordination complexes [Fe H2O 6]2+, [Fe CN 6]4-, and [FeF6]3- can be explained by the crystal field theory, which attributes the color to the energy gap between the d-orbitals of the central iron ion, influenced by the ligand field strength of the surrounding ligands.