Crystal field theory CFT is a model that helps explain the color of transition metal complexes by describing the interaction between the central metal ion and the surrounding ligands. In CFT, the ligands are treated as point charges that create an electric field around the central metal ion. This electric field causes the degeneracy of the d-orbitals in the metal ion to be broken, leading to the splitting of the d-orbitals into two energy levels: the lower energy t2g orbitals dxy, dyz, and dxz and the higher energy eg orbitals dx^2-y^2 and dz^2 .The energy difference between these two sets of orbitals is called the crystal field splitting energy . The magnitude of depends on the nature of the ligands, the geometry of the complex, and the oxidation state of the metal ion. When light is absorbed by the complex, electrons can be excited from the lower energy t2g orbitals to the higher energy eg orbitals. The energy of the absorbed light corresponds to the energy difference between these two sets of orbitals , and the remaining transmitted or reflected light is what we perceive as the color of the complex.For example, let's consider the hexaaquachromium III ion, [Cr H2O 6]^3+. In this octahedral complex, the chromium III ion is surrounded by six water molecules as ligands. The d-orbitals of the Cr^3+ ion which has a d^3 electronic configuration will be split into t2g and eg orbitals due to the crystal field created by the water ligands.In this case, all three t2g orbitals are filled with electrons, while the eg orbitals are empty. When the complex absorbs light, an electron from one of the t2g orbitals can be excited to an empty eg orbital. The energy of the absorbed light corresponds to the crystal field splitting energy . If the absorbed light has a wavelength in the visible region of the spectrum, the remaining transmitted or reflected light will give the complex its characteristic color.For the hexaaquachromium III ion, the absorbed light has a wavelength of around 590 nm, which corresponds to the orange region of the spectrum. Consequently, the transmitted light appears as the complementary color, which is blue-green. This is why the hexaaquachromium III ion appears blue-green in solution.In summary, crystal field theory explains the color of transition metal complexes by describing the splitting of the d-orbitals in the central metal ion due to the electric field created by the surrounding ligands. The absorbed light energy corresponds to the crystal field splitting energy, and the remaining transmitted or reflected light determines the observed color of the complex.