Crystal field theory CFT is a model used to describe the electronic structure and properties of transition metal coordination compounds. It helps explain the color of these compounds by considering the interaction between the central metal ion and the surrounding ligands.In a transition metal coordination compound, the central metal ion is surrounded by a certain number of ligands, which can be ions or neutral molecules. These ligands create an electric field around the metal ion, which affects the energy levels of the metal's d-orbitals. The d-orbitals are initially degenerate, meaning they have the same energy level. However, when the ligands approach the metal ion, the degeneracy is lifted, and the d-orbitals split into different energy levels.The extent of this splitting depends on the geometry of the coordination complex and the nature of the ligands. In an octahedral complex, the d-orbitals split into two groups: the lower-energy t2g orbitals dxy, dyz, and dxz and the higher-energy eg orbitals dx^2-y^2 and dz^2 . In a tetrahedral complex, the splitting is reversed, with the t2g orbitals having higher energy than the eg orbitals.The color of a transition metal coordination compound arises from the absorption of visible light, which promotes an electron from a lower-energy d-orbital to a higher-energy d-orbital. The energy difference between these orbitals corresponds to the energy of the absorbed light, which is related to its wavelength and, consequently, its color. The remaining transmitted or reflected light is what we perceive as the color of the compound.For example, if a compound absorbs red light, it will appear green, as green is the complementary color of red. The specific color observed depends on the energy difference between the split d-orbitals, which is influenced by the type of metal ion, the ligands, and the geometry of the coordination complex.In summary, crystal field theory explains the color of transition metal coordination compounds by describing the splitting of the metal ion's d-orbitals in the presence of ligands. The absorbed light's energy corresponds to the energy difference between these split orbitals, resulting in the observed color of the compound.