Crystal Field Theory CFT is a model used to describe the electronic structure of transition metal complexes and the colors they exhibit. The theory is based on the interaction between the central metal ion and the surrounding ligands, which are treated as point charges or dipoles. The ligands create an electric field that affects the energy levels of the d-orbitals of the central metal ion, causing them to split into different energy levels. The energy difference between these levels is responsible for the color observed in coordination compounds.When white light a mixture of all visible wavelengths passes through a coordination compound, some wavelengths are absorbed by the compound while others are transmitted. The absorbed wavelengths correspond to the energy difference between the split d-orbitals. The transmitted light, which is the complementary color of the absorbed light, is what we observe as the color of the compound.Several factors influence the color of coordination compounds:1. The nature of the central metal ion: Different metal ions have different d-orbital energy levels, which affect the energy difference between the split orbitals and, consequently, the color of the compound.2. The oxidation state of the metal ion: The oxidation state of the metal ion affects the energy levels of the d-orbitals and the splitting pattern, leading to different colors for different oxidation states.3. The type of ligands: Different ligands have different abilities to split the d-orbitals due to their different electron-donating or electron-withdrawing properties. Ligands with stronger electron-donating abilities cause a greater splitting of the d-orbitals, resulting in the absorption of higher energy shorter wavelength light and the transmission of lower energy longer wavelength light.4. The geometry of the complex: The arrangement of the ligands around the central metal ion affects the splitting of the d-orbitals. For example, octahedral complexes generally have a larger splitting than tetrahedral complexes, resulting in different colors.Examples:1. [Fe H2O 6]3+: This complex has an octahedral geometry with six water ligands surrounding the Fe3+ ion. The d-orbitals split into two sets: t2g and eg. The energy difference between these sets corresponds to the absorption of light in the blue region of the spectrum, resulting in the transmission of yellow-orange light, which is the observed color of the complex.2. [Cu NH3 4]2+: This complex has a square planar geometry with four ammonia ligands surrounding the Cu2+ ion. The d-orbitals split into three sets: dx2-y2 and dxy highest energy , and dz2 and dxz/dyz lowest energy . The energy difference between these sets corresponds to the absorption of light in the green region of the spectrum, resulting in the transmission of blue-violet light, which is the observed color of the complex.In summary, Crystal Field Theory explains the colors observed in coordination compounds by describing the splitting of the d-orbitals of the central metal ion due to the interaction with the surrounding ligands. The energy difference between the split orbitals determines the absorbed wavelengths of light, and the transmitted light is the complementary color, which is what we observe as the color of the compound. Factors such as the nature of the metal ion, its oxidation state, the type of ligands, and the geometry of the complex all influence the color of coordination compounds.