Crystal Field Theory CFT is a model that helps us understand the electronic structure of transition metal complexes and their corresponding properties, such as color and magnetic behavior. In this case, we will use CFT to explain the color difference between [Fe H2O 6] and [Fe CN 6] complexes and their absorption spectra.The color of a transition metal complex arises from the absorption of 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 in turn determines the color of the complex.In CFT, the interaction between the metal ion and the ligands surrounding it causes the degeneracy of the d-orbitals to be lifted, splitting them into two groups with different energy levels. The energy difference between these groups is called the crystal field splitting energy .The magnitude of depends on the nature of the ligands and the geometry of the complex. In general, strong-field ligands such as CN cause a larger splitting, while weak-field ligands such as H2O cause a smaller splitting. The spectrochemical series is a list of ligands ordered by their ability to split the d-orbitals, and it shows that CN is a stronger field ligand than H2O.Now let's consider the two complexes:1. [Fe H2O 6]: In this complex, Fe is surrounded by six H2O ligands in an octahedral geometry. Since H2O is a weak-field ligand, the splitting of the d-orbitals is relatively small small . This means that the energy required to promote an electron from the lower energy d-orbital to the higher energy d-orbital is relatively low, corresponding to the absorption of lower energy longer wavelength light. As a result, the complex appears as a complementary color to the absorbed light, which in this case is pale green.2. [Fe CN 6]: In this complex, Fe is surrounded by six CN ligands in an octahedral geometry. CN is a strong-field ligand, which causes a larger splitting of the d-orbitals large . This means that the energy required to promote an electron from the lower energy d-orbital to the higher energy d-orbital is relatively high, corresponding to the absorption of higher energy shorter wavelength light. As a result, the complex appears as a complementary color to the absorbed light, which in this case is pale yellow.In summary, the color difference between [Fe H2O 6] and [Fe CN 6] complexes is due to the difference in crystal field splitting energy caused by the different ligands H2O and CN in their respective complexes. The absorption spectra of these complexes will also differ, with [Fe H2O 6] absorbing lower energy longer wavelength light and [Fe CN 6] absorbing higher energy shorter wavelength light.