The chemical principle behind the prediction of color in transition metal complexes is the Crystal Field Theory CFT or Ligand Field Theory LFT . These theories describe the electronic structure of transition metal complexes and how the interaction between the central metal ion and the surrounding ligands influences the energy levels of the metal's d-orbitals. This interaction results in the splitting of the d-orbitals into different energy levels, which is known as crystal field splitting.When light is absorbed by a transition metal complex, an electron from a lower energy d-orbital is excited 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 color. The color we observe is the complementary color of the absorbed light.To predict the color of copper sulfate CuSO4 and cobalt chloride CoCl2 solutions with different ligands, we need to consider the following factors:1. The oxidation state of the metal ion: The oxidation state of the metal ion affects the energy gap between the split d-orbitals. A higher oxidation state generally results in a larger energy gap and a shorter wavelength of absorbed light.2. The nature of the ligand: Different ligands cause different degrees of crystal field splitting. Ligands can be classified according to the spectrochemical series, which ranks ligands based on their ability to split the d-orbitals. Strong-field ligands e.g., CN-, CO cause a larger splitting, while weak-field ligands e.g., I-, Br- cause a smaller splitting.3. The geometry of the complex: The geometry of the complex e.g., octahedral, tetrahedral, or square planar also affects the crystal field splitting. For example, octahedral complexes generally have a larger splitting than tetrahedral complexes.Using these factors, we can predict the color of copper sulfate and cobalt chloride solutions with different ligands:- Copper sulfate CuSO4 forms a blue solution due to the [Cu H2O 6]2+ complex, where Cu2+ has a 3d9 electronic configuration. The complex has an octahedral geometry with water as a weak-field ligand. The color arises from the absorption of light in the red region of the spectrum, and the complementary color observed is blue.- Cobalt chloride CoCl2 forms a pink solution due to the [Co H2O 6]2+ complex, where Co2+ has a 3d7 electronic configuration. The complex has an octahedral geometry with water as a weak-field ligand. The color arises from the absorption of light in the green region of the spectrum, and the complementary color observed is pink.When different ligands are introduced, the color of the solutions may change due to the change in crystal field splitting and the geometry of the complex. For example, if ammonia NH3 is added to a cobalt chloride solution, the color changes from pink to deep blue due to the formation of the [Co NH3 6]3+ complex, which has a larger crystal field splitting and a higher oxidation state of Co3+.