Answer :
Final answer:
Ligand strength in octahedral complexes can be inferred from their color, with red-light absorption indicating a strong-field ligand due to high-field splitting. Given the absorption colors of green, blue, and red, the increasing order of ligand strength for ligands L1, L2, and L3 is l2 < l1 < l3.
Explanation:
The observed wavelengths of light absorption in octahedral complexes can inform us about the strength of the ligands bound to the central metal. According to crystal field theory, strong-field ligands cause a large split in the energies of the d orbitals (high crystal field splitting parameter, Aoct), resulting in the absorption of higher-energy violet or blue light. Thus, compounds with strong-field ligands are perceived as yellow, orange, or red because these are the colors of light that are not absorbed.
Conversely, weak-field ligands result in a smaller split in the d orbital energies, leading to the absorption of lower-energy yellow, orange, or red light. This means that complexes with weak-field ligands appear blue-green, blue, or indigo. In the case of an octahedral complex with monodentate ligands L1, L2, and L3 that absorb green, blue, and red light respectively, L3 must be the strongest ligand (highest field strength), followed by L1, and then L2 as the weakest since the ligand absorbing the red light (lowest energy) must be a strong-field ligand.
We can infer the spectrochemical series order of ligand strength based on the color absorbed by the complexes: l2 (green-absorbing, weakest field), l1 (blue-absorbing, intermediate field), and l3 (red-absorbing, strongest field). Therefore, the correct order is l2 < l1 < l3.
