Final answer:
The correct order of the wavelength of light absorbed by the given cobalt complexes is determined by the strength of the ligands. Strong-field ligands like CN- result in higher energy absorption and shorter wavelengths, whereas weak-field ligands like I- and H2O result in lower energy absorption and longer wavelengths.
Step-by-step explanation:
The question is about determining the correct order of the wavelength of light absorbed by different cobalt complexes. According to crystal field theory (CFT), the strength of the ligand field influences the energy difference (Δ) between the split d-orbitals in a transition metal complex. Different ligands induce different splitting patterns, resulting in the absorption of light of different wavelengths.
Complexes with strong-field ligands (like CN⁻) cause a large Δ and absorb light of higher energy, which corresponds to shorter wavelengths. Therefore, such complexes generally absorb in the violet-to-blue region of the spectrum. In contrast, complexes with weak-field ligands (like I⁻ and H₂O) cause a smaller Δ and absorb lower energy light, which corresponds to longer wavelengths, in the green-to-red region of the spectrum.
Given the complexes:
[Co(H₂O)₆]³⁺
[Co(CN)₆]³⁻
[Co(I)₆]³⁻
[Co(en)₃]³⁺
CN⁻ is a stronger field ligand than I⁻, which in turn is stronger than H₂O. Therefore, we can expect the order of wavelength of light absorbed to be: (ii) > (iii) > (i), which corresponds to option 'a'.