The correct order of the wavelength of light absorbed by the following complexes is,
A. [Co(NH₃)₆]³⁺
B. [Co(CN)₆]³⁻
C. [Cu(H₂O)₄]²⁺
D. [Ti(H₂O)₆]³⁺
Choose the correct answer from the options below:

CONCEPT:

Wavelength of Light Absorbed by Complexes

• The wavelength of light absorbed by a complex depends on the energy gap (Δ_o, crystal field splitting) between the d-orbitals in the central metal ion.
• This energy gap is influenced by the nature of the ligands and their position in the spectrochemical series.
• Strong field ligands (e.g., CN⁻) cause a larger splitting of d-orbitals (higher Δ_o), leading to the absorption of light with shorter wavelengths (higher energy).
• Weak field ligands (e.g., H₂O) cause smaller splitting of d-orbitals (lower Δ_o), leading to the absorption of light with longer wavelengths (lower energy).

EXPLANATION:

• In the given complexes:
  • [Co(NH₃)₆]³⁺: NH₃ is a moderate field ligand and causes moderate splitting of d-orbitals.
  • [Co(CN)₆]³⁻: CN⁻ is a strong field ligand and causes large splitting of d-orbitals, resulting in absorption of shorter wavelengths.
  • [Cu(H₂O)₄]²⁺: H₂O is a weak field ligand and causes small splitting of d-orbitals, resulting in absorption of longer wavelengths.
  • [Ti(H₂O)₆]³⁺: H₂O is also a weak field ligand, but the metal ion Ti³⁺ has a smaller splitting than Cu²⁺.
• Comparing the strength of the ligands and the splitting:
  • [Co(CN)₆]³⁻ absorbs the shortest wavelength due to CN⁻ (strong field ligand).
  • [Co(NH₃)₆]³⁺ absorbs a shorter wavelength than [Ti(H₂O)₆]³⁺ due to NH₃ (moderate field ligand).
  • [Cu(H₂O)₄]²⁺ absorbs the longest wavelength due to H₂O (weak field ligand) and Cu²⁺ having a higher splitting than Ti³⁺.

So, the correct order is B < A < D < C.