The correct order of the wavelength of light absorbed by the following complexes is,A. [Co(NH_3)_6]^3+ B. [Co(CN)_6]^3- C. [Cu(H_2O)_4]^2+ D. [Ti(H_2O)_6]^3+Choose the correct answer from the options given below:

Answer: 2. B < A < D < C

Key idea: the larger the crystal field splitting \(\Delta\), the higher the energy absorbed and the shorter the wavelength.

• Spectrochemical series: \(\mathrm{CN^-}>\mathrm{NH_3}>\mathrm{H_2O}\).
• \(\mathrm{[Co(CN)_6]^{3-}}\) has the largest \(\Delta\), so the shortest wavelength; \(\mathrm{[Co(NH_3)_6]^{3+}}\) is next.
• \(\mathrm{[Ti(H_2O)_6]^{3+}}\) (weak field, \(\mathrm{Ti^{3+}}\), +3 charge) absorbs at about 500 nm.
• \(\mathrm{[Cu(H_2O)_4]^{2+}}\) has a +2 metal ion, water ligands and only four ligands, so it has the smallest \(\Delta\) and the longest wavelength.

Wavelength order: B < A < D < C: option (2).