Question

The correct order of the complexes
[5pt] $[\text{Co(NH}_3)_5(\text{H}_2\text{O})]^{3+}$ (A),
[3pt] $[\text{Co(NH}_3)_6]^{3+}$ (B),
[3pt] $[\text{Co(CN)}_6]^{3-}$ (C),
[3pt] $[\text{CoCl(NH}_3)_5]^{2+}$ (D)
[5pt] in terms of wavelength of light absorbed is:

• A. D $>$ A $>$ B $>$ C
• B. C $>$ B $>$ D $>$ A
• C. D $>$ C $>$ B $>$ A
• D. C $>$ B $>$ A $>$ D

Hint:

Remember the spectrochemical series and how it relates to the strength of ligands and the energy of absorbed light.

Answer 1

The Correct Option is A

\[ \text{We know } E = h\nu = \frac{hC}{\lambda} \] \[ E \propto \frac{1}{\lambda} \] Here all Co in +3 oxidation state. So, as the ligand field strength ↑, CFSE ↑ Order of field strength of ligand : \[ CN^->NH_3>H_2O>Cl^- \] CFSE order : C $>$ B $>$ A $>$ D Wavelength order : D $>$ A $>$ B $>$ C