Question

The correct order of the wavelength maxima of the absorption band in the ultraviolet-visible region for the given complexes is:

• A. \( [\text{Co(CN)}_6]^{3-}<[\text{Co(NH}_3)_6]^{3+}<[\text{Co(NH}_3)_5(\text{H}_2\text{O})]^{3+}<[\text{Co(NH}_3)_5(\text{Cl})]^{2+} \)
• B. \( [\text{Co(NH}_3)_5(\text{Cl})]^{2+}<[\text{Co(NH}_3)_5(\text{H}_2\text{O})]^{3+}<[\text{Co(NH}_3)_6]^{3+}<[\text{Co(CN)}_6]^{3-} \)
• C. \( [\text{Co(CN)}_6]^{3-}<[\text{Co(NH}_3)_5(\text{Cl})]^{2+}<[\text{Co(NH}_3)_5(\text{H}_2\text{O})]^{3+}<[\text{Co(NH}_3)_6]^{3+} \)
• D. \( [\text{Co(NH}_3)_6]^{3+}<[\text{Co(CN)}_6]^{3-}<[\text{Co(NH}_3)_5(\text{Cl})]^{2+}<[\text{Co(NH}_3)_5(\text{H}_2\text{O})]^{3+} \)

Hint:

In coordination chemistry, the stronger the ligand field (based on the spectrochemical series), the greater the crystal field splitting and the lower the wavelength of light absorbed.

Answer 1

The Correct Option is A

The wavelength of absorption in the UV-visible region is inversely proportional to the strength of the ligand field: \[ \lambda_{\text{absorbed}} \propto \frac{1}{\text{strength of ligand}} \] According to the spectrochemical series: \[ \text{CN}^->\text{NH}_3>\text{H}_2\text{O}>\text{Cl}^- \] So stronger field ligands result in a larger splitting (\( \Delta \)), and thus absorb lower wavelengths. Therefore, the correct order of increasing wavelength (i.e., decreasing ligand strength) is: \[ [\text{Co(CN)}_6]^{3-}<[\text{Co(NH}_3)_6]^{3+}<[\text{Co(NH}_3)_5(\text{H}_2\text{O})]^{3+}<[\text{Co(NH}_3)_5(\text{Cl})]^{2+} \]