Question

The wavelength of light absorbed for the following complexes are in the order
[Co(NH$_3$)$_6$]$^{3+}$ (I), [Co(H$_2$O)$_6$]$^{3+}$ (II), [Co(CN)$_6$]$^{3-}$ (III), [Co(NH$_3$)$_5$(H$_2$O)]$^{3+}$ (IV), [CoF$_6$]$^{3-}$ (V)

• A. III < I < IV < II < V
• B. III < I < II < IV < V
• C. III < IV < I < II < V
• D. III < I < IV < V < II

Hint:

Stronger ligands cause larger crystal field splitting and shorter wavelength absorption.

Answer 1

The Correct Option is A

Step 1: Concept of crystal field splitting.
The wavelength of light absorbed is inversely proportional to the crystal field splitting energy ($\Delta_o$). Stronger field ligands cause larger splitting and absorb light of shorter wavelength.
Step 2: Ligand field strength order.
CN$^-$ > NH$_3$ > H$_2$O > F$^-$.
Step 3: Arrangement based on $\Delta_o$.
[Co(CN)$_6$]$^{3-}$ has the highest splitting and hence absorbs the shortest wavelength, while [CoF$_6$]$^{3-}$ absorbs the longest wavelength.
Step 4: Conclusion.
Therefore, the correct order is III < I < IV < II < V.