Question

For the following complex (a) [Co(NH3)6]3+ (b) [Co(CN)6]3- (c) [CoCl(NH3)5]2+ (d) [Co(NH3)5(H2O)]3+ Correct order of $\lambda$ absorbed is :

• A. c > d > a > b
• B. a > b > c > d
• C. d > c > a > b
• D. c > d > b > a

Answer 1

Answer: (A)

c > d > a > b

Answer 2

1. Concept: In coordination compounds, the wavelength ($\lambda$) absorbed is inversely proportional to the ligand field splitting ($\Delta_0$). A larger $\Delta_0$ means lower $\lambda$ (blue-shifted), and vice versa.

2. Spectrochemical Series (relevant part):

   $$\text{CN}^- > \text{NH}_3 > \text{H}_2O > \text{Cl}^-$$

3. Analysis of Complexes:

   • (b) $[Co(CN)_6]^{3-}$: All strong-field $\text{CN}^-$ ligands $\Rightarrow$ highest $\Delta_0$ $\Rightarrow$ lowest $\lambda$.

   • (a) $[Co(NH_3)_6]^{3+}$: All $\text{NH}_3$ (moderately strong) $\Rightarrow$ intermediate $\Delta_0$ and $\lambda$.

   • (d) $[Co(NH_3)_5(H_2O)]^{3+}$: With one $\text{H}_2O$ (weaker than $\text{NH}_3$) $\Rightarrow$ slightly lower $\Delta_0$, so absorbs a longer $\lambda$ as compared to (a).

   • (c) $[CoCl(NH_3)_5]^{2+}$: With one $\text{Cl}^-$ (weakest) $\Rightarrow$ lowest $\Delta_0$ $\Rightarrow$ highest $\lambda$.

4. Order of $\lambda$ Absorbed (Longest to Shortest):

   $$(c) > (d) > (a) > (b)$$