Question

The correct increasing order of wavelength of absorption of the following complexes is
(i) \([\mathrm{CoCl(NH_3)_5}]^{2+}\) (ii) \([\mathrm{Co(NH_3)_5(H_2O)}]^{3+}\) (iii) \([\mathrm{Co(NH_3)_6}]^{3+}\) (iv) \([\mathrm{Co(CN)_6}]^{3-}\)

• A. (i) $<$ (ii) $<$ (iii) $<$ (iv)
• B. (i) $<$ (ii) $<$ (iv) $<$ (iii)
• C. (ii) $<$ (i) $<$ (iv) $<$ (iii)
• D. (iv) $<$ (iii) $<$ (ii) $<$ (i)
• E. (iv) $<$ (i) $<$ (ii) $<$ (iii)

Hint:

Stronger ligand $\Rightarrow$ higher $\Delta_o$ $\Rightarrow$ lower wavelength

Answer 1

The Correct Option is D

Concept:
• $\Delta_o$ (crystal field splitting) $\uparrow \Rightarrow \lambda \downarrow$
• Strong field ligand $\Rightarrow$ large $\Delta_o$ $\Rightarrow$ small wavelength

Step 1: Arrange ligands by strength.
\[ \mathrm{CN^- > NH_3 > H_2O > Cl^-} \]

Step 2: Determine splitting.

• (iv) $\mathrm{[Co(CN)_6]^{3-}}$ $\rightarrow$ strongest $\Rightarrow$ smallest $\lambda$
• (iii) $\mathrm{[Co(NH_3)_6]^{3+}}$
• (ii) $\mathrm{[Co(NH_3)_5(H_2O)]^{3+}}$
• (i) $\mathrm{[CoCl(NH_3)_5]^{2+}}$ $\rightarrow$ weakest $\Rightarrow$ largest $\lambda$

Step 3: Arrange in increasing wavelength.
\[ (iv) < (iii) < (ii) < (i) \]