Question

Which of the following orders correctly represent the strength of ligands in the spectrochemical series?
I. $\text{I}^- \lt \text{Br}^- \lt \text{S}^{2-} \lt \text{SCN}^-$
II. $\text{H}_2\text{O} \lt \text{NCS}^- \lt \text{NH}_3 \lt \text{en}$
III. $\text{Cl}^- \lt \text{F}^- \lt \text{N}^{3-} \lt \text{OH}^-$

• A. I, II only
• B. II, III only
• C. I, III only
• D. I, II, III

Hint:

Remember the general order of donor atoms in the spectrochemical series to easily solve these questions:
$\text{Halogen donors} \lt \text{Oxygen donors} \lt \text{Nitrogen donors} \lt \text{Carbon donors}$.

Answer 1

The Correct Option is A

Step 1: Recall the memorized spectrochemical series.
Roughly: $\text{I}^- \lt \text{Br}^- \lt \text{S}^{2-} \lt \text{SCN}^- \lt \text{Cl}^- \lt \text{N}_3^- \lt \text{F}^- \lt \text{OH}^- \lt \text{H}_2\text{O} \lt \text{NCS}^- \lt \text{NH}_3 \lt \text{en} \lt \text{CN}^-$.
Step 2: Check Order I against it.
$\text{I}^- \lt \text{Br}^- \lt \text{S}^{2-} \lt \text{SCN}^-$ lines up with consecutive terms of the series exactly, correct.
Step 3: Check Order II against it.
$\text{H}_2\text{O} \lt \text{NCS}^- \lt \text{NH}_3 \lt \text{en}$ also follows the same relative sequence, correct.
Step 4: Check Order III against it.
$\text{Cl}^- \lt \text{F}^- \lt \text{N}^{3-} \lt \text{OH}^-$ puts $\text{F}^-$ before $\text{N}^{3-}$, but the real series has $\text{N}_3^-$ before $\text{F}^-$, so this order is broken.
Final answer: Option 1, I and II only.