Question

Identify weak field ligand from following.

• A. $NH_{3}$
• B. $CN^{-}$
• C. $Br^{-}$
• D. $CO$

Hint:

Weak field ligands usually cause low crystal field splitting and form high-spin complexes.

Answer 1

The Correct Option is C

Step 1: Understanding the Question:
Ligands are classified based on their ability to split the d-orbitals of a central metal atom into two sets of different energies (crystal field splitting). This is summarized in the spectrochemical series.

Step 3: Detailed Explanation:
The spectrochemical series arranges ligands in increasing order of their field strength:
$\text{I}^- < \text{Br}^- < \text{SCN}^- < \text{Cl}^- < \text{S}^{2-} < \text{F}^- < \text{OH}^- < \text{C}_2\text{O}_4^{2-} < \text{H}_2\text{O} < \text{NCS}^- < \text{edta}^{4-} < \text{NH}_3 < \text{en} < \text{CN}^- < \text{CO}$
- Ligands like $\text{NH}_3$, $\text{CN}^-$, and $\text{CO}$ are considered strong field ligands as they cause large splitting and promote electron pairing.
- Halide ions like $\text{Br}^-$ are weak field ligands as they cause small splitting and usually form high-spin complexes.

Step 4: Final Answer:
$\text{Br}^-$ is a weak field ligand.