Question

Which from following is a weak field ligand?

• A. EDTA
• B. CO
• C. $\text{F}^{-}$
• D. $\text{NH}_{3}$

Hint:

Rough ordering of the Spectrochemical Series:
$\text{I}^- < \text{Br}^- < \text{S}^{2-} < \text{SCN}^- < \text{Cl}^- < \text{F}^- < \text{OH}^- < \text{C}_2\text{O}_4^{2-} < \text{H}_2\text{O} < \text{NCS}^- < \text{EDTA}^{4-} < \text{NH}_3 < \text{en} < \text{CN}^- < \text{CO}$.
Halogens are almost always weak. N and C donors are almost always strong!

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
The strength of a ligand is determined by its position in the spectrochemical series. Strong field ligands cause large crystal field splitting ($\Delta_o$), while weak field ligands cause small splitting.

Step 2: Formula Application:
Spectrochemical series (simplified): $I^- < Br^- < S^{2-} < SCN^- < Cl^- < F^- < OH^- < C_2O_4^{2-} < H_2O < NCS^- < EDTA^{4-} < NH_3 < en < CN^- < CO$.

Step 3: Explanation:
Halide ions ($F^-$, $Cl^-$, etc.) are located at the beginning of the series, making them weak field ligands. $CO$, $NH_3$, and $EDTA$ are much further along and are considered strong or intermediate-to-strong field ligands.

Step 4: Final Answer:
F$^-$ is a weak field ligand.