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 Question:
The question asks us to identify a "weak field ligand" based on the established rules of the Spectrochemical Series in coordination chemistry.

Step 2: Detailed Explanation:
The Spectrochemical Series arranges common ligands in order of increasing crystal field splitting energy ($\Delta_o$) they produce when interacting with a central metal ion.
The series generally follows the order of donor atoms:
Halogens (Weakest) $<$ Oxygen donors $<$ Nitrogen donors $<$ Carbon donors (Strongest).
Let's evaluate the options provided:
(a) EDTA: This is a hexadentate ligand primarily binding through nitrogen and oxygen. It is generally considered a moderate to strong field ligand.
(b) CO (Carbon Monoxide): This binds through carbon and is a powerful $\pi$-acceptor. It sits at the absolute far right of the series, making it the strongest field ligand known.
(d) $\text{NH_3$ (Ammonia):} This binds through nitrogen. It is a classic strong field ligand that usually forces electron pairing.
(c) $\text{F^-$ (Fluoride ion):} This binds through a highly electronegative halogen atom. It sits at the far left of the spectrochemical series. It produces a very small crystal field splitting energy ($\Delta_o$), making it a classic, textbook example of a weak field ligand. It rarely forces electron pairing, leading to high-spin complexes.

Step 3: Final Answer:
The weak field ligand is $\text{F}^{-}$, matching option (c).