Which among the following is a strong field ligand?

$NH_3$

The correct answer is Option (3) → $NH_3$.

Let us dive into the concept of ligands, the classification of ligands, and why ammonia (\( \text{NH}_3 \)) is considered a strong field ligand compared to the others listed.

Ligands are ions or molecules that coordinate to a central metal atom or ion, forming a coordination complex. They donate electron pairs to the metal, which can influence the electronic structure, geometry, and reactivity of the complex.

Ligands can vary significantly in their ability to cause splitting of d-orbitals in transition metal complexes, leading to the classification into strong field and weak field ligands based on their field strength.

Ligand Field Theory:

In coordination chemistry, transition metal complexes consist of a central metal ion surrounded by ligands. The presence of these ligands leads to the splitting of the metal's d-orbitals due to electrostatic interactions between the ligands and the d-electrons of the metal.

The amount of splitting depends on:

I. The nature of the ligands.

II. The geometry of the complex (octahedral, tetrahedral, etc.).

Classification of Ligands:

Ligands can be classified based on their ability to split the d-orbitals into two energy levels:

Strong Field Ligands:

Cause a large splitting of d-orbitals. Lead to low-spin configurations, as electrons tend to pair up in lower energy orbitals rather than occupy higher ones. Examples include:

Cyanide (\( \text{CN}^- \))

Carbon monoxide (\( \text{CO} \))

Ammonia (\( \text{NH}_3 \))

Weak Field Ligands:

Cause a smaller splitting of d-orbitals. Lead to high-spin configurations, where electrons fill the higher energy orbitals before pairing. Examples include:

Iodide (\( \text{I}^- \))

Bromide (\( \text{Br}^- \))

Chloride (\( \text{Cl}^- \))

Analyzing the Given Ligands:

Let us analyze each of the given ligands in the question:

1. I⁻ (Iodide):

Type: Weak field ligand.

Effect: Causes minimal splitting of the d-orbitals due to its larger size and poorer π-acceptor ability. It typically leads to high-spin complexes.

2. Cl⁻ (Chloride):

Type: Weak field ligand.

Effect: Similar to iodide, it causes small splitting and results in high-spin configurations. Chloride is also larger and has low π-acceptor ability.

3. NH₃ (Ammonia):

Type: Strong field ligand.

Effect: Causes significant splitting of d-orbitals. Ammonia is a good π-acceptor and causes low-spin configurations in many complexes, making it effective in stabilizing certain metal-ligand bonding arrangements.

4. SCN⁻ (Thiocyanate):

Type: Moderate field ligand (depends on coordination mode).

Effect: The thiocyanate ion can bind through sulfur (weak field) or nitrogen (strong field), leading to variability in its behavior. However, it is generally weaker than ammonia.

Why is NH₃ a Strong Field Ligand

Electronic Structure: Ammonia has a lone pair of electrons on the nitrogen atom, which it can donate to form a coordinate bond with the metal ion. This donation leads to strong metal-ligand interactions and significant splitting of the d-orbitals.

Geometric Arrangement: In an octahedral complex, ammonia ligands arrange themselves around the metal ion to maximize their interaction, which enhances the splitting of d-orbitals compared to weaker ligands.

Low-Spin vs. High-Spin: Complexes with ammonia often prefer low-spin configurations because the energy required to pair electrons in the lower orbitals is less than the energy required to promote them to higher orbitals, especially in cases with stronger metal-ligand interactions.

Conclusion:

Among the ligands listed:

I⁻ and Cl⁻ are weak field ligands that cause little splitting.

SCN⁻ can behave variably but is generally not as strong as ammonia.

NH₃ is a strong field ligand that causes significant splitting of d-orbitals, leading to its classification as such.

Thus, the correct answer to the question is NH₃ (Ammonia).