Question

Which ligand causes maximum crystal field splitting?

• A. \( F^- \)
• B. \( H_2O \)
• C. \( NH_3 \)
• D. \( CN^- \)

Hint:

Strong field ligands (\( CN^-, CO, NO_2^- \)) usually lead to "low spin" complexes because the splitting energy is greater than the pairing energy.

Answer 1

The Correct Option is D

To determine which ligand causes the maximum crystal field splitting, we need to understand the concept of crystal field theory and its implications on ligand strength.

Crystal Field Splitting: In coordination chemistry, the approach of crystal field theory helps us understand the electronic environment in transition metal complexes. The ligands that are bonded to the central metal ion affect the distribution of metal d-orbitals, resulting in a split in energy levels. The extent of this energy split is known as crystal field splitting.

Crystal Field Splitting Energy (\( \Delta \)): The value of \( \Delta \) depends on two main factors:

1. The nature of the metal ion.
2. The nature of the ligands around the metal ion.

 

Ligand Strength: Ligands can be arranged in a series known as the "Spectrochemical Series" based on their ability to split d-orbitals:

Ligands	Increasing Crystal Field Splitting (\( \Delta \))
I-, Br-, S2-	Weak Field
Cl-, F-, OH-
H2O	Intermediate Field
NH3, en (ethylenediamine)	Strong Field
CN-, CO

From the spectrochemical series, it's evident that \(CN^-\) (cyanide ion) is a strong field ligand, causing maximum crystal field splitting when compared to \( F^- \), \( H_2O \), and \( NH_3 \). This is due to its ability to engage in strong \(\pi\)-backbonding, leading to a larger separation between the d-orbitals.

Conclusion: The ligand \( CN^- \) causes the maximum crystal field splitting among the given options.

Answer 2

Step 1: Understanding the Concept:
Crystal Field Splitting (\( \Delta \)) is the energy difference between sets of d-orbitals in a coordination complex.
Different ligands split the d-orbitals to different extents.
Step 2: Key Formula or Approach:
Ligands are arranged in the "Spectrochemical Series" based on their field strength.
A strong field ligand causes a large splitting, while a weak field ligand causes a small splitting.
Step 3: Detailed Explanation:
The order of common ligands in the spectrochemical series is:
\[ I^-<Br^-<SCN^-<Cl^-<S^{2-}<F^-<OH^-<C_2O_4^{2-}<H_2O<NCS^-<edta^{4-}<NH_3<en<NO_2^-<CN^-<CO \]
From the options:
- \( F^- \) is a weak field ligand.
- \( H_2O \) is a weak field ligand.
- \( NH_3 \) is a moderate to strong field ligand.
- \( CN^- \) (Cyanide) is a very strong field ligand, appearing near the end of the series.
Because \( CN^- \) is a strong pi-acceptor ligand, it causes the greatest crystal field splitting among the choices provided.
Step 4: Final Answer:
The ligand \( CN^- \) causes maximum crystal field splitting.