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

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.