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: Recall what crystal field splitting means. 
Crystal field splitting energy $\Delta$ refers to the energy separation between different sets of d-orbitals when a metal ion forms a coordination complex.
The magnitude of this splitting depends strongly on the nature of the ligand attached to the metal ion.

Step 2: Use the spectrochemical series.
Ligands are ordered according to their ability to split d-orbitals in the spectrochemical series.
Ligands toward the left produce weak splitting, whereas ligands toward the right produce strong splitting.

Step 3: Compare the given ligands.
A commonly used spectrochemical series is:

\[ I^- < Br^- < SCN^- < Cl^- < S^{2-} < F^- < OH^- < H_2O < NH_3 < en < NO_2^- < CN^- < CO \] 

From the options:

• $F^-$ produces weak crystal field splitting.
• $H_2O$ is also a weak-field ligand.
• $NH_3$ causes moderate splitting.
• $CN^-$ is a very strong-field ligand and lies near the extreme right of the series.

Because $CN^-$ is a strong π-acceptor ligand, it interacts strongly with metal d-orbitals and produces the largest energy separation.

Step 4: Final conclusion.
Among the given ligands, the one that produces the maximum crystal field splitting is:

\[ \boxed{CN^-} \]