Question

According to crystal field theory, the correct order of ligands with respect to their decreasing order of field strength is

• A. \( Cl^- \gt NH_3 \gt H_2O \gt CO \)
• B. \( CO \gt NH_3 \gt H_2O \gt Cl^- \)
• C. \( CO \gt H_2O \gt NH_3 \gt Cl^- \)
• D. \( Cl^- \gt H_2O \gt NH_3 \gt CO \)

Hint:

A frequently asked sequence from the spectrochemical series is: \[ CO \gt CN^- \gt NH_3 \gt H_2O \gt F^- \gt Cl^- \gt Br^- \gt I^- \] Moving toward the right side of the spectrochemical series means increasing field strength.

Answer 1

The Correct Option is B

Step 1: Recall the basis of comparison.
In crystal field theory the field strength of a ligand is its ability to split the d-orbitals; this is captured by the spectrochemical series.
Step 2: Write the relevant series.
\[ Cl^- < H_2O < NH_3 < CO \] with field strength increasing left to right.
Step 3: Place the strongest ligand.
CO is a sigma donor and pi acceptor, giving the largest splitting, so it ranks first in a decreasing order.
Step 4: Order the middle pair.
From the series \(NH_3\) is stronger than \(H_2O\), so \(NH_3 > H_2O\).
Step 5: Place the weakest ligand.
\(Cl^-\) lies low in the series, so it is the weakest and comes last.
Step 6: Write the decreasing order.
Combining everything: \(CO > NH_3 > H_2O > Cl^-\).
\[ \boxed{CO > NH_3 > H_2O > Cl^-} \]