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

Concept: According to Crystal Field Theory (CFT), ligands are classified as weak-field ligands and strong-field ligands based on their ability to split the d-orbitals of a central metal ion. A stronger ligand produces a larger crystal field splitting energy \((\Delta)\). The spectrochemical series provides the experimentally observed order of ligand field strength. A portion of the spectrochemical series is: \[ I^- \lt Br^- \lt Cl^- \lt F^- \lt OH^- \lt H_2O \lt NH_3 \lt CN^- \lt CO \] Field strength increases from left to right.

Step 1: Identify the strongest ligand. Among the given ligands, \[ CO \] is a very strong-field ligand because it acts as both a sigma donor and a pi acceptor. It causes maximum crystal field splitting. Therefore, \[ CO \] must be placed first.

Step 2: Compare \(NH_3\) and \(H_2O\). From the spectrochemical series: \[ H_2O \lt NH_3 \] Therefore, ammonia is a stronger field ligand than water. Hence, \[ NH_3 \gt H_2O \]

Step 3: Compare \(Cl^-\). Chloride ion is a weak-field ligand and appears much lower in the spectrochemical series. Therefore, it has the smallest crystal field splitting among the given ligands. \[ Cl^- \] must come last.

Step 4: Write decreasing order of field strength. Combining all observations: \[ CO \gt NH_3 \gt H_2O \gt Cl^- \] Hence the correct option is \[ \boxed{(2)} \]