Which of the following statements is correct?

for weak field ligand $Δ_o < P$

The correct answer is Option (1) → for weak field ligand $Δ_o < P$

In crystal field theory, the behavior of ligands depends on the comparison between:

• $Δ_0$ (crystal field splitting energy)
• P (pairing energy)

Weak field ligands cause small splitting, strong field ligands cause large splitting.

Option 1: For weak field ligand, $Δ_o < P$

Weak field ligands produce small splitting of d-orbitals. Because $Δ_0$ is small, electrons prefer to remain unpaired rather than pair up. Hence $Δ_0$ is less than pairing energy (P) - this statement is correct.

Option 2: $CN^-$ is a weak field ligand

$CN^-$ (cyanide ion) lies at the strong field end of the spectrochemical series. It produces large splitting and usually forms low-spin complexes, so calling it weak field is wrong.

Option 3: Weak field ligands yield low spin complex

Weak field ligands cause small $Δ_0$, so electrons avoid pairing and occupy higher orbitals singly. This leads to high-spin complexes, not low spin. So this statement is incorrect.

Option 4: $I^-$ is a strong field ligand

$I^-$ (iodide) is toward the weak field end of the spectrochemical series. It produces small splitting and forms high-spin complexes, so this statement is false.