What is the decreasing order of field strength of given ligands?

(A) $S^{2-}$
(B) Ethylenediamine
(C) $NCS^-$
(D) $CN^-$

Choose the correct answer from the options given below:

(D), (B), (C), (A)

The correct answer is Option (3) → (D), (B), (C), (A)

The decreasing order of ligand field strength (crystal field splitting parameter Δ) is determined by the spectrochemical series, an empirically derived order based on spectroscopic data of transition metal complexes.

Standard Spectrochemical Series (relevant portion, increasing order):

I⁻ < Br⁻ < S²⁻ < SCN⁻ (S-bonded) < Cl⁻ < ... < H₂O < NCS⁻ (N-bonded) < ... < NH₃ < en (ethylenediamine) < ... < CN⁻ < CO

Positions of the given ligands:

• (A) S²⁻: Weak field ligand (early in the series, similar to halides).
• (C) NCS⁻: Medium field ligand (typically N-bonded in most complexes; stronger than H₂O but weaker than NH₃).
• (B) Ethylenediamine (en): Strong field ligand (chelate effect enhances strength beyond NH₃).
• (D) CN⁻: Very strong field ligand (near the top, excellent σ-donor and π-acceptor).

Decreasing order:

(D) CN⁻ > (B) Ethylenediamine > (C) NCS⁻ > (A) S²⁻

Correct option: (D), (B), (C), (A)