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:

The correct option is (D), (C), (B), (A).

Explanation:
According to Crystal Field Theory (CFT), the ability of ligands to cause crystal field splitting (${\Delta }_o$) is represented in the spectrochemical series. In this series, ligands are arranged in increasing order of their field strength (or splitting ability).

The general order of field strength for the given ligands in the spectrochemical series is:
Halides/Sulfur-donors < Oxygen-donors < Nitrogen-donors < Carbon-donors

Let us identify the donor atoms and position of each given ligand:
1. CN^- (D): A carbon-donor ligand. Carbon-donor ligands are strong-field ligands because carbon is less electronegative and has a high ability to donate electrons, leading to the strongest field strength among the choices.
2. NCS^- (C): A nitrogen-donor ligand (isothiocyanate), which has a higher field strength than sulfur-donor ligands.
3. Ethylenediamine (B): Also a nitrogen-donor chelating ligand (en), which typically acts as a strong-field ligand.
4. S^2- (A): A sulfur-donor ligand, which is a weak-field ligand and has the lowest field strength among the given options.

Therefore, arranging them in the decreasing order of their ligand field strength gives:
CN^- (D) > NCS^- (C) > Ethylenediamine (B) > S^2- (A)

Thus, the correct decreasing order sequence is (D), (C), (B), (A).