Identify the reagent used in the test

Sodium nitroprusside is the complex: $\text{Na}_2[\text{Fe}(\text{CN})_5\text{NO}]$ The coordinated ligands around iron are five cyanide ligands and one nitrosyl ligand, $\text{NO}$.

What happens in the sulphide ion test?

Sulphide ion $\text{S}^{2-}$ reacts with sodium nitroprusside to give a violet/purple coloured complex. In this reaction, the coordinated nitrosyl ligand is transformed into a thionitrosyl-type ligand.

Determine the converted ligand

The nitrosyl ligand $\text{NO}$ is converted to: $\text{NOS}^-$ Thus the relevant ligand formed during the test is $\text{NOS}^-$.

Check options

• A: $\text{NOS}^-$ — correct
• B: $\text{SCN}^-$ — thiocyanate, not formed here
• C: $\text{SNO}^-$ — not the conventionally accepted representation in this test
• D: $\text{NCS}^-$ — isothiocyanate, not formed here

Final answer

The ligand coordinated to the metal ion is converted to: $\boxed{\text{NOS}^-}$