During sodium nitroprusside test of sulphide ion in an aqueous solution, one of the ligands coordinated to the metal ion is converted to

In the sodium nitroprusside test the reagent used is sodium nitroprusside, $$Na_2[Fe(CN)_5NO]$$. In the anion complex $$[Fe(CN)_5NO]^{2-}$$ the six ligands present are five cyanide ions, $$CN^-$$, and one nitrosyl ligand, $$NO$$.

When sulphide ion $$S^{2-}$$ is added to an aqueous solution of sodium nitroprusside, the deep-purple coloured species $$[Fe(CN)_5NOS]^{4-}$$ is produced:

$$[Fe(CN)_5NO]^{2-} + S^{2-} \;\longrightarrow\; [Fe(CN)_5NOS]^{4-}$$

Comparing the coordination spheres before and after the reaction, we see that one of the original ligands, $$NO$$, is replaced by the thionitrosyl (thiocyanato-N-oxide) ligand $$NOS^-$$. The five $$CN^-$$ ligands remain unchanged.

Thus, during the test one coordinated ligand is converted to $$NOS^-$$.

Option A which is: NOS$$^{-}$$