JEE Main · 2021 · 26 Aug 2021 · Shift 1 · Coordination Compounds

Sodium nitroprusside, $\mathrm{N}{\mathrm{a}}_2[\mathrm{Fe}(\mathrm{CN}{)}_5\mathrm{NO}]$, reacts with sulphide ion, the ${\mathrm{S}}^{2-}$ adding to the nitrogen of the coordinated nitrosyl to give the thionitrosyl complex $[\mathrm{Fe}(\mathrm{CN}{)}_5\mathrm{NOS}{]}^{4-}$, and the intense violet colour of this ion is the standard laboratory test for a soluble sulphide. Prussian blue, $\mathrm{F}{\mathrm{e}}_4[\mathrm{Fe}(\mathrm{CN}{)}_6{]}_3$, is blue; the hexacyanoferrate(II) ion is pale yellow because the strong field of cyanide pushes its $d$-$d$ absorption out of the visible region; and the thiocyanato complex of iron(II) is not violet, the blood-red thiocyanate colour belonging to iron(III). The violet species is $[\mathrm{Fe}(\mathrm{CN}{)}_5\mathrm{NOS}{]}^{4-}$.