Which one of the following complexes is violet in colour?

🧠 The Violet Complex Is Sodium Nitroprusside-Sulphide

The species $[\mathrm{Fe(CN)_5NOS}]^{4-}$ is formed when sodium nitroprusside, $\mathrm{Na_2[Fe(CN)_5NO]}$, reacts with sulfide ions ($\mathrm{S^{2-}}$) in alkaline solution. The product is a deep violet colour — the basis of the classical qualitative test for sulfide.

🗺️ Why the Other Options Aren't Violet

| Complex | Colour | |---|---| | $\mathrm{Fe_4[Fe(CN)_6]_3 \cdot H_2O}$ | deep blue (Prussian blue) | | $[\mathrm{Fe(CN)_5NOS}]^{4-}$ | violet ✓ | | $[\mathrm{Fe(SCN)_6}]^{4-}$ / $[\mathrm{Fe(SCN)}]^{2+}$ | blood red (Fe³⁺ + SCN⁻ test) | | $[\mathrm{Fe(CN)_6}]^{4-}$ (ferrocyanide) | yellow / pale |

⚡ The Sulfide Test Reaction

The qualitative-analysis test:

$\mathrm{Na_2[Fe(CN)_5NO]} + \mathrm{Na_2S} \rightarrow \mathrm{Na_4[Fe(CN)_5NOS]}$ (violet)

The S²⁻ adds to the nitrosyl nitrogen, forming an N–S linkage that gives the intense purple colour. This is the standard classroom test for hydrogen sulfide / sulfide ions.

⚠️ Don't Confuse Nitroprusside with Ferrocyanide

• $\mathrm{Na_2[Fe(CN)_5NO]}$ — nitroprusside, has 5 CN⁻ + 1 NO ligand.
• $\mathrm{Na_4[Fe(CN)_6]}$ — ferrocyanide, has 6 CN⁻.

They look similar on paper but behave very differently. Only nitroprusside reacts with sulfide to give the violet [Fe(CN)₅NOS]⁴⁻.

$\boxed{\text{Answer: (2) } [\mathrm{Fe(CN)_5NOS}]^{4-}}$