Deep Violet / Purple coloration

$$\text{S}^{2-} + [\text{Fe(CN)}_5\text{NO}]^{2-} \to [\text{Fe(CN)}_5\text{NOS}]^{4-} \quad \text{(Deep Violet / Purple)}$$

$$\text{Na}_2\text{S} + \text{Na}_2[\text{Fe(CN)}_5\text{NO}] \to \text{Na}_4[\text{Fe(CN)}_5\text{NOS}]$$

Concept: When alkaline sodium fusion extract (Lassaigne's extract) containing sulfide ions ($$\text{S}^{2-}$$) reacts with sodium nitroprusside, a coordination complex called sodium thionitroprusside is generated. The intense purple/violet color is due to a charge-transfer transition within this newly formed thionitroprusside complex.