In the confirmatory test for the sulphide ion (), passing gas through an alkaline solution of sodium — Qualitative and Quantitative Analysis Chemistry Question
Question
In the confirmatory test for the sulphide ion ($S^{2-}$), passing $H_2S$ gas through an alkaline solution of sodium nitroprusside results in a purple coloration. What is the coordination complex responsible for this color?
Answer: B
💡 Solution & Explanation
Sodium nitroprusside is $Na_2[Fe(CN)_5NO]$. When it reacts with a sulphide ion ($S^{2-}$), the nucleophilic sulphide attacks the nitrosyl ($NO^+$) ligand. The coordination sphere changes, incorporating the sulphur to form the thionitrosyl complex $[Fe(CN)_5NOS]^{4-}$. The sodium salt of this complex, $Na_4[Fe(CN)_5NOS]$, exhibits a deep purple or violet color.
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