Question

Which one of the following complexes is violet in colour ?

• A. \([Fe(SCN)_6]^{3-}\)
• B. \([Fe(CN)_5 NOS]^{4-}\)
• C. \(Fe_4[Fe(CN)_6]_3 \cdot H_2O\)
• D. \([Fe(CN)_6]^{4-}\)

Hint:

The sodium nitroprusside test for sulfide ions is one of the few tests in inorganic chemistry that produces a distinctive violet/purple color. Remember the addition of sulfur to the NO ligand.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
The color of coordination complexes is a result of specific electronic transitions. Many of these are used as diagnostic tests in qualitative analysis.
Step 2: Detailed Explanation:
1. \([Fe(SCN)_6]^{3-\):} This complex (or specifically \([Fe(H_2O)_5(SCN)]^{2+}\)) is responsible for the characteristic **blood-red** color in the test for \(Fe^{3+}\) ions with thiocyanate.
2. \([Fe(CN)_5 NOS]^{4-\):} This is the purple/violet colored complex formed when sodium nitroprusside \([Fe(CN)_5 NO]^{2-}\) reacts with sulfide ions (\(S^{2-}\)) in alkaline solution.
\[ [Fe(CN)_5 NO]^{2-} + S^{2-} \to [Fe(CN)_5 NOS]^{4-} \]
3. \(Fe_4[Fe(CN)_6]_3 \cdot H_2O\): This is known as **Prussian Blue** and has a deep blue color.
4. \([Fe(CN)_6]^{4-\):} This is the ferrocyanide ion, which is pale yellow in solution.
Step 3: Final Answer:
The complex \([Fe(CN)_5 NOS]^{4-}\) is violet in colour.