Which one of the following complexes is violet in colour?

Step 1: Identify the Colours of Each Complex

| Complex | Name | Colour | |---|---|---| | $\ce{[Fe(CN)5NOS]^{4-}}$ | Pentacyanonitrosylferrate sulphide | Violet/Purple ✅ | | $\ce{[Fe(SCN)6]^{4-}}$* | Iron(II) thiocyanate | Blood red in +3 state | | $\ce{Fe4[Fe(CN)6]3}$ | Prussian blue | Dark blue | | $\ce{[Fe(CN)6]^{4-}}$ | Hexacyanoferrate(II) | Yellow/amber |

Step 2: Origin of the Violet Complex

$\ce{[Fe(CN)5NOS]^{4-}}$ is formed in the Lassaigne sulphur test: $\ce{Na2S + Na2[Fe(CN)5NO] -> Na4[Fe(CN)5NOS]}$

When $\ce{S^{2-}}$ ion from the sodium extract reacts with sodium nitroprusside $\ce{Na2[Fe(CN)5NO]}$, the sulphide ion replaces the oxygen in the NO ligand to form the violet-coloured complex.

Step 3: Conclusion

Answer: (a) $\ce{[Fe(CN)5NOS]^{4-}}$ is violet in colour.

Key Points to Remember:

• Violet/purple = $\ce{[Fe(CN)5NOS]^{4-}}$ → indicates sulphur in Lassaigne's test
• Blood red = $\ce{[Fe(SCN)]^{2+}}$ → formed when N + S both present
• Prussian blue = $\ce{Fe4[Fe(CN)6]3}$ → indicates nitrogen in Lassaigne's test
• Yellow = ammonium phosphomolybdate → indicates phosphorus