Question

The formula of the compound which gives violet colour in Lassaigne's test for sulphur with sodium nitroprusside is:
(A) $N{a_4}[Fe{(CN)_6}S]$
(B) $N{a_4}[Fe{(CN)_5}NCS]$
(C) $N{a_4}[Fe{(CN)_5}NOS]$
(D) $N{a_2}[Fe{(CN)_5}NOS]$

Answer 1

In order to answer this question, you must be aware of the concepts of the coordinate compounds and also with the Lassaigne’s Test. Use The Lassaigne’s process to determine the colour and hence choose the correct answer.

Complete step-by-step answer: Step 1: In this step we will enlist all the Lassaigne’s Test:
Nitrogen, Sulphur, and halogens present in organic compounds are detected by Lassaigne’s test. Here, a small piece of Na metal is heated in a fusion tube with the organic compound. The principle is that, in doing so, Na converts all the elements present into ionic form.
The formed ionic salts are extracted from the fused mass by boiling it with distilled water. This is called sodium fusion extract.
Step 2: In this step, we will carry out the Lassaigne’s test on the given compound:
While preparing Lassaigne's extract, sulphur from the organic compound reacts with sodium to form sodium sulphide.
$Na + \,\,S\,\, \to \,\,N{a_2}S$
Sodium sulphide reacts with sodium nitroprusside to form a violet colour compound, which confirms the presence of Sulphur.

$N{a_2}S\,\, + \,\,N{a_2}[Fe{(CN)_5}NO]\,\, \to \,\,N{a_4}[Fe{(CN)_5}NOS]$
And hence we got our answer.

Clearly, the correct answer is Option (C).

Note: Coordination compounds of transition metals with weak-field ligands are often blue-green, blue, or indigo because they absorb lower-energy yellow, orange, or red light. Each of these complex ions has the same metal with the same oxidation state, so the ligand field is the relevant factor. However transition metals are special in that the energy difference between the non-degenerate d orbitals corresponds to the energy of radiation of the visible light spectrum.