During sodium nitroprusside test of sulphide ion in an aqueous solution, one of the ligands coordina

Q: Why doesn't the sulphide ion just replace the cyanide ligand?

Cyanide ($CN^-$) is a very strong-field ligand and forms highly stable bonds with Iron. The nitrosyl ($NO^+$) ligand, however, is highly electrophilic, making it a prime target for the nucleophilic attack by the $S^{2-}$ ion.

Q: Does the oxidation state of Iron change from +2 to +3 during this test?

No, this is a classic misconception! In the reactant $[Fe(CN)_5(NO)]^{2-}$, Iron is $+2$ because $NO$ is $NO^+$. In the product $[Fe(CN)_5(NOS)]^{4-}$, Iron is still $+2$ because the new ligand is $NOS^-$. The redox change happens within the ligand, not the metal.

Q: Can this test be used to detect sulphate ($SO_4^{2-}$) or sulphite ($SO_3^{2-}$) ions?

Absolutely not. The sodium nitroprusside test is highly specific to the sulphide ($S^{2-}$) ion because only $S^{2-}$ has the correct nucleophilic character to attack the $NO^+$ ligand and form the purple thionitrosyl complex.

Q: What exactly causes the intense purple color of the final complex?

The brilliant purple color arises from a phenomenon called Ligand-to-Metal Charge Transfer (LMCT). The new electronic structure of the $NOS^-$ ligand allows for specific absorption of light in the visible spectrum, reflecting purple.

Q: Is the $NO$ ligand neutral in the initial complex?

No, in sodium nitroprusside, the nitrosyl ligand exists as the nitrosonium ion ($NO^+$). This positive charge is what makes it so reactive towards the incoming negatively charged sulphide ion.

Question

During sodium nitroprusside test of sulphide ion in an aqueous solution, one of the ligands coordinated to the metal ion is converted to

Options:

(A) NOS^{-}

(B) SCN^{-}

(C) SNO^{-}

(D) NCS^{-}

Sigma Percentile · Accepted Answer

The qualitative analysis of inorganic salts is often a visual treat, filled with vibrant color changes and fascinating coordination chemistry. One of the most famous and visually striking tests is the **Sodium Nitroprusside Test** used to detect the presence of sulphide ions ($S^{2-}$).

Let's dive deep into the molecular mechanics of this beautiful reaction and understand exactly what happens to the ligands coordinated to the metal ion.

### Analyzing the Setup

The reagent we use is sodium nitroprusside, which has the chemical formula $Na_2[Fe(CN)_5(NO)]$. When dissolved in an aqueous solution, it dissociates to give the active complex ion, $[Fe(CN)_5(NO)]^{2-}$.

If we look closely at this octahedral complex, the central metal atom is Iron ($Fe$). It is surrounded by six ligands: five cyanide ions ($CN^-$) and one nitrosyl ion ($NO^+$).

A common trap for students is miscalculating the oxidation state of Iron here. Since the five $CN^-$ ligands contribute a total charge of $-5$ and the $NO^+$ ligand contributes $+1$, the Iron must be in the **$+2$ oxidation state** to give the complex an overall charge of $-2$.

### The Master Reaction

When an aqueous solution containing sulphide ions ($S^{2-}$) is added to the sodium nitroprusside solution, a fascinating chemical attack occurs.

You might intuitively think that the $S^{2-}$ ion would displace one of the ligands. However, the cyanide ligands are strong-field and tightly bound, and the nitrosyl ligand is highly electrophilic. Instead of a substitution, the sulphide ion directly attacks the $NO^+$ ligand!

$$[Fe(CN)_5(NO)]^{2-} + S^{2-} \rightarrow [Fe(CN)_5(NOS)]^{4-}$$

The $NO^+$ and $S^{2-}$ combine to form a brand new ligand: the **thionitrosyl group** ($NOS^-$).

### Final Conclusion and The Trap

This transformation yields a new complex ion, $[Fe(CN)_5(NOS)]^{4-}$, which corresponds to the sodium salt $Na_4[Fe(CN)_5(NOS)]$.

The formation of this specific complex is accompanied by a dramatic color change to a **brilliant purple**, which serves as the confirmatory signal for sulphide ions.

Let's revisit our oxidation state trap. In the new complex, we have five $CN^-$ (total $-5$) and one $NOS^-$ (total $-1$). To balance the overall $-4$ charge of the complex, the Iron must still be in the **$+2$ oxidation state**. The metal's oxidation state does not change; only the ligand transforms!

Therefore, during the sodium nitroprusside test, the $NO$ ligand is converted to $NOS^-$.

Animated Solution for Chemistry - Salt Analysis: During sodium nitroprusside test of sulphide ion in an aqueous solution, one of the ligands coordinated to the metal ion is converted to

Select Answer:

Visualized Solution

Visual Anchor: The Reactant

Logic Bridge: Complex Composition

Raw Setup: Introducing Sulphide

Atomic Compute: The Attack

Final Answer: The Purple Complex

The Way Forward: Oxidation State Trap

The Sigma Insight: Salt Analysis

Analyzing the Setup

The Master Reaction

Final Conclusion and The Trap

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