Aqueous solution of nickel sulphate on treating with pyridine and then adding a solution of sodium nitrite gives dark blue crystals of :

Options

A. `[Ni(py)_(4)]SO_(4)`
B. `[Ni(py)_(2)(NO_(2))_(2)]`
C. `[Ni(py)_(4)](NO_(2))_(2)`
D. `[Ni(py)_(3)(NO_(2))]_(2)SO_(4)`

Explanation

To solve the question, we need to analyze the reaction that occurs when an aqueous solution of nickel sulfate is treated with pyridine and then sodium nitrite. Here’s a step-by-step breakdown of the solution:

### Step 1: Identify the reactants
The first reactant is nickel sulfate, which in aqueous solution can be represented as NiSO₄·6H₂O (hydrated form). The second reactant is pyridine (C₅H₅N), which is a strong field ligand.

**Hint:** Look for the chemical formula of nickel sulfate and understand its hydrated form.

### Step 2: Understand the role of pyridine
Pyridine is a stronger field ligand compared to water (H₂O). In coordination chemistry, stronger ligands can displace weaker ligands from the coordination sphere of the metal ion. Here, pyridine will displace the water molecules coordinated to nickel.

**Hint:** Recall the spectrochemical series to determine the strength of ligands.

### Step 3: Formation of the complex
When pyridine displaces water, it forms a complex with nickel. The complex formed can be represented as [Ni(C₅H₅N)₄]²⁺. In this complex, four pyridine molecules coordinate to the nickel ion.

**Hint:** Consider how many pyridine molecules are needed to replace the water molecules around the nickel ion.

### Step 4: Addition of sodium nitrite
After forming the nickel-pyridine complex, sodium nitrite (NaNO₂) is added. The nitrite ions can act as counter ions in the complex, leading to the formation of [Ni(C₅H₅N)₄](NO₂)₂.

**Hint:** Think about how counter ions balance the charge of the complex ion formed.

### Step 5: Color of the complex
The resulting complex [Ni(C₅H₅N)₄](NO₂)₂ is known to exhibit a dark blue color. This is characteristic of certain nickel complexes, especially those with strong field ligands like pyridine.

**Hint:** Remember that the color of coordination compounds can often indicate the type of ligands and the oxidation state of the metal.

### Conclusion
Thus, the dark blue crystals formed from the reaction of aqueous nickel sulfate with pyridine and sodium nitrite are [Ni(C₅H₅N)₄](NO₂)₂.

### Final Answer
The dark blue crystals formed are [Ni(C₅H₅N)₄](NO₂)₂.