Brown ring complex known as:

Explanation

To solve the question regarding the brown ring complex, we will follow these steps:

### Step 1: Identify the Components
The brown ring complex is formed from the reaction of iron(II) sulfate (FeSO4) with nitric oxide (NO) and water (H2O).

### Step 2: Write the Reaction
When FeSO4 is mixed with NO and water, a complex is formed. The reaction can be represented as:
\[ \text{FeSO}_4 + \text{NO} + 5 \text{H}_2\text{O} \rightarrow \text{[Fe(NO)(H}_2\text{O})_5]^{2+} + \text{SO}_4^{2-} \]

### Step 3: Formulate the Complex
The resulting complex is known as the brown ring complex, which can be represented as:
\[ \text{[Fe(NO)(H}_2\text{O})_5]^{2+} \text{SO}_4^{2-} \]

### Step 4: Name the Complex
The brown ring complex is specifically known as "Nitrosyl iron(II) complex" or simply "Brown ring complex".

### Final Answer
The brown ring complex is known as the **Nitrosyl iron(II) complex**.

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