Which of the following combines with `Fe^(2+)` ions to form brown complex ?

Options

A. `NO`
B. `N_2 O`
C. `N_2 O_3`
D. `N_2 O_5`

Answer

To solve the question of which compound combines with Fe²⁺ ions to form a brown complex, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Relevant Ions**: We need to determine which of the given compounds can react with Fe²⁺ ions. The common candidates in this context are nitrogen oxides, particularly NO (nitric oxide).

2. **Understand the Reaction**: When Fe²⁺ ions react with NO, a redox reaction occurs. The Fe²⁺ ions are oxidized to Fe³⁺ ions, while NO is reduced to N₂O or other nitrogen species.

3. **Formation of the Brown Complex**: The reaction between Fe²⁺ and NO leads to the formation of a complex known as the "brown ring complex." This complex can be represented as [Fe(H₂O)₅NO]²⁺. The presence of NO in the complex gives it a characteristic brown color.

4. **Conclusion**: Based on the reaction and the formation of the brown ring complex, we conclude that NO is the compound that combines with Fe²⁺ ions to form the brown complex.

### Final Answer:
The compound that combines with Fe²⁺ ions to form a brown complex is **NO (Nitric Oxide)**.

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