In test of `NO_(3)^(-)` ion, the dark brown ring complex is forme,d which is true of thic complex?

Options

A. the colour is due to charge transfer spectra
B. iron and NO both have +1 charge
C. The complex species can be represented as `[Fe^(1)(H_(2)O)(5)NO]^(3+)`
D. Iron has +2 oxidation state and NO is neutral

Explanation

To answer the question regarding the dark brown ring complex formed in the test for the nitrate ion (NO₃⁻), we will analyze the formation and characteristics of this complex step by step.

### Step-by-Step Solution:

1. **Understanding the Reaction**:
The dark brown ring complex is formed when nitrate ions (NO₃⁻) react with concentrated sulfuric acid (H₂SO₄) in the presence of ferrous sulfate (FeSO₄). The reaction leads to the formation of a complex known as the brown ring complex.

**Hint**: Identify the reactants involved in the formation of the brown ring complex.

2. **Formation of the Complex**:
The complex can be represented as [Fe(H₂O)₅(NO)]²⁺, where NO represents nitrosyl, and it is formed when Fe²⁺ ions interact with NO in the presence of water molecules. The complex appears brown due to the presence of charge transfer transitions.

**Hint**: Write the formula of the complex formed and note the ligands involved.

3. **Oxidation States**:
In the complex, iron (Fe) is in the +1 oxidation state. This can be determined by analyzing the charges: the nitrosyl (NO) is in the +1 oxidation state, and the overall charge of the complex is +2. Therefore, Fe must be +1 to balance the charge.

**Hint**: Calculate the oxidation states of the metal and ligands in the complex.

4. **Color of the Complex**:
The brown color of the complex is attributed to ligand-to-metal charge transfer (LMCT). This occurs when electrons are transferred from the ligand (in this case, the nitrosyl) to the metal center (Fe), which is deficient in electrons.

**Hint**: Explain the reason behind the color of the complex in terms of electronic transitions.

5. **Conclusion**:
The brown ring complex has the following characteristics:
- It is formed from Fe²⁺ and NO in the presence of H₂O and H₂SO₄.
- Iron is in the +1 oxidation state.
- The complex exhibits a brown color due to charge transfer spectra.

**Hint**: Summarize the properties of the complex and the significance of its formation.

### Final Answer:
The dark brown ring complex formed in the test for nitrate ions is characterized by:
- The presence of Fe in the +1 oxidation state.
- The formation of the complex [Fe(H₂O)₅(NO)]²⁺.
- The brown color due to ligand-to-metal charge transfer (LMCT).