Problem 22 - A Complete Resource Book in Chemistry for JEE Main

Question

The brown ring complex compound is formulated as \(\left[\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{5} \mathrm{NO}\right] \mathrm{SO}_{4} .\) The oxidation state of iron in the compound is (a) 1 (b) 2 (c) 3 (d) 0

Step-by-Step Solution

Answer

The oxidation state of iron in the compound is 2.

Step 1: Understanding the Ligands

The compound given is \(\left[\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{5}\mathrm{NO}\right] \mathrm{SO}_{4} \). This is a coordination complex with \(\mathrm{H}_{2}\mathrm{O}\) and \(\mathrm{NO}\) as ligands. \(\mathrm{H}_{2}\mathrm{O}\) is a neutral ligand, and \(\mathrm{NO}\) can vary in its oxidation state (nitrosyl ligand can be either \(\mathrm{NO}^{+}\), \(\mathrm{NO}\)\(\bullet\) or \(\mathrm{NO}^{-}\)). In many complexes, \(\mathrm{NO}\) is treated as \(\mathrm{NO}^{+}\) with a charge of +1.

Step 2: Analyzing Sulfate

The \(\mathrm{SO}_{4}^{2-}\) is the counterion, which carries a -2 charge. Since the compound is electrically neutral, the charges of the complex and the \(\mathrm{SO}_{4}\) have to balance each other.

Step 3: Calculating the Charge on the Complex

Given that \(\mathrm{SO}_{4}^{2-}\) carries a -2 charge, the coordination complex \([\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{5}\mathrm{NO}]\) must carry a +2 charge to balance the sulfate ion. The coordinated ligands \(\mathrm{H}_{2}\mathrm{O}\) are neutral and do not add to the positive charge.

Step 4: Determining the Oxidation State of Iron

Assuming \(\mathrm{NO}^{+}\) contributes +1 charge, and the water ligands being neutral, the charge balance equation becomes: \(\text{charge of Fe} + 1 = +2\). Solving this gives the oxidation state of iron: \(\mathrm{Fe}^{2+}\).

Step 5: Final Conclusion

Since the calculations show that Fe has an oxidation state of +2, option (b) is correct.

Key Concepts

Brown Ring Complex

The brown ring complex is a fascinating example of a coordination compound. It is often used in qualitative analysis to detect the presence of nitrate ions in solution. The complex is represented by the formula \( \left[\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{5}\mathrm{NO}\right] \mathrm{SO}_{4} \), which showcases how iron forms a coordination sphere with various ligands. This coordination allows for the formation of a stable compound, notable for its distinctive brown color when nitrate ions are present.

• In these types of compounds, the metal center, which is iron (Fe) in the brown ring complex, is surrounded by a certain number of molecules or ions, known as ligands.
• The formation process is often demonstrated through the reaction of iron sulfate with a solution that has nitrate ions under acidic conditions.

Formation of this complex indicates the coordination of different ligands around a single metal, contributing to the overall stability and unique properties of the compound.

Oxidation State

The oxidation state, sometimes referred to as oxidation number, is a critical concept in understanding chemical compounds. It indicates the number of electrons that an atom gains, loses, or appears to use when bonding with other atoms. In coordination chemistry, determining the oxidation state of a metal center, such as iron in the brown ring complex, is essential for understanding the chemistry involved.
For the brown ring complex \( \left[\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{5}\mathrm{NO}\right] \mathrm{SO}_{4} \), determining the oxidation state involves analyzing the charges of the ligands and counterions:

• \(\mathrm{H}_{2}\mathrm{O}\) is a neutral ligand, contributing no charge.
• \(\mathrm{NO}\) is typically treated as \(\mathrm{NO}^{+}\) with a +1 charge.
• The \(\mathrm{SO}_{4}^{2-}\) is a doubly charged ion, acting as a counterbalance in the compound.

By using these pieces of information, we find that the oxidation state of iron must be +2 to ensure charge neutrality. The knowledge of oxidation states helps predict the chemical reactivity and properties of coordination complexes.

Coordination Complex

A coordination complex, also known as a complex compound, refers to a structure in which a central atom, typically a metal, is bonded to surrounding molecules or ions called ligands. These complexes play a pivotal role in various fields, including chemistry, biology, and materials science.
In the case of the brown ring complex, iron is the central metal, surrounded by water and nitrosyl ligands. Here’s why coordination complexes are significant:

• They allow metals to form more stable compounds by sharing electrons with ligands.
• Complexes often exhibit unique chemical properties, such as color changes, that can be exploited in chemical assays.
• Coordination complexes are crucial in the development of catalytic systems and biological processes, where they act as enzymes or enzyme mimics.

Understanding coordination complexes involves considering both the geometry of the complex and the electronic environment created by the ligands. These factors combine to give each complex its unique characteristics.

Ligands in Chemistry

Ligands are ions or molecules that donate a pair of electrons to a metal atom to form a coordination bond. In the realm of coordination chemistry, ligands are diverse and play a crucial role in the formation and stability of coordination complexes.
In the brown ring complex, two types of ligands are present: water (\(\mathrm{H}_{2}\mathrm{O}\)) and nitrosyl (\(\mathrm{NO}\)).

• \(\mathrm{H}_{2}\mathrm{O}\) is a neutral ligand, and it forms part of the coordination sphere by donating lone pairs to the iron center without contributing to the overall charge.
• \(\mathrm{NO}\), as a nitrosyl ligand, can adopt different oxidation states, but in this complex, it is often found as \(\mathrm{NO}^{+}\), contributing a +1 charge to the system.

Ligands can significantly affect the physical and chemical properties of the coordination complex, including its geometry, color, and reactivity. The choice and arrangement of ligands around the metal center are pivotal in the functionality of the complex, influencing all these vital attributes.