Question

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

Short answer

The oxidation state of iron in this compound is +1 (Option a).

Step-by-step solution

Understand the Composition of the Complex

The complex compound given is \(\left[\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{3}\mathrm{NO}^{+}\right] \mathrm{SO}_{4}\). It comprises an inner complex \(\left[\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{3}\mathrm{NO}^{+}\right]\) and a sulfate ion \(\mathrm{SO}_{4}^{2-}\). The overall charge of the compound is neutral.

Identify Charges on Known Ligands and Ions

In the complex, \(\mathrm{H}_{2}\mathrm{O}\) (water) is a neutral ligand with no charge. The \(\mathrm{NO}^{+}\) is a positively charged cation with a charge of \(+1\). The sulfate ion \(\mathrm{SO}_{4}\) is outside the square brackets and has a charge of \(-2\).

Calculate the Charge on the Inner Complex

The overall compound must be neutral, implying the charge of \[\left[\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{3}\mathrm{NO}^{+}\right]\] is \(+2\) to balance the \(-2\) charge from the \(\mathrm{SO}_{4}^{2-}\).

Determine the Oxidation State of Iron

Let's denote the oxidation state of \(\mathrm{Fe}\) as \(x\). The charge of the inner complex is the sum of the charges of iron and the ligands: \[x + 3 \times 0 + 1 = +2\] Simplifying this, \[x + 1 = +2 \Rightarrow x = +1\]. So, the oxidation state of iron (\(\mathrm{Fe}\)) in this complex is \(+1\).

Key concepts

Complex Compounds

Complex compounds are fascinating molecular structures that consist of a central atom or ion surrounded by a set of molecules or ions known as ligands. Typically, the central atom in these complexes is a metal, often a transition metal due to their ability to accommodate various ligands. This feature makes these complexes especially versatile in chemistry.

In the case of the brown ring complex mentioned, it consists of a central iron (Fe) atom connected to ligands like water ( H_2O ) and the nitrosonium ion ( NO^{+} ). These components together form an inner coordination sphere that is often enclosed within square brackets in the chemical formula.

• The formation of complex compounds allows metals to exhibit various oxidation states and coordination geometries.
• Complexes play a crucial role in the stability and reactivity of materials in chemical reactions.

Understanding the composition and characteristics of these compounds is essential for appreciating their behavior and reactivities.

Ligands

Ligands are crucial to the structure of complex compounds, acting as the atoms, ions, or molecules that donate at least one pair of electrons to the central metal to form a coordination bond. Ligands can vary dramatically in size and nature, from simple ions to more complicated organic molecules.

In the example of the brown ring complex, the compound includes ligands such as water ( H_2O ), which is neutral, and the nitrosonium ion ( NO^{+} ), which is positively charged. These ligands influence the properties and characteristics of the complex.

• Neutral ligands, like water, do not contribute to the overall charge of the complex.
• Cationic ligands, like the nitrosonium ion, contribute positively to the charge.

The nature and number of ligands influence coordination numbers, bond angles, and the overall geometry of the complex, which affects its reactivity and interactions.

Charge Calculation

Charge calculation is a critical procedure for understanding the properties of complex compounds. It helps determine the oxidation state of the central metal atom, balance equations, and predict the stability of the compound. Charge calculation involves a consideration of the charges on all the components within the complex.

For example, in the given brown ring complex: \[\left[\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{3}\mathrm{NO}^{+}\right] \mathrm{SO}_{4}\] the \(\mathrm{H}_{2}\mathrm{O}\) ligands are neutral, the \(\mathrm{NO}^{+}\) ion carries a \(+1\) charge, and the sulfate ion (\mathrm{SO}_{4}^{2-}) has a \(-2\) charge.

• The inner complex must have an overall charge that balances the charge of the sulfate ion, resulting in a neutral compound.
• Using known charges, we find the inner complex must have a \(+2\) charge to neutralize the \(-2\) sulfate charge.

These calculations lead to determining the oxidation state of the metal atom, a fundamental step in complexes' formation and functionality.

Transition Metals

Transition metals are a unique subset of elements found in the d-block of the periodic table. They are versatile cations known for their ability to form various oxidation states due to partially filled d orbitals. This enables them to bond with diverse ligands, forming complex compounds.

Iron (Fe) is a transition metal found in the brown ring complex. Its properties allow it to exhibit different oxidation states, making it highly applicable in multiple chemical processes and materials. Transition metals like iron are characterized by:

• Variable oxidation states, allowing diverse interactions and compound formations with different ligands.
• Formation of colored compounds due to electronic transitions within the d-orbitals.
• The ability to form coordination complexes involving a wide range of ligands, impacting the compound's chemistry and applications.

Understanding transition metals' behavior in complexes is key to comprehending their role in a myriad of applications from catalysis in industrial processes to biological systems.