Problem 97 - A Complete Resource Book in Chemistry for JEE Main

Question

A 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 here is (a) 1 (b) 2 (c) 3 (d) 4

Step-by-Step Solution

Answer

The oxidation state of iron in the complex is (a) 1.

Step 1: Identify Charges

Determine the known charges for each component in the compound. Water molecules \(\mathrm{H}_2\mathrm{O}\) are neutral, nitrosonium ion \(\mathrm{NO}^+\) has a charge of +1, and sulfate ion \(\mathrm{SO}_4^{2-}\) carries a -2 charge.

Step 2: Set Up the Equation

Let the oxidation state of iron (Fe) be \(x\). The compound is electrically neutral, so the sum of charges in the coordination complex and sulfate has to equal zero. Create an equation: \([x + 5(0) + 1] + (-2) = 0\). This equation represents the charges from \(\mathrm{Fe}^{x}, \,\mathrm{5\,H_{2}\,O} (0),\,\mathrm{NO}^+ (1),\,\text{ and } \,\mathrm{SO}_{4}^{2-} (-2)\).

Step 3: Solve for x

Substitute known values into the equation: \(x + 1 - 2 = 0\). Simplify the equation to find \(x\), which represents the oxidation state of iron.

Step 4: Calculate

Rearrange and solve for \(x\): \(x + 1 - 2 = 0\) simplifies to \(x - 1 = 0\). Therefore, \(x = 1\).

Key Concepts

Oxidation States

In coordination chemistry, oxidation states are crucial for understanding the electron configuration of a central atom within a complex. The oxidation state, often expressed as a Roman numeral, indicates the charge on an atom assuming that all the bonds involving ions are entirely ionic, with full electron transfer. This is a simplified way to predict the charge distribution in a compound.
To calculate the oxidation state of an atom within a complex ion, such as iron in \([\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{5} \mathrm{NO}^{+}\right]\), we account for the charges contributed by each ligand and any overall charge on the complex or compound.
Here's a quick method for determining oxidation states:

• Annotate the known charges of all present ions and ligands.
• Write an equation based on the neutrality of the whole compound, where the sum of the oxidation states equals the net charge.
• Solve this equation to find the unknown oxidation state.

For complex ions like the brown ring complex, understanding oxidation states helps in predicting chemical reactions and the stability of the complex.

Complex Ions

Complex ions consist of a central metal atom or ion surrounded by molecules or anions termed as ligands. These are formed by coordinate covalent bonds, where the ligand donates a pair of electrons to the metal center. The architecture of complex ions, like \([\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{5} \mathrm{NO}^{+}\right]\), exemplifies the fascinating ways in which transition metals can form intricate structures.
Ligands are crucial players in complex ion stability and function. They can be neutral molecules like water, or charged species like nitrosonium \(\mathrm{NO^+}\). In the studied complex, water molecules are neutral, contributing no charge; however, the nitrosonium ion adds a positive charge. It is important to distinguish the total charge distribution, as this influences properties such as solubility, reactivity, and magnetic behavior.
When analyzing complex ions:

• Identify the primary metal and count its coordination number—the number of ligand attachments.
• Assess the charges of ligands and their impact on the overall charge of the complex.
• Consider the geometry and spatial arrangement which influences the properties of the complex.

Understanding these principles allows chemists to predict how such complexes will behave in different chemical environments.

Transition Metals

Transition metals are found in the d-block of the periodic table and are characterized by their ability to form various oxidation states. This diversity stems from their partially filled d-orbitals, which can participate in bonding. These elements, like iron in the brown ring complex, are central to coordination chemistry.
Transition metals are highly adaptable, forming a wide range of compounds with different properties. The variable oxidation states, such as the +1 state of iron in our example, can significantly alter the chemical and physical characteristics of the compounds they form.
Here are some defining features of transition metals:

• They often display high melting and boiling points.
• They can form colored compounds due to electronic transitions between d-orbitals.
• Their paramagnetic or diamagnetic nature is due to unpaired d-electrons.
• They are excellent catalysts as they facilitate a wide array of chemical reactions.

Understanding transition metals' role in creating complex ions provides essential insights into their reactivity and applications in both natural and industrial processes.