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

Fe has an oxidation state of +1.

Step 1: Review the Complex Formula

The brown ring complex is given as \(\left[\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{5}\mathrm{NO}^{+}\right]\mathrm{SO}_{4} \). It consists of an iron (\(\mathrm{Fe}\)) ion, five water molecules (\(\mathrm{H}_{2} \mathrm{O}\)), a nitrosonium ion (\(\mathrm{NO}^{+}\)), and a sulfate ion (\(\mathrm{SO}_{4}^{2-}\)).

Key Concepts

Oxidation State

Oxidation state, also known as oxidation number, is a concept used in chemistry to describe the distribution of electrons among the atoms in a molecule or complex compound. It is a way of tracking how many electrons each atom has gained or lost in a chemical reaction. In the context of the brown ring complex compound \[\left[\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{5}\mathrm{NO}^{+}\right] \mathrm{SO}_{4} \]we need to determine the oxidation state of the iron (\(\mathrm{Fe}\)) ion. The sulfate (\(\mathrm{SO}_{4}^{2-}\)) ion carries a charge of \(-2\), and the nitrosonium ion (\(\mathrm{NO}^{+}\)) has a charge of \(+1\).
The water molecules (\(\mathrm{H}_{2} \mathrm{O}\)) are neutral. Thus, the overall charge of the complex part \(\left[\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{5}\mathrm{NO}^{+}\right]\) must equal \(+2\) to offset the sulfate ion. Solving for the oxidation state \(\mathrm{X}\) of iron gives\[\mathrm{X} + (0) + (+1) = +2\]So, \(\mathrm{X} = +1\).This shows that the oxidation state of the iron in the brown ring complex is \((\mathrm{Fe}^{1+})\).However, typically for traditional nomenclature, it might be incorrectly presumed to be \(+2\) without considering the special stability or complex interactions in experimental setups. Knowing this helps us answer such problems in exams and better understand reactions involving complex compounds.

Coordination Chemistry

Coordination Chemistry is a field of chemistry that focuses on the study of complex compounds that have a central atom connected to surrounding molecules or ions, termed as ligands. These central atoms are usually metals, and the ligands are bound to the metal center by coordinate covalent bonds. This chemistry helps in understanding the structure, properties, and reactivity of these compounds.In the brown ring complex, \[\left[\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{5}\mathrm{NO}^{+}\right] \]coordination occurs through water molecules (\(\mathrm{H}_2\mathrm{O}\)) and a nitrosonium ion (\(\mathrm{NO}^+\)). Each of these ligands provides a lone pair of electrons to form a bond with the central iron ion (\(\mathrm{Fe}\)). The total coordination number of the iron in this compound is six, as it interacts with five water molecules and one nitrosonium ion. These interactions are governed by principles like crystal field theory and ligand field theory, which explain how ligand electron pairs affect the metal center and influence its properties.Understanding coordination chemistry of this complex aids in comprehending how such interactions facilitate certain properties like the distinctive color of the complex, which leads to its common name, the "brown ring".

Complex Compounds

Complex compounds, also known as coordination complexes, are molecules consisting of a central metal atom bonded to surrounding ions or molecules, known as ligands. These compounds play a significant role in biological systems and industrial applications.In the case of the Brown Ring Complex, \[\left[\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{5}\mathrm{NO}^{+}\right] \mathrm{SO}_{4} \]this coordination compound consists of an iron ion at its center bonded to different ligands:

• Five water molecules (\(\mathrm{H}_2\mathrm{O}\))
• A nitrosonium ion (\(\mathrm{NO}^+\))

It is noteworthy for its brown color when it forms as a precipitate during certain chemical reactions, particularly in tests for nitrate ions in solution.
Complex compounds exhibit unique properties as they combine various ligands and metal ions in diverse structures. The interactions in these compounds involve changes in electronic arrangements that lead to distinct colors and spectral properties, which can be explained further by the study of electronic transitions and ligand field effects.The study of complex compounds like the brown ring complex allows chemists to create new materials with specific properties, useful in everything from magnetic materials to catalysts in chemical reactions.