Question

Which of the following combines with \(\mathrm{Fe}(\mathrm{II})\) ions to form a brown complex? (a) \(\mathrm{N}_{2} \mathrm{O}_{3}\) (b) \(\mathrm{N}_{2} \mathrm{O}_{4}\) (c) \(\mathrm{NO}\) (d) \(\mathrm{N}_{2} \mathrm{O}\)

Short answer

Option (c) \( \text{NO} \) forms a brown complex with \( \text{Fe(II)} \).

Step-by-step solution

Identify the Metal Ion

We are working with the iron ion specifically in the +2 oxidation state, denoted as \( \text{Fe(II)} \). This means the iron ion has a charge of \( +2 \).

Understand Complex Formation

A complex is formed when a central metal ion like \( \text{Fe(II)} \) is surrounded by molecules or ions which can donate pairs of electrons, known as ligands. Different ligands can change the color of the complex formed.

Consider Ligand Binding

We need to determine which option among \( \text{N}_2\text{O}_3 \), \( \text{N}_2\text{O}_4 \), \( \text{NO} \), and \( \text{N}_2\text{O} \) can act as a ligand to \( \text{Fe(II)} \) and produce a brown complex. Not all ligands will bind to \( \text{Fe(II)} \) or will produce a brown color.

Identify Known Brown Complex

Nitric oxide, \( \text{NO} \), is known to combine with \( \text{Fe(II)} \) to form a complex. This complex is historically recognized as having a brown color. This reaction can be seen in circumstances like the formation of a brown ring during analytical tests for nitrates.

Key concepts

Iron(II) Oxidation State

Iron in the +2 oxidation state is referred to as Iron(II) or \( ext{Fe}^{2+}\). This means that the iron atom has lost two electrons, giving it a positive charge of +2. This oxidation state is common in many iron complexes and is crucial in biological systems and industrial applications.
Apart from having lost two electrons, \( ext{Fe}^{2+}\) is smaller compared to metallic iron, making it a compact and efficient core for forming complexes with other molecules.
In chemistry, this iron state is relatively reactive and can form complexes with various ligands, which are pivotal to many chemical reactions and processes. Understanding how iron behaves in this state is essential for learning about its role in both nature and technology.

Ligand Binding

Ligands are molecules or ions capable of donating a pair of electrons to a central metal ion, forming a complex. In our context, they interact with \( ext{Fe(II)}\) to create stable and colorful complexes.
When a ligand binds to a metal ion, it typically acts through a lone pair of electrons, creating a coordinate covalent bond. Ligands come in many forms, from simple ions like chloride (Cl-) to more complex molecules like ethylenediamine.

• The nature of the ligand affects the color and stability of the complex.
• Strong field ligands often result in low-spin complexes.
• Weak field ligands result in high-spin complexes.

Ligand binding is fundamental for understanding various reactions, including biological processes and analytical tests.

Nitric Oxide Complex

Nitric oxide, \( ext{NO}\), is a small, yet significant molecule in chemistry and biology. It serves as a ligand that can bind effectively with \( ext{Fe(II)}\) to form complexes known for their distinct brown color. This binding is significant in certain chemical tests, such as the brown ring test used to detect nitrates.
This complex formation is characterized by the donation of an electron pair from the nitrogen atom in \( ext{NO}\) to the iron center, resulting in a bond that stabilizes the \( ext{Fe-NO}\) complex.
The unique properties of the \( ext{Fe-NO}\) complex, such as its color, make it easily recognizable and useful in analytical chemistry.

Color Change in Complexes

Color changes in complexes arise from the way they interact with light. The color we perceive often results from certain wavelengths being absorbed while others are reflected or transmitted.
In the case of \( ext{Fe(II)}\) complexes, different ligands can result in different colors. This is due to the changes in the electronic environment around the iron ion.

• The type of ligand involved can alter the energy levels of electrons in the metal ion.
• Light absorption occurs when electrons transition between these energy levels.
• The specific energies of these transitions determine the color we observe.

Understanding the principles of these color changes helps us identify compounds and predict the outcome of complex formation in various chemical reactions.