Problem 59 - A Complete Resource Book in Chemistry for JEE Main

Question

Formula of ferrocene is (a) \(\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]^{4}\) (b) \(\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]^{3+}\) (c) \(\left[\mathrm{Fe}(\mathrm{CO})_{3}\right]\) (d) \(\left[\left(\mathrm{C}_{6} \mathrm{H}_{5}\right)_{2} \mathrm{Fe}\right]\)

Step-by-Step Solution

Answer

The correct option is most closely related to (d), despite the misnamed ligands.

Step 1: Understanding the Question

The problem asks for the correct chemical formula of ferrocene. We are given four different options, and we need to choose the right one.

Step 2: Identifying Ferrocene

Ferrocene is a well-known organometallic compound, characterized by its sandwich structure. It is composed of an iron (Fe) atom sandwiched between two cyclopentadienyl ( C5H5) ligands.

Step 3: Analyzing the Options

Now we analyze each option: (a) \[\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]^{4-}\]is hexacyanoferrate(II), not ferrocene. (b) \[\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]^{3+}\]is hexacyanoferrate(III), also not ferrocene.(c) \[\left[\mathrm{Fe}(\mathrm{CO})_{3}\right]\]is iron carbonyl, not ferrocene.(d) \[\left[(\mathrm{C}_{6} \mathrm{H}_{5})_{2} \mathrm{Fe}\right]\]This more closely resembles the known formula for ferrocene, but it should have cyclopentadienyl rings instead of phenyl groups.

Step 4: Identifying the Correct Formula

The correct formula for ferrocene involves the iron atom bonded to two cyclopentadienyl anions, structurally represented as \[\text{Fe(C}_5\text{H}_5)_2\]. None of the options match this exactly, but options often rely on structural similarity, and (d) is often mistaken for such due to notations in answer keys.

Step 5: Concluding the Analysis

Since we know the structural formula, (d) is typical to see in questions testing understanding of ligand bonding in structures like ferrocene. The structural resemblance makes (d) the closest option given to the true ferrocene configuration.

Key Concepts

Organometallic Compounds

Organometallic compounds are fascinating molecules that contain bonds between a metal and organic ligands—molecules that donate electrons to the metal. These compounds are integral in numerous chemical processes.
They serve important roles in organic synthesis and catalysis.

• Organometallic compounds bridge the gap between organic and inorganic chemistry.
• Their unique structures allow them to perform functions that purely organic or inorganic compounds cannot.

These often include metals from the d-block of the periodic table, which grants them a variety of oxidation states and coordination environments. This allows the design of highly versatile chemical reactions.

Sandwich Structure

A distinctive feature of some organometallic compounds is the sandwich structure. This unique configuration occurs when a metal atom is positioned between two parallel, flat organic rings. Ferrocene, the focus of our exercise, is one of the most famous examples of a compound with a sandwich structure.
In this structure:

• The metal center is typically a transition metal, like iron in ferrocene.
• Two organic rings, such as cyclopentadienyl ligands, encase the metal symmetrically.

The rings do not bond directly with each other, yet they tightly surround the metal, resembling a sandwich. This arrangement confers both stability and unique chemical properties to the compound.

Cyclopentadienyl Ligand

The cyclopentadienyl ligand is an essential component of many organometallic compounds, especially those with a sandwich structure. In ferrocene, the iron atom is bonded with two cyclopentadienyl rings. These rings are derived from cyclopentadiene, a simple five-carbon ring with alternating double bonds.
Key characteristics include:

• The cyclopentadienyl ion ( ext{C}_5 ext{H}_5^{-}) is aromatic, granting it substantial electronic stability.
• It coordinates to metals through its negative charge, donating electron density and stabilizing the metal center.

This ligand plays a critical role in defining the properties and reactivity of ferrocene and similar compounds.

Chemical Formula Analysis

Chemical formula analysis involves understanding the structural details that lead to the correct representation of a compound on paper. Getting a chemical formula right is important for both educational and practical purposes.
When analyzing ferrocene's formula, it's important to recognize the defining structural elements:

• Iron (Fe) as the central metal atom.
• Two cyclopentadienyl rings depicted as ( ext{C}_5 ext{H}_5).

In its standard representation, ferrocene is shown as ext{Fe(C}_5 ext{H}_5)_2, highlighting the central iron atom flanked by two organic ligands. This depiction forms the basis of how chemists write and talk about organometallic compounds.