Problem 33 - A Complete Resource Book in Chemistry for JEE Main

Question

The structure of iron pentacarbonyl is (a) square planar (b) trigonal bipyramid (c) triangular (d) none

Step-by-Step Solution

Answer

The structure of iron pentacarbonyl is trigonal bipyramidal (b).

Step 1: Identify the Molecular Geometry

To determine the molecular geometry of iron pentacarbonyl \((Fe(CO)_5)\), identify the number of ligand atoms attached to the central iron atom. Iron pentacarbonyl consists of 5 carbonyl \((CO)\) groups bonded to iron \((Fe)\).

Step 2: Apply VSEPR Theory

According to VSEPR (Valence Shell Electron Pair Repulsion) theory, a central atom with five bonding pairs of electrons will arrange these bonds to minimize repulsion, leading to a trigonal bipyramidal geometry.

Step 3: Conclusion on Geometry

Therefore, based on VSEPR and the number of bonds, iron pentacarbonyl has a trigonal bipyramidal molecular structure.

Key Concepts

Iron Pentacarbonyl

Iron pentacarbonyl, denoted as (Fe(CO)_5) , is a fascinating compound in the world of chemistry. It belongs to a class of compounds known as metal carbonyls, which consist of a metal center surrounded by carbon monoxide ligands.
The compound Fe(CO)_5 features five CO groups bonded to a central iron atom.
This specific arrangement is key to understanding its molecular geometry and properties.

• The ligands : The carbon monoxide ligands attach to iron using both their carbon and oxygen ends, driving the molecular stability.
• Ligand arrangement: All five CO groups are symmetrically distributed around the iron atom, maximizing the compound’s stability through uniform electron distribution.

Understanding iron pentacarbonyl’s structure is crucial for recognizing how molecular geometry affects chemical behavior and interactions.

VSEPR Theory

VSEPR Theory, short for Valence Shell Electron Pair Repulsion, is a reliable model used to predict the geometry of molecules based on electron pair interactions. In essence, VSEPR considers that electron pairs surrounding a central atom tend to position themselves as far apart as possible to minimize repulsion.
This theory is central to determining the molecular geometry of compounds, including iron pentacarbonyl (Fe(CO)_5) .

• Electron pairs as determining factors: VSEPR theory identifies bonding pairs and lone pairs to predict shape.
• Application on iron pentacarbonyl: With five CO groups as bonding pairs and no lone pairs on the iron atom, the VSEPR model predicts a specific spatial arrangement.

By applying this theory, we can deduce the expected trigonal bipyramidal structure due to the symmetrical placement of bonding pairs.

Trigonal Bipyramidal Structure

The trigonal bipyramidal structure is a well-defined geometric arrangement for molecules with five electron pairs around a central atom. In the case of iron pentacarbonyl (Fe(CO)_5) , this structure plays a pivotal role in the molecule’s properties and reactivity.

• Geometry insight: The structure comprises two distinct positions: three equatorial bonds at 120-degree angles from each other, and two axial bonds positioned at 90 degrees to the equatorial plane.
• Synergy of placement: This geometrical orientation helps evenly distribute electron repulsion and stabilize the molecule.
• Impact on behavior: Due to this arrangement, the compound exhibits specific chemical reactions and interactions, characteristic of its trigonal bipyramidal shape.

Understanding such molecular architecture aids in predicting the chemistry of related metal carbonyls, enhancing knowledge about structure-dependent properties.