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 a trigonal bipyramid (b).

Step 1: Understanding the Problem

We need to determine the molecular geometry of iron pentacarbonyl. Iron pentacarbonyl has the chemical formula Fe(CO)_5, where iron is bonded to five carbon monoxide (CO) ligands.

Step 2: Investigating Molecular Shape

Iron pentacarbonyl is a coordination complex. To determine the geometry, we consider the coordination number and electron geometry around the central atom, iron.

Step 3: Analyzing Electron Pair Geometry

The coordination number of the central atom, iron, in Fe(CO)_5 is 5 because it is bonded to five CO ligands. According to the VSEPR theory, a neutral complex with a coordination number of 5 typically has a trigonal bipyramidal geometry.

Step 4: Matching Geometry to Options

Among the given options, a trigonal bipyramidal geometry corresponds to option (b), which is the correct description of the molecular geometry of iron pentacarbonyl.

Key Concepts

Coordination Complex

A coordination complex is a fascinating concept in chemistry where a central atom—often a metal—binds to a group of molecules or ions, forming a distinct entity. This bonded group, known as ligands, surround the metal atom, creating a unique arrangement. In the case of iron pentacarbonyl, the central metal is iron, while the ligands are carbon monoxide molecules. Coordination complexes have diverse applications, from industrial catalysis to biological processes. Each complex's structure depends on factors such as the size, charge, and electron configuration of the metal, as well as the nature of the ligands.

• Central atom: A metal, here it's iron.
• Ligands: Molecules or ions like CO.
• Bonds: Formed through coordination bonds.

The geometry of a coordination complex can vary significantly, which is where theories like VSEPR come into play.

VSEPR Theory

The VSEPR (Valence Shell Electron Pair Repulsion) theory is a simple yet powerful model used to predict the shape of molecules based on electron pair interactions. It posits that electron pairs around a central atom will arrange themselves as far apart as possible to minimize repulsion, influencing the molecular geometry. For coordination complexes, like iron pentacarbonyl, VSEPR helps in determining the overall structure by considering the coordination number—the number of ligand bonds to the central atom.

• Coordination number: Indicates the number of ligand bonds.
• Electron pair repulsion: Determines the spatial arrangement.

In the specific case of iron pentacarbonyl, this theory concludes that with a coordination number of 5, the geometry is most likely trigonal bipyramidal.

Trigonal Bipyramidal

The trigonal bipyramidal shape is one of the fundamental geometries predicted by VSEPR theory, especially for compounds with a coordination number of 5. This shape features two different positions for ligands:

• Axial positions: Two ligands are located along a vertical axis.
• Equatorial positions: Three ligands are placed in a plane perpendicular to the axis.

The structure allows for minimal repulsion between the ligands, ensuring stability. In a trigonal bipyramidal arrangement, the equatorial positions are slightly more favorable energetically than axial ones. This geometry is essential for understanding various molecular behaviors and interactions.

Iron Pentacarbonyl

Iron pentacarbonyl, with the formula Fe(CO)_5, serves as a classic example of a coordination complex with a trigonal bipyramidal geometry. It consists of an iron atom at the center bound to five CO ligands. This unique arrangement results in a well-defined molecular shape, crucial for its chemical interactions and applications, such as in organic synthesis and catalysis.

• Chemical formula: Fe(CO)_5
• Geometry: Trigonal bipyramidal

Understanding its structure gives insights into its reactivity and functional capabilities in various scientific and industrial fields. The precise arrangement of CO ligands around the iron ensures optimal stability and efficiency in its applications.