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.

Step 1: Understand the Molecule

The molecule in question is iron pentacarbonyl, which has the chemical formula \( Fe(CO)_5 \). This molecule consists of a central iron atom surrounded by five carbon monoxide \((CO)\) ligands.

Step 2: Consider the Known Structure

Iron pentacarbonyl is a well-known example of a coordination complex with iron in the zero oxidation state.

Step 3: Identify Possible Geometries

With five ligands around a central atom, the two common geometries are square planar and trigonal bipyramidal. The square planar geometry would have the iron atom in the center of a square with four \( CO \) ligands at the corners and one more \( CO \) ligand either above or below the plane.

Step 4: Analyze the Coordination Geometry

Iron pentacarbonyl is known to adopt a trigonal bipyramidal shape where three of the carbon monoxide ligands occupy equatorial positions (in the same plane), and two occupy axial positions (perpendicular to that plane).

Step 5: Confirm the Correct Option

Based on molecular geometry knowledge, iron pentacarbonyl (\( Fe(CO)_5 \)) clearly takes on a trigonal bipyramidal structure.

Key Concepts

Molecular Geometry

Molecular geometry is all about the three-dimensional arrangement of atoms in a molecule. It helps us understand how molecules will look in space. For coordination complexes, like iron pentacarbonyl, the shape is often determined by the number of ligands—atoms or groups of atoms attached to a central atom. In coordination chemistry, knowing the correct molecular geometry allows scientists to predict physical and chemical properties of a molecule. It includes properties like polarity (how charges are distributed within a molecule) and reactivity (how easily a molecule reacts with others).

• Bond angles, distances, and spatial orientations all contribute to a molecule's geometry.
• Using VSEPR theory (Valence Shell Electron Pair Repulsion), one can predict geometric shapes.
• The shape can greatly influence how a molecule behaves in chemical reactions.

Understanding molecular geometries allows us to decipher structures of complex molecules and their potential functions. This knowledge also provides insights into how molecules interact with each other and with other substances in their environment, making it a crucial topic in chemistry.

Trigonal Bipyramidal Structure

A trigonal bipyramidal structure is one type of molecular geometry that can occur in molecules with five ligands around a central atom. This means the central atom has five atoms or groups attached to it. These structures are common in coordination complexes and can be visualized as having two distinct positions for the ligands:

• Equatorial positions: Three of the ligands lie in the same plane. They form a triangle around the central atom.
• Axial positions: The remaining two ligands are located above and below the plane. They're often 180° apart.

The equatorial bonds are typically spaced at 120°, while the axial bonds are 90° from the equatorial ones. Balancing these angles gives the molecule its unique shape. This configuration allows for separation or sharing of different electronic and steric influences effectively within a molecule. Trigonal bipyramidal geometry is key in chemistry, as it lets us predict how the molecule might look and behave. Understanding these structures aids in designing new molecules, especially in drug design and materials science.

Iron Pentacarbonyl

Iron pentacarbonyl, denoted as \(Fe(CO)_5\), is a popular example of a coordination complex where iron (Fe) is in the zero oxidation state. With five carbon monoxide (\(CO\)) ligands attached to a central iron atom, this complex has significant applications in various fields.

• The ligands attached are highly symmetrical, creating a balanced structure.
• Iron pentacarbonyl has a unique trigonal bipyramidal shape, which is confirmed through experimental evidence.
• This complex is pivotal in organometallic chemistry, where metal-carbon bonds are key.

Iron pentacarbonyl serves as a useful reagent in chemical synthesis and as a precursor to other iron compounds. It's also important industrially and is used in processes requiring iron catalysts. Understanding its geometry helps chemists exploit its properties for innovative solutions in various chemical reactions.