Question

The number of CO stretching bands in the infrared spectrum of $Fe(CO)_5$ is

• A. 1
• B. 2
• C. 3
• D. 4

Answer 1

The Correct Option is B

To determine the number of CO stretching bands in the infrared (IR) spectrum of \(Fe(CO)_5\), we need to consider the molecular geometry and symmetry of the complex.

The iron pentacarbonyl, \(Fe(CO)_5\), has a trigonal bipyramidal geometry. This structure has a central iron atom surrounded by five carbon monoxide (CO) ligands. Two of the CO ligands are axial, and three are equatorial.

For such geometries, the vibrational modes can be analyzed using group theory. Each type of CO bond can have different vibrational modes based on their positions:

1. The axial CO bonds are symmetrical and equivalent.
2. The equatorial CO bonds are symmetrical among themselves.

Therefore, by applying group theoretical analysis, we find:

• There are 2 types of CO stretching vibrations in this structure:
  • One type corresponds to the stretch of the axial COs, and
  • Another type corresponds to the stretch of the equatorial COs.

Each type of stretching will result in a distinct band in the IR spectrum. Hence, the number of distinct CO stretching bands in the IR spectrum of \(Fe(CO)_5\) is 2.

Therefore, the correct answer is 2.