Chapter 3, Problem 32P

Interpretation Introduction

Interpretation:

Difference between cis-${\text{M(CO)}}_{\text{2}}{\text{X}}_{\text{2}}$ and trans-${\text{M(CO)}}_{\text{2}}{\text{X}}_{\text{2}}$ by using information from CO stretching region of IR spectra is to be explained and a derivation of the number of ${\nu }_{\text{CO}}$ modes for each molecule is to be included in answer.

Concept Introduction:

Raman and infrared (IR) spectroscopies are branches of vibrational spectroscopy.

For a mode of vibration to be infrared (IR) active, it must give rise to a change in the molecular electric dipole moment.

For a mode of vibration to be Raman active, it must give rise to a change in the polarizability of the molecule.

If the symmetry label (${\text{A}}_{\text{1}}\text{,}{\text{B}}_{\text{1}}\text{,}\text{E}$) of normal mode of vibration is associated with a product term (${\text{x}}^{\text{2}}\text{,}\text{xy}$) in the character table, then the mode is Raman active.

If the symmetry label (${\text{A}}_{\text{1}}\text{,}{\text{B}}_{\text{1}}\text{,}\text{E}$) of normal mode of vibration is associated with x, y or z in the character table, then the mode is IR active.

The number and nature of the symmetry elements of a given molecule are conveniently represented by its point groups and give rise to labels such as ${\text{D}}_{\text{3h}}$, ${\text{T}}_{\text{d}}$, $O_h$, ${\text{I}}_h$, ${\text{D}}_{\text{2d}}$, ${\text{C}}_{\text{2}}$, ${\text{C}}_{\text{3v}}$ etc.