Analysis of the vibrational spectra, force fields, and molecular structures of pentacarbonyl(methyl)manganese(I) and pentacarbonyl(methyl)rhenium(I)

Vibrational spectra of CH3Mn(CO)5 and CH3Re(CO)5 have been reinvestigated in particular in the far-IR (500–50 cm−1) region. Raman (3500–50 cm−1) solid phase (at ambient and liquid nitrogen temperature) and solution (in CH2Cl2) polarization data for the Re compound are presented. Based on the available and the present experimental data and the results of the normal coordinate calculations, the assignment of the frequencies has been reviewed and some modifications in the low frequency region have been suggested. Force-field calculations have been performed on the basis of vibrational data of the –CH3, –CD3, and –13CH3 species of gaseous, dissolved, and solid samples. Density functional theory calculations show that the rotational barrier of the methyl group is less than 0.5 kcal mol−1. The study of pressure effect on CO and M–CH3 stretching force constants proved that the stronger CO bonds have a relatively smaller pressure effect than the much weaker M–CH3 bonds. As a characteristic of pressure sensitivity ‘bond compressibility’ has been introduced.