Infrared spectroscopic studies on metal carbonyl compounds. Part XVII. A comparative study of the vibrational spectrum of ditechnetium decacarbonyl and of the decacarbonyls of manganese and rhenium in the C–O stretching region

Abstract

The i.r. spectra of the M₂(CO)₁₀ carbonyls (M = Mn, Tc, or Re) have been reinvestigated in the C–O stretching region with special emphasis on the weak isotopic bands. In the case of the Mn compound the effects of isotopic enrichment have also been examined. The i.r.-inactive (A₁, E₂, and E₃) fundamentals were calculated from the isotopic satellites and the results agree within ±1 cm^–1 with recent Raman data. For the technetium compound no Raman study seems to exist and thus the A₁(2123 and 1999·5 cm^–1), E₂(2029 cm^–1), and E₃(1990·5 cm^–1) frequencies reported here are the first data on these i.r.-inactive modes. The force and interaction constants of all three decacarbonyls have been calculated in a C–O factored force field without a priori neglections.

Both the equatorial and the axial C–O force constants of Tc₂(CO)₁₀[16·642 and 16·316 mdyn Å^–1, respectively (1 mdyn Å^–1= 10² N m^–1)], are the highest in the triad, whereas most of its interaction constants have intermediate values between those of Mn₂(CO)₁₀ and Re₂(CO)₁₀. For Re₂(CO)₁₀ the effect of the eclipsed configuration (D_4h) on the eq ⋯ eq′ interaction constants has also been studied.