Abstract: The Far-IR and Far-Raman spectra of [Mn₂(CO)₁₀], [Mo(CO)₅] and [Re₂(CO)₁₀] were investigated by both experiments and density functional theory (DFT) calculations. The present study consisted of three main parts. (1) Binary systems consisting of [Mn₂(CO)₁₀], [Mo(CO)₅] and [Re₂(CO)₁₀] in toluene under argon were experimentally measured in flow-through mode with far-IR spectroscopy using diamond windows. The BTEM algorithm, a spectral reconstructed method, was applied to these far-IR spectroscopic data to accurately reconstruct pure component spectra of [Mn₂(CO)₁₀], [Mo(CO)₅] and [Re₂(CO)₁₀]. (2) The same binary systems were measured by Raman spectroscopy in flow-through mode using a quartz cell. Again, the BTEM algorithm was applied to these Raman spectroscopic data to reconstruct pure component spectra of [Mn₂(CO)₁₀], [Mo(CO)₅] and [Re₂(CO)₁₀]. In above two experiments, the concentrations of organometallic systems under investigation are very dilute (ca. 100ppm) – similar to the concentrations typically used in metal-mediated homogeneous catalysis. (3) DFT calculations were performed, using different functionals and basis sets, for the optimized geometries of [Mn₂(CO)₁₀], [Mo(CO)₅] and [Re₂(CO)₁₀]. Then, both the far-IR and far-Raman spectra were predicted. The predicted far-IR and far-Raman spectra of [Mn₂(CO)₁₀], [Mo(CO)₅] and [Re₂(CO)₁₀] are rather consistent with the reconstructed IR and Raman spectra from experiments. In particular, the M-C vibrations in the IR and Raman could be assigned as well as the M-M vibrations in the Raman. The present results indicate that theoretical calculations combining with the experimental spectroscopies is a valuable tool to validate the vibrations of metal-metal, metal-carbon bonds of metal carbonyls at low wavenumber. Such a combined approach seems applicable to a wide range of structural identification problems in reactive organometallic systems, such as homogeneously catalyzed systems, in order to identify new species in solution.

Key words: Density functional theory, BTEM, Metal carbonyl, Far-Raman and Far-IR