Vibrational spectra and structure of the cyclopentadienyl-anion (Cp^-), the pentamethylcyclopentadienyl-anion (Cp*^-) and of alkali metal cyclopentadienyls CpM and Cp*M (M=Li, Na, K)

Structural, electronical and vibrational properties of Cp^- and Cp*^-, and of alkali metal cyclopentadienyl (CpM, M=Li, Na, K) and pentamethylcyclopentadienyl (Cp*M, M=Li, Na) complexes have been studied. The main goals of the study were to investigate the influence of the CH₃ groups on the spectral features and on the M-C force constants and the change of ionic character of the M-C bond for different metals. FT-IR, FT-FIR and FT-Raman spectra of LiCp* and NaCp* compounds were recorded. Density functional theory calculations have been performed in order to obtain optimized geometries, vibrational frequencies and IR intensities. Calculated vibrational data were systematically compared to the experimental ones. Based on the calculations and experimental data, the vibrational spectra of Cp^- and CpM were revised and reinterpreted, and a complete assignment of Cp*^- and Cp*Li, Cp*Na vibrations was proposed. Correlations have been determined for the different metal atoms and the charge distribution, bond orders, bond energies and force constants.