Electronic structures of organometallic complexes of f elements LXXVII: Complementary information of polarized Raman spectra of oriented single crystals and model calculations on the basis of density functional theory for assigning the vibrational spectra

The far and mid infrared (FIR/MIR) spectra of powdered Fe(η5-C5Me5)2 (FeCp∗2) as well as the polarized Raman spectra of an oriented single crystal (where the four FeCp∗2 molecules of D5d symmetry in the unit cell are pairwise perpendicular to each other) have been recorded. The polarization selection rules were developed for this situation which allow the assignment of the observed Raman lines to irreducible representations (irreps) which agree well with the predictions of a calculation applying density functional theory. This finding suggests additional correlations to the observed (unpolarized) bands in the FIR/MIR spectra with the calculated wavenumbers (and their irreps) of IR active normal modes. Neglecting the νCH vibrations, a r.m.s. deviation of 7.0 cm−1 for 41 assignments (IR and Raman) could be achieved. The frequencies of the calculated νCH vibrations, however, are on the average 55 cm−1 higher than the experimental ones. Skeletal and intra-ligand vibrations could be separated and all previous assignments of the former had to be revised. Because of mixing, the graphical representations of the two skeletal modes of E1u symmetry do not resemble the idealized ones of a typical sandwich complex but those of FeCp2 shown in the literature. In-phase and out-of-phase intra-ligand motions of the Fe[(C5C5)]2 moiety have calculated wavenumber differences between 3 and 42 cm−1. Additionally, because of mixing with skeletal modes some of the low frequency intra-ligand vibrations display a noticeable dependence on the mass of the central atom which prevents the direct transfer of the identified βCCH3 and γCCH3 normal modes from MCp∗2 (M = Fe, Ru) to LnCp∗3 (Ln = La, Ce, Pr, Nd, Sm) complexes.

The results of density functional theory calculations on the frequencies of the vibrational spectra of Fe(η5-C5Me5)2 are compared to the frequencies and intensities of IR (pellets) and Raman active normal modes (assigned on the basis of polarization measurements of oriented single crystals).Figure optionsDownload as PowerPoint slideHighlights
► DFT calculations on normal modes of an organometallic sandwich complex.
► Polarized single crystal Raman spectra of an organometallic sandwich complex.
► Excellent agreement of experimental spectra and DFT predictions.