The vibrational spectra and assignment of bis-(π-cyclopentadienyl)-iron(II) and the bis-(π-cyclopentadienyl)cobalt(III) cation
Raman and i.r. spectra (4000–70 cm.–1) are reported for solid ferrocene and for the isoelectronic cation bis-(π-cyclopentadienyl)cobalt(III)+, in the solid state as the tribromide, [Co(C5H5)2]+Br3–, and in solution as the chloride. Raman polarization measurements assist the assignment, which agrees with that originally proposed for ferrocene by Lippincott and Nelson rather than with the revised assignment recently put forward by Fritz. A reason is suggested for the unusually low Raman intensity of the totally symmetric C–H bending fundamental ν2 at ca. 800–850 cm.–1 in bis(cyclopentadienyl)metal compounds. In comparison with ferrocene, the metal–ring stretching force constant is slightly higher in the cobalticinium ion, but is dependent on the physical state of the sample. Although most of the ring fundamentals are similar in the two compounds, the frequencies of the out-of-plane C–H deformation modes of the cobalticinium ion are markedly higher. The possible origins of these effects are discussed.