Bis(dicarbonyl-pi-cyclopentadienyliron)-A Solid-State Vibrational Spectroscopic Lesson

FT-Raman spectroscopy is increasingly important as an investigatory tool, especially for chemical species in the solid state. Despite a general increase in the study of crystalline materials, the vibrational spectroscopy of molecular solids is covered in few chemistry courses, perhaps because of perceived theoretical difficulties. The present paper shows how easy it is to cover the subject by detailing a discussion of the spectral characteristics of the terminal ν(CO) modes of the crystalline forms of the cis and trans isomers of (η5-C5H5)2Fe2(CO)4. These molecules belong to different molecular point groups (C2v, cis; C2h, trans) but crystallize in the same space group (P21/c, C2h5) and so provide two different examples of vibrational analyses appropriate to this space group. Use of infrared and Raman data for these species shows that it is no more difficult to make vibrational predictions for the solid state than for isolated species. Indeed, it can be not only easier but also more relevant to the data to hand, since Raman spectra, in particular, are commonly observed using solid samples.