Metal sandwich compounds such as ferrocene (FeCp2) and ruthenocene (RuCp2) exhibit the six skeletal modes shown in Fig. 1. Among them, the ring tilt (v5), the metal–ring stretch (v3), and the ring–metal–ring bending (v6) are infrared active under D5h (or D5d) symmetry.
K. Nakamoto, Carl Udovich, John R. Ferraro, and A. Quattrochi, "Metal Isotope and High Pressure Effects on the Infrared Active Skeletal Vibrations in Metal Sandwich Compounds," Appl. Spectrosc. 24, 606-608 (1970)