Many vibrations which occur in the far-infrared region of spectrum are similar to the types of vibrations found in other regions of the spectrum.  They fall in the far-infrared because the atomic masses are large and/or the forces resisting the deformation are small.  Most of the vibrations of metal carbonyls fall into this category and hence they give a good example of what can be expected.  A number of metal carbonyls e.g. BrMn(CO)5, ClMn(CO)5, HMn(CO)5, DMn(CO)5, CH3Mn(CO)5, CD3Mn(CO)5 and CF3Mn(CO)5, have been studied in the vapour phase for first time and it is now possible to discuss the general nature of the vibration in this region.  The metal-CO stretching frequencies are found between 350 and 475 cm-1.  The metal-C-O bending frequencies are more scattered and fall between 275 and 650 cm-1 depending in part upon the phase of motion. The OC-M-CO bending modes occur between 50 and 150 cm-1.  Other vibrations are founds in this region when one or more CO group are replaced by groups in which heavy atoms are attached to the metal. Factor which shifts the positions of the frequencies, such as mass or charge on the metal carbonyl moiety, will be discussed.

Zaid Mahmood Zaid ,