Vibrational spectroscopy metal-carbon stretching vibrations

One of the classic examples of an area in which vibrational spectroscopy has contributed to the understanding of the surface chemistry of an adsorbate is that of the molecular adsorption of CO on metallic surfaces. Adsorbed CO usually gives rise to strong absorptions in both the IR and HREELS spectra at the (C-O) stretching frequency. The metal-carbon stretching mode ( 400 cm-1) is usually also accessible to HREELS. 

Carbon monoxide on metals forms the best-studied adsorption system in vibrational spectroscopy. The strong dipole associated with the C-O bond makes this molecule a particularly easy one to study. Moreover, the C-0 stretch frequency is very informative about the direct environment of the molecule. The metal-carbon bond, however, falling at frequencies between 300 and 500 cm1, is more difficult to measure with infrared spectroscopy. First, its detection requires special optical parts made of Csl, but even with suitable equipment the peak may be invisible because of absorption by the catalyst support. In reflection experiments on single crystal surfaces the metal-carbon peak is difficult to obtain because of the low sensitivity of RAIRS at low frequencies [12,13], EELS, on the other hand, has no difficulty in detecting the metal-carbon bond, as we shall see later on. 

Carbon monoxide on metals forms the best-studied adsorption system in vibrational spectroscopy. The strong dipole associated with the C-O bond makes this molecule a particularly easy one to study. Moreover, the C-O stretch frequency is... 

Where X in M—H—X is a main group element, different behavior is observed. The case of carbon is particularly important, and here a weak or medium weak asymmetric stretch is observed at a frequency slightly lower than v(C—H) for the uncomplexed ligand. For example, v(MHC) as appears at 2510 cm for TaCp(CFTMe3)Cl. In metal borohydride complexes (X = B), the v(BHM)as vibrations are ca. 200cm to lower wavenumber than the v(B—H) terminal vibrations. Raman spectroscopy has not been used routinely on metal hydrides, but useful conclusions can be drawn from this technique. ... 

Infrared spectroscopy of olefin complexes is a less useful probe of n-bonding than infrared spectroscopy of CO complexes. Binding of an olefin to an electron-rich metal center does reduce the C-C stretching frequency, as one would expect from the reduction of the C-C bond order due to Ti-backbonding. However, the C-C stretch of a coordinated olefin is weaker than that of coordinated carbon monoxide because the vibration of the olefin creates a smaller change in the dipole moment. (Recall that symmetric vibrations are not observed in the infrared spectrum because of a lack of change in the dipole moment.) Thus, the olefin stretch is weak and lies at a frequency that overlaps with other bands. 

---