Metal Carbonyl - Analytical Characterization

Analytical Characterization

Important analytical techniques for the characterization of metal carbonyls are infrared spectroscopy and 13C NMR spectroscopy. These two techniques provide structural information on two very different time scales. Infrared active vibrational modes, such as CO-stretching vibrations are often fast compared to intramolecular processes, whereas NMR transitions occur at lower frequencies and thus sample structures on a time scale that, it turns out, is comparable to the rate of intramolecular ligand exchange processes. NMR data provides information on "time-averaged structures," whereas IR is an instant "snapshot." Illustrative of the differing time scales, investigation of dicobalt octacarbonyl (Co₂(CO)₈) by means of infrared spectroscopy provides 13 ν_CO bands, far more than expected for a single compound. This complexity reflects the presence of isomers with and without bridging CO-ligands. The 13C-NMR spectrum of the same substance exhibits only a single signal at a chemical shift of 204 ppm. This simplicity indicates that the isomers quickly interconvert.

Iron pentacarbonyl exhibits only a single 13C-NMR signal owing to rapid exchange of the axial and equatorial CO ligands by Berry pseudorotation.