Apart from X-ray crystallography, important analytical techniques for the characterization of metal carbonyls are infrared spectroscopy and C 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 provide information on "time-averaged structures", whereas IR is an instant "snapshot". Illustrative of the differing time scales, investigation of dicobalt octacarbonyl (Co(CO)) by means of infrared spectroscopy provides 13 ν bands, far more than expected for a single compound. This complexity reflects the presence of isomers with and without bridging CO ligands. The C 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 (on the NMR timescale) interconvert.
Iron pentacarbonyl exhibits only a single C NMR signal owing to rapid exchange of the axial and equatorial CO ligands by Berry pseudorotation.