Vibrational spectroscopy transition-metal carbonyl complexes

In IR-spectroscopy, the basic concept is that the vCO vibration is sensitive to the amount of electron density available on the metal, if a transition metal carbonyl complex like [M(CO)jL] (M=Cr, Mo, W L=phosphane, phosphite) is considered. We use [M(CO)jL] rather than the more familiar Tohnan system [Ni(CO)3L] simply because P-NMR data is more readily available for the former. 

For carbonyl complexes, a combination of IR (p. 11) and 13C NMR spectroscopy will often reveal the molecular symmetry and also provide further information about the nature of the metal centre to which the CO ligand(s) is bound. Flowever, the spectroscopic events involved occur within completely different time frames. Molecular vibrations (IR and Raman spectroscopy) are rapid relative to molecular fiuxional processes. NMR transitions, however, are slow, often comparable in rate to intramolecular fluxionality and even intermolecular ligand exchange processes. This can lead to time-averaged chemical environments being observed on the NMR timescale . So long as this is borne in mind, 13C NMR spectroscopy is a very valuable technique and can provide thermodynamic and kinetic data about such processes over a temperature range [variable temperature (VT) NMR]. 

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