Metal carbonyl complexes vibrational spectroscopy

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. 

Reviews containing material relevant to this chapter have appeared dealing with the synthesis of organometallic complexes by conventional and metal-vapour methods, thermochemical studies, vibrational and photoelectron spectroscopy of metal carbonyl complexes, nitrogen fixation, catalytic, insertion, and ligand-transfer reactions, and alkene metathesis. Several authors have contributed chapters to a volume dealing with the uses of organometallic complexes in... 

Two other publications on Ir (73 keV) Mossbauer spectroscopy of complex compounds of iridium have been reported by Williams et al. [291,292]. In their first article [291], they have shown that the additive model suggested by Bancroft [293] does not account satisfactorily for the partial isomer shift and partial quadrupole splitting in Ir(lll) complexes. Their second article [292] deals with four-coordinate formally lr(l) complexes. They observed, like other authors on similar low-valent iridium compounds [284], only small differences in the isomer shifts, which they attributed to the interaction between the metal-ligand bonds leading to compensation effects. Their interpretation is supported by changes in the NMR data of the phosphine ligands and in the frequency of the carbonyl stretching vibration. 

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]. 

Since polynuclear carbonyls take a variety of structures, elucidation of their structures by vibrational spectroscopy has been a subject of considerable interest in the past. The principles involved in these structure determinations were described in Sec. 1-10. However, the structures of some polynuclear complexes are too complicated to allow elucidation by simple application of selection rules based on symmetry. Thus the results are often ambiguous. In these cases, one must resort to X-ray analysis to obtain definitive and accurate structural information. However, vibrational spectroscopy is still useful in elucidating the structures of metal carbonyls in solution. 

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