Carbonyls, metal Stretching frequencies

This simple picture of bonding is convenient to use, and often completely acceptable. However, it does lack sophistication and may not be used to explain some of the subtleties of these systems. One obvious point in this regard concerns infrared spectral data. Coordination of carbon monoxide to a metal invariably leads to a lower carbonyl stretching frequency (vco). implying a lower CO bond order as predicted. However, the values for vcn may be considerably higher for metal complexes of an isocyanide than are the values for the ligand itself. The valence-bond picture cannot rationalize... 

A84. L. H. Jones, Inorganic Vibrational Spectroscopy, volume 1. Dekker, New York, 1971. Chapters Metal carbonyls, pp. 141-161 (38). Discussion of carbonyl stretching frequencies, force constants, etc., for binary carbonyls. 

Table Stoichiometry and corresponding IR stretching frequencies of different metal carbonyls. Charges for monocarbonyls were estimated from the correlation between IR stretching frequency and charge of the metal center as inferred from theoretical calculations (for details see [15,17]) ...

For the study of mixed oxides, one should characterize the various sites. In this case, the first step is to characterize the CO adsorption at various equilibrium pressures at low temperature, followed by evacuation at increasing temperatures to obtain information about the stabilities of the various species. Although the C—O stretching frequency is the most informative parameter, the data determining the stabilities of the various species can be decisive for the assignment of the bands. Multiple carbonyls adsorbed on the same metal cation are possible, and in order to identify them isotopic mixtures should be used. Sometimes the polycarbonyls are very stable and in this case, if 12CO is adsorbed first and then 13CO introduced, mixed species may not form at ambient temperature. 

All the group 4B metallocene dicarbonyl complexes exhibit two strong metal carbonyl stretching frequencies (Ax and Bf) with the exception of bis(i7-indenyl)dicarbonyltitanium (34) and bis(i7-tetrahydroindenyl)dicar-bonyltitanium they exhibit three frequencies due to splitting of the Bt... 

Comparison of the C-O stretching frequencies for a series of metal carbonyl complexes can reveal interesting trends. The complexes listed below all obey the 18-electron rule, but with different numbers of CO ligands attached, the metal atoms do not have the same increase in electron density on them because the coordination numbers are different. 

In this case, the product is the fac isomer, in which all NH3 ligands are trans to the CO molecules. Ammonia does not form ty bonds to metals because it has no orbitals of suitable energy to accept electron density. Thus, the back donation from Cr in Cr(NH3)3(CO)3 goes to only three CO molecules, and the bond order is reduced even more than it is in Cr(CO)s, where back donation occurs equally to six CO molecules. There is, of course, an increase in Cr-C bond order and stretching frequency in Cr(NH3)3(CO)3 compared to Cr(CO)s. Based on the study of many mixed carbonyl complexes, it is possible to compare the ability of various ligands to accept back donation. When this is done, it is found that the ability to accept back donation decreases in the order... 

The preparation of carbonyl-lr—NHC complexes (Scheme 3.1) and the study of their average CO-stretching frequencies [7], have provided some of the earliest experimental information on the electron-donor power of NHCs, quantified in terms of Tolman s electronic parameter [8]. The same method was later used to assess the electronic effects in a family of sterically demanding and rigid N-heterocyclic carbenes derived from bis-oxazolines [9]. The high electron-donor power of NHCs should favor oxidative addition involving the C—H bonds of their N-substituents, particularly because these substituents project towards the metal rather than away, as in phosphines. Indeed, NHCs have produced a number of unusual cyclometallation processes, some of which have led to electron-deficient... 

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