Infrared Intensities of Metal Carbonyl Stretching Vibrations

Infrared Intensities of Metal Carbonyl Stretching Vibrations, 10, 199 Infrared and Raman Studies of w-Complexes, 1, 239 Insertion Reactions of Compounds of Metals and Metalloids, 5, 225 Insertion Reactions of Transition Metal-Carbon o-Bonded Compounds I Carbon Monoxide Insertion, 11, 88... 

Hydrogenation Reactions Catalyzed by Transition Metal Complexes, 17, 319 Infrared Intensities of Metal Carbonyl Stretching Vibrations, 10, 199... 

Infrared Intensities of Metal Carbonyl Stretching Vibrations, 10, 199... 

Having established both a probable mathematical form of the true shape of an infrared absorption band and its relationship to its apparent or observed profile, Ramsay outlined three methods for determining true integrated absorption intensities. As these methods have been used exclusively in all reported studies of absolute intensities of metal carbonyl stretching vibrations, they are now described in some detail. ... 

Finally, although Noack (799) pioneered the use of the absolute intensities of carbonyl stretching vibrations for structural information as far back as 1962, the results of very few studies have appeared subsequently. The primary purpose of this article is to attempt to assess the relevance of both relative and absolute intensities of metal carbonyl infrared-active stretching vibrations to structural problems which are known to be characteristic of metal carbonyl chemistry. Although the study of absolute intensities is still beset with practical and theoretical difficulties, these are due, at least in part, to the lack of activity in this field of research. It is hoped, therefore, that this article will help to stimulate a greater interest in the topic. 

In general, the position, shape, and intensity of an infrared-active absorption band are solvent-sensitive. In particular, carbonyl stretching vibrations of metal carbonyl complexes exhibit large frequency shifts 10, 11, 21, 21, 28, 37, 64, 69, 12, 115, 120) and sizable broadening of bands... 

Hydrogen induces a decrease in the carbonyl stretching frequency of some 40 cm". At high temperature there is a further irreversible decrease, which the authors relate to a modification of the metal-CO interaction by surface residues. Exposure to CO/Hj at 25O°C results in IR-detectable adsorbed species. Bands at 1585/1378 cm" and around 3000 cm" are assigned to formate and hydrocarbon ad-species, respectively. The authors conclude that these surface species are not reaction intermediates. This very important conclusion is based on the observations that (1) the infrared bands continue to grow in intensity even after the FT reaction has reached steady state, and (2) substitution of Dj for Hj does not result in a significant rate of decrease in intensity of H-derived vibrations (Fig. 10). 

A general discussion of the infrared spectra of transition metal complexes has been published (23). Although correlations of CO bending modes (24) and of band intensities (25,26) have appeared and vibrations of other ligands are often noted, the number of CO stretching modes and their exact frequencies are the data most frequently cited for metal carbonyl com-... 

Infrared spectroscopy is especially useful for metal carbonyls because the intense C=0 stretching vibration at 1700-2100 cm appears a region that is relatively free of other bands. The intensity is large thanks to the polarization on binding (M-C +=0 ) and consequent large d ildr. In polycarbonyls, the v(CO) bands are coupled in a way that depends on the symmetry of the M(CO) fragment. 

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