INTENSITIES OF CO STRETCHING MODES IN INFRARED SPECTRA OF ADSORBED CO AND METAL CARBONYLS
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The presently available data relating to the infrared intensity of CO stretching motion in adsorbed CO and in metal carbonyls are reviewed. For adsorbed CO which exhibits higher CO stretching frequency than gaseous CO, it is proposed that the intensity is predominantly controlled by the strength of bond interaction. A reversal in sign of d/drco at an intermediate CO-adsorbate bond strength accounts for the relationship between frequency and intensity. Adsorbed CO with lower frequency than gaseous CO, and metal carbonyls, exhibit enormously enhanced infrared intensity in the CO stretching mode. This enhancement arises from extensive -bond interaction with the adsorbate or central metal. It is possible to derive certain rules for the ratios of intensities of CO modes of different symmetry species in metal carbonyls, assuming local MCO bond moment derivatives, and assuming separability of the CO stretches from other normal modes. These rules are tested and found inadequate for substituted octahedral carbonyl systems. Estimation of bond angles in monosubstituted carbonyl compounds from intensity ratios is therefore an uncertain procedure. An alternative approach is proposed which makesuse of data in a series such as M(CO)6, M(CO)5L, trans-M(CO)4L2. 1967, American Chemical Society. All rights reserved.