DETAILED ANALYSIS OF CARBONYL STRETCHING VIBRATIONS IN AXIAL AND EQUATORIAL SUBSTITUTED IRON CARBONYL-COMPOUNDS - ABSOLUTE INFRARED INTENSITIES AND FORCE CONSTANTS OF CARBONYL LIGANDS
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Force constant calculations employing a restricted force field of CO stretching vibrations have been carried out for axially mono- and disubstituted phosphine, phosphite, and carbene and equatorially monosubstituted olefin derivatives of iron pentacarbonyl. The computations were made possible with the aid of frequency data obtained from 13CO photochemically enriched species. For the axially substituted LFe(CO)4 species the force constant for the CO ligand opposite L (phosphines, phosphites, or carbene) was found to be greater than those cis to L by approximately 0.50 mdyn/. For the equatorially substituted (olefin)Fe(CO)4 derivatives the two CO groups opposite the olefin ligand exhibited somewhat larger force constants as compared with the two CO ligands trans to one another. The calculated force constants for the equatorially substituted (olefin)Fe(CO)4 species were all significantly larger than the corresponding values in the axially substituted LFe(CO)4 derivatives. Absolute infrared intensities of vco were determined and used to calculate characteristic MCO group dipole moment derivatives in these complexes. The results of the intensity studies are discussed in terms of vibrational coupling and geometrical and electronic properties of the CO ligands as well as the substituent ligands. In addition, a combination of vco force constants and dipole moment derivatives have been employed in understanding and comparing the electron distributions in the metal carbonyl framework of the two isomeric forms of monosubstituted iron pentacarbonyl derivatives. These studies provide as well some criteria for distinguishing between the two types of infrared spectra commonly observed in the isomeric forms of monosubstituted iron pentacarbonyl species. 1974, American Chemical Society. All rights reserved.