Vibrational spectra carbonyl complexes

The carbonyl complex 33 is obtained in a clean displacement reaction when carbon monoxide is passed through a solution of 32 in methanol. Due to the increased charge on the cation, the product precipitates in the process. The rather low frequency of the C O stretching vibration in the IR spectrum of the solid (KBr disc) at 1960 cm-1 points to a strong Fe-CO interaction (100), as expected for the highly basic NN4 ligand environment. Similarly low values (1940-1960 cm-1) were reported for the pentaamine iron(II) complexes... 

Raman data are beginning to appear more frequently (see for example 1, 3, 4, 8, 9, 12, 14, 16, 17, 18, 26) thus making assignments much more convincing. The value of polarized infrared spectra has been long realized but only recently (7) has the first example of the polarized infrared spectrum of a carbonyl complex appeared Mn2(CO)io and Re2(CO)io were studied in a nematic liquid crystal. The value of Cotton-Kraihanzel force constants has been further discussed (10,15) and a reasonably complete assignment of the vibrational spectrum of the cation Re(CO)e has been published (1). A number of isotopically substituted species have been studied (5, 6, 16, 23, 25). As a result of one of these studies Bor (5) reassigned the spectrum of Mn2(CO)io. 

In carbonyl complexes, the number of C — O stretching bands cannot exceed the number of CO ligands. The alternative is possible in some cases (more CO groups than IR bands), when vibrational modes are not IR active (do not cause a change in dipole moment). Examples are given in Table 13-7. Because of their symmetry, carbonyl complexes of Tj and symmetry have a single carbonyl band in the IR spectrum. 

Carbonyl complexes provide convenient examples of vibrations that are visible and invisible in the IR spectrum. Identical reasoning applies to other linear monodentate ligands (such as CN and NO), and also to more complex ligands. We begin by considering several simple cases. 

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