MC-stretching frequencies

Although not all bands can be assigned in the 300-700 cm region of these complex systems, certain bands can still be associated with the appropriate symmetry species. For instance, the e bands of many carbonyl compounds were split in the solid state spectra and could consequently be identified (5, 25, 80). Also, where Raman data were available, e.g., for the derivatives Ni(CO)4 L , the ai fundamentals were readily identified from polarization measurements (31, 35, 43, 131, 222). Finally, correlation curves of the MC-stretching frequencies of the derivatives Ni(CO)4 L have also been plotted and, in an analogous way to those for the CO-stretching frequencies, used to establish certain assignments (35, 43, 131, 222). 

Table 2. MC-stretching frequencies of chromium- and nickel-carbonyls calculated at different reference geometries...

The next examples are the metal-carbon and carbon-oxygen stretching frequencies of Cr(CO)6 and Ni(CO)4 at experimental, LDA and LDA/NL reference geometries. Table 2 includes the MC-stretching frequencies, and Table 3 includes the CO-stretching frequencies of these systems. The experi-... 

MC and C°0 bonds. The decrease in CO bond order or force constant is estimated by studying the CO stretching frequency in IR spectroscopy. The IR spectra is characterised by frequency of vibration, which is related to force constant (k) as... 

Free CO has IR stretching frequency of 2143 cm-1. In the case of terminal CO groups in neutral molecules, the CO frequency has been found to be 2125-1850 cm-1. This suggests that the bond strength of CO has decreased or MC bond strength has increased. This is only possible if there is back bonding from filled metal... 

The pi bond between the metal and carbon atom arises as a result of a backbonding interaction between a filled metal rf-orbital of the appropriate symmetry and the empty pi star orbital of the carbonyl. Any factor that increases the electron density at the metal will increase the strength of the MC pi bond, and thus the contribution from resonance form b. The result is a decrease in the CO bond order and a resulting decrease in the CO stretching frequency in the infra-red spectrum of the complex. Aspects which affect the DR. spectrum of metal carbonyl derivatives include the cf-electron count of the metal, the donor properties of the ligands and the charge on the complex. For example, in case of [W(CO)5Cl] and... 

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