M=C stretching frequency

A.J. Carty, Guelph-Waterloo Centre What are the frequencies of the metal-carbon (v(M=C)) stretching frequencies in the tungsten alkylidyne compounds One might expect these to be quite high in view of the x-ray data showing short M=C bonds and evidence of multiple bond reactivity. 

For example, in a series of Fe(CN)5Xn- complexes, the changes in the M-C stretching frequencies are less than 30 cm-1 when the iron is oxidized from Fe(II) to Fe(III) (26). The large observed excited state frequencies are consistent with the large increase in metal formal charge expected in the excited state. 

More direct evidence for M—CO n bonding can be obtained by photoelectron spectroscopy. For Cr(CO)6 and W(CO)6, the photoionization of one of those electrons that must be responsible for whatever M—CO 77 bonding exists was carried out and the effect of removing such an electron on the frequency of the totally symmetric M—C stretching vibration in the resulting M(CO)6 ion measured. The frequency of this vibration was, in each case, found to be significantly lower than that of the corresponding vibration in the neutral M(CO)6 molecule, that is, by 10%... 

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