Electronic Spectra of Arene Complexes

Like many cyclopentadienyl complexes, a series of arene derivatives, particularly those of the sandwich type, do not obey Sidgwick s rule. Some of them have 

Investigations of IR spectra of [Cr(C6H6)(CO)3] and [Cr(C6D6)(CO)3] showed that the benzene ligand has C symmetry. The bands of the benzene spectrum which lie at ca 1600 and 1500 cm become shifted to lower frequencies after the benzene molecule is coordinated to the metal. 

The NMR signal for the free benzene molecule occurs at t = 2.7. It becomes shifted toward higher fields after coordination to the metal by approximately 3 ppm. For benzene complexes, the singlet resonance usually occurs in the range of t = 4.5-6, 

Chapter 10 Table 10.4. Proton Chemical Shifts of Arene Complexes of Transition Metals  

The chemical shift of coordinated benzene is lower (the shift of the signal due to coordination toward higher fields) compared to the free ligand. This is in agreement with the lowering of the multiplicity of the C —C bond due to coordination of the arene molecule (Table 10.5). 

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