Ring whizzers

As summarized by Cotton 31), considerations of line shape have generally led to the conclusion that migration of metals about conjugated rings proceeds via a series of 1,2-shifts. Thus for the (Tj -CjHsjM complexes listed in Table I, shifts such as those below occur. Similar 

Thus up to mid-1974 a proliferation of ring-whizzer papers further augmented the data suggesting that 1,2-shifts were the dominant, if not universal, mode of transition metal migrations about polyene rings. 

The complexity of the proton spectra of (t7 -C8H8)Mo(CO)3 species was such that line-shape analysis and inference of the rearrangement pathway were not feasible. In 1974 however. Cotton, Hunter, and Lahuerta (30) showed that the four C signals of the CsH8 ring broadened equally at temperatures about 0°C. This result rules out the 

Recently, [i7 -C7H7Ge(CH3)3]Fe(CO)3 has been shown to be fluxional and appears to be a second exception (85a). 

Steric interactions also influence the ground-state stability and barrier heights of the (R,R)-(S,S) and (R,S)-(S,R) diastereomers found in (rf-C5H5)Fe(CO)[Sn(CH3)3](CH3CH==CH2) (56). The olefin in one pair of diastereomers is significantly more labile. Diastereomers can also be detected by both IR and NMR in (Tj -CsHsIMfCOKNOKolefin) where M = Cr, Mo, and W (57, 60, 66). Approximately equal amounts of the diastereomers of the chromium complexes may be prepared readily by irradiation of the nitrosyl dicarbonyl complex in the presence of an olefin, such as propene or styrene (66). 

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Many other examples of ring whizzers have been established, and careful NMR studies over a range of temperatures yield much information about the activation barriers and detailed mechanisms. 

FIGURE 13.54 Ring-Whizzer Mechanism and Variable Temperature NMR Spectra of (CjHjljFelCOlj. The central peak at 4.5 ppm due to the... 

The term ring whizzers was coined by Cotton to describe the phenomenon that was first studied quantitatively by Bennett et al. in the compound Fe(Ti -C5Hj)(T -C5H5)(CO)2, which has the following structure ... 

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