Rotation barrier alkylidenes

Table 13.1 gives some activation parameters for the rotation about metal-alkylidene bonds in cationic complexes. In some cases the rotation is too fast for the nmr spectra to be frozen out at reasonably low tern-peratures then an upper limit for AG is given. Two isomers of the CHMe complex of iron would have been generated at -90°C but only the most stable was observed because of rapid rotation. Again an upper limit for AG may be set. While in most cases the major origin of the rotational barrier may be steric, the barriers appear to be modulated by electronic effects. Larger barriers are found on replacing CO by a tertiary phosphine and for the more basic phosphines even where these are less bulky. For... 

No coalescence of NMR signals was observed for the pseudotetrahedral alkylidene and vinylidene complexes [Re(C5H5)(NO)(PPh3)(=CHPh] and the related CH2 and C=CH2 complexes which sets lower limits for rotation barriers about the Re = C bonds of 18-19 kcal mol". " However, slow isomerizations of kinetically formed isomers of the CHPh and C=CMePh complexes to thermodynamic mixtures could be followed by NMR methods and this has allowed kinetic activation parameters for the ligand rotation to be determined in these cases. 

Later studies by Hillier and coworkers suggested that this was related to the need for concomitant rotation of the alkylidene ligand. It is therefore important to survey all steps of the catalytic cycle, as many steps present barriers that may affect the overall rate of reaction. 

Table 13.1. Experimental Barriers for Rotation about Metal-Alkylidene Bonds...

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