Tautomerism and Valence Isomerism

In solution some olefin Ti-complexes have been found to exist in dynamic forms. Cyclo-octatetraeneiron tricarbonyl, (C8H8)Fe(CO)3, serves as an example. The Fe(CO)3 group in this complex shifts about the cyclo-octatetraene ring (Fig. 6-9). 

These structures, which possess different valence bonds, are isomers. The phenomenon is called valence tautomerism (also referred to as ring whizzing). Other species also are known to exhibit valence tautomerism in solution (Fig. 6-10). 

If the energy barrier between two forms is small, we may anticipate valence tautomerism. However, if the energy barrier is high, we are dealing with valence isomerism. 

At room temperature only one MNR peak was observed for 7T-C7H7V-(CO)3. This was interpreted to mean that the entire seven-electron system complexed with the vanadium metal atom [6-41] (delocalized molecular 

A final example of valence isomerism involves a a-n rearrangement. 7c-Allyl palladium chloride [6-43] becomes a a-allyl complex in dimethyl sulfoxide as determined by NMR study. The rearrangement is facilitated by the interaction of the solvent with the metal. When a stronger donor ligand, such as PPh3, was used in different ratios (1 1 and 2 1), structures [6-44] and [6-45] were obtained, equations (6-77 and 6-78). A u-allylic complex [6-45] exists with [6-46] as a valence tautomer. 

---