Arenes and Other Alicyclic Ligands

Arenes usually bind in the 6e, ri -form 5.27, but (5.28) and (5.29) structures are also seen. An t or rj arene is planar, but the T) ring is strongly folded. The C-C distances are usually essentially equal, but slightly longer than in the free arene. Arenes are much more reactive than Cp groups, and they are also more easily lost from the metal so arenes are more often actor rather than spectator ligands. 

Typical synthetic routes resemble those used for alkene complexes  

From the arene, a metal salt and a reducing agent C6H6,A1,A1C13 

For naphthalene, t] binding is still common, but the tendency to go V is enhanced because this allows the uncomplexed ring to be fully aromatic. If one ring is differently substituted from the other, isomers called haptomers have the metal bound to one ring or the other, often with metal exchanging between sites. 

TpW(NO)(PMe3) gives an t complex with naphthalene, where the stabler 1,2-bound form is in equilibrium with the 2,3-form, which has the character of a quinodimethane and can give the Diels-Alder reaction of Eq. 5.43.  

The commonest synthetic method is abstraction of H from an rf cyclohep-tatriene complex with PhsC (Eq. 5.48) or EtsO the oxonium cation is the reagent of choice because the by-products, Et20 and EtH, are both volatile. 

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