Complexes of Arenes and Other Alicyclic Ligands

Arenes usually bind to transition metals in the 6e, ii -form 5.41, but ti (5.42), and 1] (5.43) structures are also known.In the ti form the ring is usually strongly folded, while an ti arene tends to be flat. The C—C distances are usually essentially equal, but slightly longer than found in the free arene. Arenes are much more reactive than Cp groups, and they are also more easily lost from the metal. This means that arenes cannot be used as inert stabilizing ligands for a whole series of metal complexes as can Cp. 

The NMR is perhaps the most useful method of characterization, the metal-bound carbons showing a 25 ppm shift to high field on coordination, due to the increased shielding from the nearby metal. 

Synttwsis Typical synthetic routes differ little from those used for alkene complexes  

From the arene, a metal salt and a reducing agent  

Other T Ugarkis Polycyclic arenes such as naphthalene also bind to low-valent metals. In this case t) binding is still common but the tendency to bind T) is enhanced because, as we saw for indenyl, this allows the uncomplexed ring to be fully aromatic. If one ring is different in some way from the other, different isomers, called haptomers, can exist in which the metal is bound to one or the other ring. The metal can migrate from one ring to the other in a haptomeric equilibrium. 

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