Arene and Related Complexes

Arenes form ir-complexes with all the transition metals. Many ir-arene complexes have been isolated, structurally characterized, and studied by a plethora of spectroscopic techniques. The first T -arene complexes were prepared by Hein in 1919, but the structures of these compoimds were not fully recognized until 1954. The prototype sandwich complex, bis(benzene)chromium, Cr(CjH )2, was prepared by E. 0. Fischer in 1955.  

The coordinated arene serves many fimctions in transition metal chemistry. It can be a spectator ligand, a labile ligand that temporarily occupies and then releases vacant coordination sites, a catalyst poison, or a substrate for catalytic and stoichiometric transformations. An arene is a stronger electron donor than three CO ligands. For example, the strong 

Arene complexes are usually prepared by the displacement of more weakly bound, monodentate ligands, by oxidation or evolution of CO ligands, or by reducing a metal from a higher oxidation state in the presence of an arene. These methods have been reviewed. - In some cases, one arene will displace another. For example, hexamethylbenzene replaces p-diisopropyl benzene in the tungsten complex of Equation 2.13. This reaction is imdoubt-edly controlled by steric effects. The mechanisms of these arene exchange reactions have been clarified by Traylor and are discussed in Section 4.55. 

Arene complexes can also be prepared by metal-atom vapor techniques. This method is most practical with lower-boiling (less-refractory) metals, but chromium arene complexes have been prepared by this method. A metastable iron-benzene complex has been prepared on a 10-20 g scale by this method.- Such complexes are often useful inorganic synthons in subsequent reactions. 

1 OH Effect of acid-base properties from arene binding. 

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