Ring structures arene complexes

The only reported X-ray structure of a it-bonded diiodine exists in the 12/coronene associate [75], which shows the I2 to be located symmetrically between the aromatic planes and to form infinite donor/acceptor chains. -Coordination of diiodine over the outer ring in this associate is similar to that observed in the bromine/arene complexes (vide supra), and the I - C separation of 3.20 A is also significantly contracted relative to the stun of their van der Waals radii [75]. For the highly reactive dichlorine, only X-ray structures of its associates are observed with the n-type coordination to oxygen of 1,4-dioxane [76], and to the chlorinated fullerene [77]. 

We have also recently explored some ruthenium-arene complexes that depart markedly from the general structure described above. For instance, full-sandwich ruthenium complexes have been synthesized, in which the positions X, Y, and Z are taken by an rj6-arene ring of a biologically active ligand, such as aspartame, to assess the influence of a metal complex as a modulating substituent on the properties of the bioactive ligand (66). 

Arenes are attractive building blocks for the synthesis of complex carbocyclic species. In addition to being inexpensive and available in a variety of substitution patterns, they possess ring structures composed entirely of unsaturated carbons. Because of this unsaturation, they have the potential for extensive functionalization. However, the realization of this potential requires synthetic methods that overcome the energetic barrier associated with aromatic stabilization. 

The electronic difference between benzene and borazine is further supported by the properties of compounds of the type (R<,BjN j)Cr(CO)j. Although the.se are formally analogous to (rj< -QRf,)Cr(CO)j. the bonding is not nearly so strong in the borazine complex—its ring-metal dissociation energy appears to be about one-half that of the arene complex. In addition, there is considerable evidence that the borazine molecule is puckered in the.sc complexes. The actual structure appears to be intermediate between a true tt complex and the extreme rr-only model ... 

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