Ferrocene, bonding electrophilic aromatic

Ruthenocene and osmocene, however, crystallize in the eclipsed form. The C5H5 rings in w-cyclopentadienyl complexes are planar and all the C—C bond distances are equal. As will be seen later (p 219) they possess considerable aromatic character, and in favourable cases, e.g. ferrocene, undergo electrophilic substitution reactions such as acetylation. 

Its aromaticity cannot, of course, be tested by attempted electrophilic substitution, for attack by X would merely lead to direct combination with the anion. True aromatic character (e.g. a Friedel-Crafts reaction) is, however, demonstrable in the remarkable series of extremely stable, neutral compounds obtainable from (15), and called metallocenes, e.g. ferrocene (16), in which the metal is held by n bonds in a kind of molecular sandwich between the two cyclopentadienyl structures ... 

The first transition element compound containing only carbocyclic rings as ligands was bis(ri5-cyclopentadienyl)iron, [Fe(r 5-C5H5)2], which has a sandwich structure with two parallel p5- or Ji-bonded rings. The recognition that this compound was amenable to electrophilic substitution, similar to the aromatic behaviour of benzene, led to the suggestion of the non-systematic name ferrocene and to similar names for other metallocenes . 

Ferrocene, Fe(Ti5-C5H5)2, and related cyclopentadienyl complexes of transition metals in fact are far more thermally stable, less reactive substances than ionic cyclopentadienides, and have an extensive derivative chemistry that is typically aromatic in that their C-H bonds can undergo such electrophilic substitution reactions as Friedel-Crafts alkylation or acylation, nitration, and so on. Moreover, as a substituent, the ferrocenyl group (ri -f sl l5)Fc(ri -( 5l I4) (=R) is even more effective than a phenyl substituent in stabilizing carbenium ions [RCH2]+. The redox and photochemical properties of many metaUocenyl residues make them versatile substituents with many chemical and materials applications. ... 

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