Ferrocene, bonding substitution

An X-ray structural study of the ferrocenylphosphonium salt (147) shows a significant shortening of the phosphorus-ferrocene bond, lending support to an earlier suggestion by McEwen that there may be overlap of the filled hog MO of the ferrocenyl group with an empty 3d orbital of phosphorus. The possible existence of an equilibrium between the substituted methylphosphonium salts (148) and the cationic phosphorane system (149) has been ruled out following spectroscopic and structural studies of a series of such salts. ... 

Ferrocene compounds containing heteroelements are versatile building blocks for polynuclear complexes. They possess electrochemically active iron centers as well as unique cylindrical shapes. An example of ferrocene-substituted thiolato ligands is l,l -ferrocenedithiolate (S2fc). Its complex with Pd(II) lacks a Pd—Fe bond. 

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 ... 

Over the past decade a number of new covalently bonded TTF/ferrocene adducts have been reported [77, 78]. The crystal structure of the l,l -bis(l,3-dithiole-2-ylidine)-substituted ferrocene derivative has been published [77]. In this complex, ferrocene has essentially been incorporated as a molecular spacer between the two l,3-dithole-2-ylidene rings forming a stretched TTF molecule. This adduct, and its methyl-substituted derivative, have been combined with TCNQ to form charge-transfer complexes with room temperature powder conductivities of 0.2 S cm-1. Similar diferrocenyl complexes have been prepared with bis (dithiolene) metal complexes [79, 80]. 

Ruthenocene is an example of a stable x-bonded organometallic compound which undergoes substitution reactions similar to those displayed by ferrocene. Because ruthenocene has heretofore been relatively unavailable, its chemistry has not been extensively studied. 

We have seen how a substituent on one cyclopentadienyl ring in ferrocene markedly affects subsequent ring substitution reactions. While certain transformations of this type are at least partially determined by steric factors, it seems certain that resonance and inductive interactions also play a very important role in determining orientation. Electronic transmissions both within and across the cyclopentadienyl rings are not fully understood, since the precise nature of the bonding in ferrocene is still uncertain. 

The dicyclopentadienyl metal compounds undergo Friedel-Crafts alkylation and acylation, sulfonation, metalation, arylation, and formyla-tion in the case of ferrocene, dicyclopentadienyl ruthenium, and dicyclopentadienyl osmium, whereas the others are unstable to such reactions ( ). Competition experiments (128) gave the order of electrophilic reactivity as ferrocene > ruthenocene > osmocene and the reverse for nucleophilic substitution of the first two by n-butyl lithium. A similar rate sequence applies to the acid-catalysed cleavage of the cyclopentadienyl silicon bonds in trimethylsilylferrocene and related compounds (129), a process known to occur by electrophilic substitution for aryl-silicon bonds (130). 

There are some reports in the literature concerning the cleavage of bonds between silicon and an sp2-hybridized carbon in silylcyclopentadienyl compounds of type 4 (Section II.B). For example, equation 4697 shows hydrolytic fission in the reaction of a tris(trimethylsilyl) substituted ferrocene derivative with aqueous tetrabutylammonium fluoride. 

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