Cyclopentadienyl derivatives transition metal chemistry

The organometallic chemistry of the first-row transition metals generally starts with the binary metal carbonyl organometallic complexes. Noncarbonyl organometallic complexes starting with other easily accessible binary compounds provide entries to a broader spectrum of complexes. In this context, we describe the synthesis of the mixed sandwich complex (tj5-pentamethylcyclopentadienyl) ( j5-cyclopentadienyl) iron as an example of the synthetic utility of the solution-stable derivative (>j5-pentamethyl-cyclopentadienyl) (2,4-pentanedionate) iron. 

Cyclopentadiene (1), a simple organic compound first found in the volatile parts of coal tar, has become one of the most important ligands used in organometallic chemistry. More than 80% of all known organometallic complexes of the transition metals contain the cyclopentadienyl fragment or a derivative thereof. 

Monomeric, paramagnetic chlorohydrides snch as MH2CI2 (PR3)4 derive from facile oxidative addition of H2 to MCl2(PR3)4 complexes. Of these 17e dodecahedral complexes, (32) is the most interesting (eqnation 17). Its thermolysis in solution offers a qnadruply bridged hydride (33) and it can also be rednced to a monomeric Ta 16e species (34). Ta hydrides display a hydridic character (see Hydride Complexes of the Transition Metals), as shown by the reduction of CO by cyclopentadienyl derivatives. (see Niobium Tantalum Organometallic Chemistry)... 

It was soon noted that iron was by no means unique in forming tt-cyclopentadienyl derivatives, and research groups in several countries set out to determine the nature and scope of cyclopentadienyl-metal chemistry. At the present time, practically all the metals of the short transition series, as well as nearly all the metals and metalloids of the main group series, form one or more cyclopentadienyl compounds. In addition, cyclo-pentadienyl derivatives of over one-half the lanthanides have now been described, and even cyclopentadienyl derivatives of U, Th, and several mns-uranium elements are known. The present status of cyclopentadienyl-metal chemistry is illustrated in part in Figure 1. Elements designated by shaded areas are known to form one or more cyclopentadienyl derivatives. 

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. ... 

In this article the term organometallic compound includes alkyl and aryl derivatives of the rare earths—the transition metals of group III, scandium, yttrium, lanthanum and the lanthanides cerium to liitetium with covalent metal-to-carbon a-bonds, as well as the so-called 77-complexes with more than monohapto metal-to-carbon bonds, for example cyclopentadienyl and olefin complexes, metal acetylides, but not carbonyls, cyanides and isocyanide complexes. Derivatives of scandium, yttrium and lanthanum are included and discussed together with the compounds of the lanthanides, because of many similarities in the synthesis and the chemistry of these organometallic derivatives of the rare earths. 

Transition metal complexes self-assembled via Tc-interactions are not large in number, although some very interesting examples have been reported. This area of organometallic chemistry is far from being systematically investigated and it is quite difficult to make any predictions concerning possible supramolecular structures of particular compounds. With the exception of several polymeric olefin and arene complexes, the majority of the complexes described in this section are cyclopentadienyl derivatives. 

The cyclopentadienyl ligand, generally abbreviated as "Cp," is a common anionic ancillary ligand in organometallic chemistry. Complexes of this ligand or its substituted derivatives are known for all transition metals and for most /-block metals. - Ferrocene is the classic cyclopentadienyl compound, and the identification of the structure of this species by Woodward, Wilkinson, and co-workers, after initial misassignment, is legendary7 °... 

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