Cyclopentadienyl compounds

Mono(cyclopentadienyl) complexes of the rare earth metals have been comprehensively reviewed by Okuda and Arndt.38 

5 Cyclopentadienyl compounds. An inq)roved synthesis of CpNa has been rqrarted. Crystal structures have been determined for the following cyclopentadiMiyl-metal species (i) 

Metallation of l,l-Ph-q iro[2,4]hqita-4,6-diene with Naplr-,Li provided [C5H4CH2CPl p-2U. Dynamic processes (induding occhange of U and rotation of cyclopentadienyl rings) in a ferocoie bridged li [(vni cyclopentadienyl)2li] spedes have been studied by NMR spectroscopy. 

The cyclopentadienyl compound (15,8S)-7,7,9,9-Me4-tricyclo-[6.1.1.0 ]deca-2,5-dienyl-lithium) exists almost exclusively as the exo-Li-monomer in THF at 26°C but at -80°C a tanaiy equilibrium of an exo-Li monomer, an exo-li sandwich dima and an endo,endo- A sandwich dima exists in a S. 1 2.8 1.0 ratio as shown NMR spectroscopy ( Li-, HOESY) and supported by MNDO calculations.  

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A comprehensive study of 15 zirconecene monomethyl monochloro compounds and their exchange of chloride for methyl with AlMe3 indicates that the exchange process is a function of the electron deficiency at the metal.62 With low electron densities, the zirconium is more likely to exchange chloride for methyl. The study revealed low electron densities at the metal for the indenyl compounds by comparison to the cyclopentadienyl compounds. The study also revealed that steric effects are minor compared to the electronic effects of the ligands on the zirconium. 

Figure 19 Examples of some cyclopentadienyl compounds of aluminum.

An unexpected endo selectivity in addition of certain carbon and sulfur nucleophiles to the O, / -unsaturated (arene)ruthenium(II)cyclopentadienyl compound (154) has been reported. This stereochemistry has been compared with that of the 5 n2 reactions but a detailed theoretical approach is yet to be undertaken. ... 

Cyclopentadienyl Compounds. [(7t-Cp)2Zr(Bz)2] has been prepared in 60% yield by the reaction of [(ji-Cp)2ZrCl2] with BzMgCl in ether. The compound... 

The chemistry of organometalhc compounds has been studied for almost 200 years [78] and the cyclopentadienyl compounds have been known from the beginning of the 1950s [79,80], In recent years the volatility and high reactivity of some these compounds have also been exploited in ALD. 

Cyclopentadienyl compounds (i.e. metallocenes) (Fig. 5), which have at least one direct metal-carbon bond to the C5H5 ligand, were first synthesised in the 1950s [79,80]. Since then, reactions of cyclopentadienyl reagents have been applied for almost every element [123]. The main application of metallocenes is their use as catalysts in the polymerisation of olefins by Ziegler-Natta polymerisation processes. As many metallocene compounds are volatile and thermally stable, they are also suitable for use as precursors in MOCVD [124-127]. Although cyclopentadienyl compounds have attracted considerable interest as precursors in CVD depositions they are sometimes too reactive [128]. However, high reactivity and thermal stability make cy-... 

Although there are several potentially volatile cyclopentadienyl compounds for other transition metals [123], which could be utilised for AID processing, only a few have been studied in this respect. For example, volatile (C5H5)2Mn and (C5MeH4)Mn(CO)3 have been used as manganese sources for doping ZnS thin films to produce yellow-emitting thin-film electroluminescent devices [156]. 

In addition to metal alkyls and cyclopentadienyl compounds other types of true organometallic precursors have also been applied in ALD depositions. For example, ( 6115)361 [162,163] and (C6H5)4Pb [164] have been used for the preparation of bismuth- and lead-containing oxide thin films. These compounds are nof exfremely reactive and therefore ozone is needed to obtain completely oxidised films [162,163]. H2O2 as a second precursor does not oxidise (C6H5)3Bi completely, since metallic bismuth was observed in the deposited films [162]. 

To overcome this disorder, the same synthetic procedure was used with the substituted cyclopentadienyl compounds bis(terr-butylcyclopentadienyl)zinc and bis[(trimethylsilyl) cyclopentadienyljzinc. Indeed, the corresponding Zn4Ni2 clusters were formed, in which the geometry of the central cluster core and the bonding of the cyclopentadienyl groups... 

In a number of classes of systems, the catalytic and other chemical effects of metal ions on reactions of organic and inorganic molecules are generally recognized the catalysis of nucleophilic reactions such as ester hydrolysis the reactions of alkenes and alkynes in the presence of metal carbonyls (8, 9, 69) stereospecific polymerization in the presence of Ziegler catalysts (20, 55, 56) the activation of such small molecules as H2 (37), 02 (13), H202 (13), and possibly N2 (58) and aromatic substitution reactions of metal-cyclopentadienyl compounds (59, 63). 

One path of research resulting from the discovery of ferrocene has been concerned with the synthesis and structural elucidation of cyclopentadienyl derivatives of other metals and metalloids. At the present time, over 60 chemical elements have been found to form one or more cyclopentadienyl compounds. This topic has recently been reviewed by the present author as well as by others (17, 83, 125). 

The successful acylation of ferrocene set off a vigorous research effort that has resulted in the establishment of the present field of metallocene chemistry, ir- Cyclopentadienyl compounds of ruthenium, osmium, manganese, vanadium, and chromium also exhibit certain aromatic-type reactions in varying degrees. These metallocenes thus represent a new class of heterocyclic compounds in which transition metals, akin to nitrogen, oxygen, and sulfur in classical heterocycles, not only are an integral part of the structure but actually participate directly in many reactions. 

The observed moments of the cyclopentadienyl compounds are consistent, as pointed out by McConnell, with a pattern of three d orbitals (d0 and d 2) lying considerably below the remaining two (d x). In fact, the calculations of Dahl and Ballhausen (70) lead to a similar explanation in that various electron configurations within the a g and e2g molecular orbitals explain the observed magnetic moments, whereas the e g lies too high in energy to... 

Cp2Mo2(CO)4] is an important member of a group of cyclopentadienyl compounds and, like its chromium and tungsten analogues, this species is considered to involve a metal-metal triple bond.7,220 These and related compounds manifest a rich and novel chemistry,221 but this falls outside the field of this review. 

Highlights in the chemistry of cyclopentadienyl compounds have been reviewed.65 Trends in the metallation energies of the gas-phase cyclopentadienyl and methyl compounds of the alkali metals have been studied by ab initio pseudopotential calculations. Whereas there is a smooth increase in polarity of M-(C5H5) bonds from Li to Cs, lithium appears to be less electronegative than sodium in methyl derivatives. The difference between C5H5 and CH3 derivatives is attributed to differences in covalent contributions to the M-C bonds. In solution or in the solid state these trends may be masked by the effects of solvation or crystal packing.66 The interaction between the alkali metal ions Li+-K+ and benzene has also been discussed.67... 

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