Bonding cyclopentadienyl compounds

The reaction of the ionic cyclopentadienides with metal compounds is by far the most general method for synthesis of cyclopentadienyl metal compounds. This method is applicable both to transition metals, which form 7T-bonded cyclopentadienyl compounds principally, and to main group metals which form a-bonded cyclopentadienyl compounds. 

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-... 

Fig.5 Examples of simple metallocene structures a parallel sandwich, b multi-decker sandwich, c half-sandwich, d bent/tilted sandwich and e compounds with differently bonded cyclopentadienyl ligands...

The substituted cyclopentadienylmagnesium compounds 62a and 62b have been prepared from the corresponding fulvenes (equation 8) and were structurahy characterized in the solid state by X-ray crystallography . The structures are, as expected, (tj -bonded cyclopentadienyl groups) for bis(cyclopentadienyl)magnesium compounds. 

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... 

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... 

Gold(I) forms simple cyclopentadienyl compounds as well as more complex species involving 3-center 2-electron (3c-2e) bonds, and these together with analogous 3c-2e arylgold(I) compounds are discussed here. 

The types of compounds formed by gold(I) and gold(III) often differ from those of other metals due to the constraints imposed by coordination number and electron count at the metal. Thus, for example, whereas 7r-bonded cyclopentadienyl complexes of palladium and platinum are numerous (336), and a copper(I) species of this type is known (337), cyclopentadienyl complexes of univalent (94, 96, 97) and trivalent (228) gold have invariably been found to be fluxional behavior, similar to that in dicyclopentadienylmercury, was involved (228). 

Dynamic processes, such as those in equations 1-4, are of great importance in synthetic cyclopentadienylsilicon chemistry. They allow design of multiply substituted cyclopentadienyl compounds using the regioselectivity in silicon-carbon and also in carbon-carbon bond formation (see Sections n.D and HE). 

C5Hs)2Ti]2, and the unexpected diamagnetism was confirmed. A further unusual feature, deduced from infrared spectral data, was that in one case the compound was not of the 7r-sandwich type, but instead the dimer apparently contained both v- and rr-bonded cyclopentadienyl groups (8). 

The mass spectra of a considerable number of 77--bonded complexes of the group VIA metals have been reported, but in many cases mass spectrometry has only been used to determine the molecular weight, so that a detailed examination of the fragmentation processes involved has not been attempted, and only the molecular ion and perhaps a few other major peaks are reported. Within this section it is more convenient to discuss the compounds in terms of the attached ligands rather than in terms of the central metal atom. The classifications are (A) cyclopentadienyl compounds (B) arene compounds and (C) olefin, acetylene, and allyl compounds. 

Although most alkali metal compounds have M—C a bonds, cyclopentadienyl 7r-complexes are known for Li, Na, and K. These are commonly polymeric but solvation can break the chains and even result in cation-anion separation.44... 

Cyclopentadienyl compounds of mercury are limited to (CsHj Hg, which is a molecular compound with linear C—Hg—C bonding and C5Me5HgCl, also having linear (C—Hg—Cl) bonds to Hg. (C5Me5)2Hg apparently cannot be made.102... 

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