Sandwich structure, of ferrocene

The correct identification of the sandwich structure of ferrocene led Fischer to consider the possibility of arenes acting as hexahapto 6VE ligands. By simple arithmetic, a neutral bis(arene) sandwich complex of a zerovalent Group 6 element, e.g. chromium (Figure 6.71), was anticipated, a line of reasoning which led Fischer to develop the synthesis of dibenzenechromium. His approach resulted in the comparatively general Fischer-Hafner synthesis (1955), which is applicable to many metals and arenes (devoid of Lewis-basic substituents, Figure 7.34). 

The first metal carbonyl compounds Ni(CO)4 and Fe(CO)s were prepared and characterized by L. Mond and his group in 1889-91 and this work has burgeoned into the huge field of metal carbonyl cluster compounds which is still producing results of fundamental importance. Even more extensive is the field of organometal-lic chemistry which developed rapidly after the seminal papers on the sandwich structure of ferrocene (E. O. Fischer and W. Pfab, 1952 ... 

The sandwich structure of ferrocene was suggested in 1952, soon after its discovery, by G. Wilkinson and independently by E.O. Fischer, and confirmed... 

In the 1970 s Kaminsky, Sinn, and others discovered that bis(cyclo-pentadienyl)dimethyltitanium when mixed with trimethyl aluminum and water provided a catalyst system capable of polymerizing ethylene [5]. The titanium structure bears some resemblance to that of ferrocene. Ferrocene was reported in 1951 [6,7] and the following year, the correct structure reported [8]. A brief, interesting account of the early days of the research on the structure proof of ferrocene has been written [9]. Because the Kaminsky catalysts have the same sandwich structure of ferrocene, they are referred to as metallocene catalysts. Just as the ferrocene ushered in a new era of organometallic chemistry, the Kaminsky metallocene spurred a tremendous amount of research in olefin polymerization catalysts. 

The sandwich structure of ferrocene and its extraordinary thermal stability (up to 500 °C) have contributed to the extensive and still lively research in the field. 

The structure of ferrocene is shown in Figure 14-16. The iron ion is sandwiched between two cyclopentadienyl rings, which are freely rotating in solution. 

Metallocenes are sometimes referred to as sandwich compounds because the metal ion is sandwiched between two planar cyclic alkenes as shown in Figure 1.2. Although metallocenes had been synthesized some years earlier, their true structures were not established until the structure of ferrocene was reported in 1951. 

FIGURE 7 Structures of ferrocene (18), the related iron(ll) bisdicarbollide anion (19), and a hexadecker, mixed cyclopentadienyl/triboracyclopentadienyl pentametal sandwich complex (20). 

As the most prevalent structure in organometallic chemistry, sandwich compounds, defined as two cyclic aromatic hydrocarbon ligands flanking one metal center, has played a central role on the stage of organometallic chemistry and catalysis ever since the seminal structure of ferrocene (CpjFe) was elucidated in 1952 [161]. However, this chemistry had never been extended into compound mono-layers of multiple metal atoms in place of the one metal center as the filling of the sandwich until... 

As a consequence of the molecular orbital interactions, ferrocene adopts an axially symmetrical sandwich structure with two parallel Cp ligands with a distance of 3.32 A (eclipsed conformation) and ten identical Fe-C distances of 2.06 A as well as ten identical C-C distances of 1.43 A [12]. Deviation of the parallel Cp arrangement results in a loss of binding energy owing to a less efficient orbital overlap [8]. All ten C-H bonds are slightly tilted toward the Fe center, as judged from neutron-diffraction studies [13]. 

The discovery in 1951 of the transition metal rr-complex, ferrocene or bis-cyclopentadienyl-iron, Fe(Cp)2, (1, 2) led to enormous interest being shown in the possible structures of such compounds and in the nature of the metal-ring bonding. Within a year the sandwich type structure (Fig. 1) had been proposed (2), and an outline treatment of the metal 3d-ligand 7r-orbital interaction by Jaffe (4) was soon followed by a more detailed molecular... 

Unlike the slipped octa- or non-methyl derivatives, however, 10 exhibits a ferrocene-like rf/rf-parallel-sandwich structure. The Be-CsMes centroid distance (1.655(1)A) and the Be-C distance (av. 2.049A) are noticeably longer than in 9 (by 0.12 A). There is some spread in the Be-C bond lengths in 10 (A = 0.15 A), and there are inter-ring Me-Me contacts as close as 3.63 A (cf. the sum of the van der Waals radii of 4.0 A57). 

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