Ferrocenes sandwich structure

While dicyclopentadienyl compounds of transition metals (see Transition Metals) typically have a ferrocene-hke (see Ferrocene) sandwich structure, the electron-deficient cyclopentadienides of the main group metals tend to form polymers with bridging cyclopentadienyl (Cp) groups. Although exceptions are known, the tendency is rather consistent. The element zinc, owing to its borderline position between transition metals and main group metals, presents a variety of coordination (see Coordination Numbers Geometries) modes with the Cp anion. 

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]. 

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). 

METALLOCENES. A class of neutral transition meial compounds containing two eyclopeniadicnyl tC Ho ligands. 7 -bonded to a ccmral metal atom in a "sandwich" structure, as exemplified by ferrocene (sec belosvi. 

Elements of the first transition series from vanadium to nickel form stable bis(7j-cyclopentadienyl) complexes having the normal ir-sandwich structure typical of ferrocene. In contrast, bis(Tj-cyclopentadienyl)ti-tanium, (i7-CsH5)2Ti (1), is not even known as a discrete compound. 

Stannocene, bis-Tj5-cyclopentadienyltin(II), and plumbocene, bis-Tj5-cy-clopentadienyllead(II), were first reported by Fischer and Gruber in 1956 (12). They observed the compounds to be monomeric in benzene and to have nonzero dipole moments and proposed the a-bonded structure shown in Fig. 1A. Wilkenson and co-workers reported the similarity between the IR spectrum of stannocene, plumbocene, and ferrocene and suggested the presently accepted angular sandwich structure for stannocene and plumbocene shown in Fig. IB (13). 

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). 

Beryllocene, Cp2Be, prepared in 1956 by E. O. Fischer, was intensively examined to determine its structural features. This work quickly abandoned a structural model based on ferrocene, and different bonding modes for the two Cp rings were demonstrated by Cp2Be s high dipole moment in solution. The slipped sandwich structure, with... 

The BCr molecule is larger than that of ferrocene or cobaltocene, but all these complexes have a rather compact structure which diminishes the interaction of solvent molecules with the metal ion screened from two sides by ligands (sandwich structure). They have also a very delocalized multielectron system and, in consequence, when one electron is removed, the change in polarizability of these species is rather small. 

The smallest rings that are able to incorporate a ferrocene building block without deformation of the sandwich structure (with its two planar and parallel cyclopenta-dienyl rings) are [3]ferrocenophanes. This becomes evident in the series of sila-ferrocenophanes fc[(SiMe2) ] ( = 2, 3) (Scheme 5-41) [243]. 

The first transition element compound containing only carbocyclic rings as ligands was bis(ri5-cyclopentadienyl)iron, [Fe(r 5-C5H5)2], which has a sandwich structure with two parallel p5- or Ji-bonded rings. The recognition that this compound was amenable to electrophilic substitution, similar to the aromatic behaviour of benzene, led to the suggestion of the non-systematic name ferrocene and to similar names for other metallocenes . 

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