Ring structures sandwiched metallocenes

In all cases, the l,l -diheteroferrocenes have metallocene-type structures in which the Fe atom is sandwiched between two symmetric tj5-heterolyl rings. Although the heterolyl rings are not planar, the four C... 

Owing to the 7tCc-ocrc CT interactions, the usual equivalence between the two Kekule structures of benzene is broken and the ring distorts strongly to D3h symmetry, with pronounced alternation (by 0.02 A) of C=C bond lengths. Complexes of this type are evidently closely related to the metallocene sandwich compounds discussed elsewhere in this book (Section 4.9.5), with the benzene molecule described as a tridentate ligand in the language of metal coordination chemistry. 

Triple-decker sandwiches that are capped at one or both ends by C2B3 rings are important synthons for the construction of larger stacks via coordination to metal ions (see the following subsection), and many of these have been synthesized and their reactivity investigated.75,78 Bent triple-deckers such as 6879,80 that resemble bent metallocene complexes have been prepared, structurally characterized, and shown to be amenable to the introduction of halo- and organosubsitutuents at boron.80... 

Metallocenes have a transition metal sandwiched between two cyclopentadienyl rings. The cyclopentadienyl rings may not necessarily be parallel because of bridging. They may be further substituted to restrict access to the metal. The catalyst structure can be changed to make different polymeric properties. A typical example would be the zirconium metallocene shown below. 

The alkaline-earth metallocenes exhibit various structures, depending on the sizes of the metal atoms. In Cp2Be, the Be2+ ion is if /if coordinated between two Cp rings, as shown in Fig. 12.7.3(a). The mean Be-C distances are 193 and 183 pm for rj5 and rj1 Cp rings, respectively. In contrast, Cp2Mg adopts a typical sandwich structure, as shown in Fig. 12.7.3(b). The mean Mg-C distance is 230 pm. In the crystalline state, the two parallel rings have a staggered conformation. 

A unique type of R2Pb compound is the cyclopentadienyl derivative of divalent lead. Only two such compounds are known, bis(cyclopenta-dienyl)lead and bis(methylcyclopentadienyl)lead. From dipole moments and infrared and NMR spectra, these compounds are postulated to possess a sandwich-type configuration, similar to that of ferrocene and the other metallocenes, but one in which the cyclopentadienyl rings are located angularly about the lead atom. This type of structure is the first... 

Metallocenes (Fig. 2) are sandwich structures, typically incorporating a transition metal such as titanium, zirconium, or hafnium in the center. The metal atom is linked to two aromatic rings with five carbon atoms and to two other groups—often chlorine or alkyl. The rings play a key role in the polymerization activity (23-27). Electrons associated with the rings influence the metal, modifying its propensity to attack carbon-carbon double bonds of the olefins. The activities of these metallocenes combined by aluminum alkyls, however, are too low to be of commercial interest. Activation with methylaluminoxane, however, causes them to become 10-100 times more active than Ziegler-Natta catalysts. 

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