Symmetrical Sandwich Systems

We discuss in this section the principal bonding characteristics shown in the metallocene and bis-benzene series and the way in which these trends influence the splittings within the set of 3d-orbitals and consequently determine the electronic ground states of these complexes. A brief account of the bonding of the recently discovered symmetrical bis-cyclobutadiene complex NiCbj will also be summarised. 

Evidently the highest bonding interactions to the ring n-orbitals are those in- 

A 1 and A 2 are given in Table II, and these support the conclusions outlined above 

Complex Calculated Calculated Assumed Derived Derived 

In so far as a consistent rationalisation of the observed ground states in molecular orbital terms may be made, it is pertinent to note that apart from early suggestions that additional electrons beyond the eighteen electrons of ferrocene might be accommodated in the metal 4 s or 4p orbitals, there has really only been any ambiguity, and therefore controversy, over the ground states of complexes containing four or five valence d electrons. 

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Chapter 2 dealt with so-called symmetric sandwiched metallocenes. In addition to these more traditional symmetric metallocenes, a wide variety of metal-containing macromolecules exist where the metal is bonded to a lone ring system or to an asymmetric sandwich-like system. As in the case of the symmetric metallocenes, attachment of the metal is via it or n-type bonding. Here, the metal is... 

The ability of the crown ether ligand, 12-crown-4, to separate the lithium cation from the organic moiety, thus stabilizing the SSIP structure, was also observed in the study dealing with cyclopentadienyllithium systems . As described in Section II.C.3, SSIP structures of these compounds are characterized by low x( Li) values. However, it is important to reahze that the variation found for x( Li) is basically caused by the local symmetry around the lithium cation and other highly symmetric situations will also lead to small x( Li) values. Examples are the sandwich compounds mentioned in Section n.C.3. It is thus necessary to consider also / Li and C chemical shift data in order to classify a certain complex as SSIP or CIP. 

An important class of organo-metallic compounds are the so-called metal-sandwich compounds. These compounds have the formula (CnHw) M and consist of a transition metal atom M sandwiched symmetrically between two parallel carbocyclic ring systems, e.g. ferrocene, or, to give it its proper name, dicyclopentadienyliion [(C H Fe], and... 

The aromatic character of ferrocene originates in the anionic cydopen-tadienide system (18), which has six 7t-electrons delocalised over a symmetrical cyclic five-carbon system. Coordination of two such ring systems with a metal ion (iron in the case of ferrocene) in a sandwich-like structure (20) gives rise to a remarkably stable compound. 

Two cyclopentadienyl-sandwiched Cp2ZrX2/MAO complexes (Cp = cyclopentadienyl X = halogen or alkyl) with C2v-symmetry form the earliest metallocene system that has been used for ethylene homopolymerization with remarkably high activity. The application in propylene polymerization is less useful. They produce PP with low activity and low molecular weight.f Unexpectedly, the Cp2TiPh2/MAO system can be used for the synthesis of predominately i-PP below room temperature. The stereochemical structure of the resulting polymer indicates a chain-end-controlled model in the polymerization. Bridged C2v-symmetric... 

For simplicity we employ a model system sketched in Fig. 5.7. It consists of a film composed of spherically symmetric molecules that is sandwiched between the surfaces of two. solid substrates. The substrate surfaces are planar, parallel, and separated by a distance along the 2-axis of the coordinate system. They are semi-infinite in the 2-direction, occupying the half spaces Sz/2 < 2 < oo and -oo < z < -s /2, and infinite in the x- and y-directions. Each substrate comprises alternating slabs of two types strongly adsorbing and weakly adsorbing. The strong and weak slabs have widths dg and (/w, respectively, in the x-direction and are infinite in the j/-direction. The... 

Walton et al. went on to study the orientation of block copolymers as a function of interaction with blocks when walls are preferential for one of the blocks, parallel orientation forms.In the case of symmetric wetting of preferential walls, unfrustrated spacing occurs for wall spacing h,h = ndg, which is the bulk spatial period do multiplied by the number of layers n. Alternatively, for asymmetric preferential walls (the walls attract opposite blocks), the unfmstrated system forms when the plate spacing corresponds with h = n + Vi)do-This behavior was confirmed experimentally with PS-b-PMMA sandwiched between two walls treated with random copolymers of polystyrene-ran-poly(methyl methacrylate) (PS-r-PMMA) with varying composition and thus varying block interaction. ... 

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