Aromaticity metal atom role

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. 

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... 

Elemental analysis of fuel oil often plays a more major role that it may appear to do in lower-boiling products. Aromaticity (through the atomic hydrogen/carbon ratio), sulfur content, nitrogen content, oxygen content, and metals content are all important features that can influence the use of residual fuel oil. 

Since metal ions play key roles in cellular physiological processes many specific reporter molecules have been developed, mostly as fluorescent indicators incorporating extended aromatic and conjugated structures, where the wavelength of fluorescence depends upon specific binding of a metal ion. Several F NMR reporters have been created by addition of fluorine atoms (Table 5). 

Chls and all tetrapyrroles are heteroaromatic compounds and the aromatic character of the underlying tetrapyrrole moiety and the reactivity of the functional groups in the side chains govern their chemistry. Three different classes of tetrapyrroles, differentiated by their oxidation level, occur in nature porphyrins (11, e.g. hemes), chlorins (12, e.g. chls) and bacteriochlorins (13, e.g. bchls). As a cyclic tetrapyrrole with a fused five-membered ring, the overall reactivity of chi is that of a standard phytochlorin 7. Such compounds are capable of coordinating almost any known metal with the core nitrogen atoms. Together with the conformational flexibility of the macrocycle and the variability of its side chains, this accounts for their unique role in photosynthesis and applications ... 

In this last section of the chapter other van der Waals interactions in which —H bonds and/or carbon atoms or, more correctly, electron density, are involved will be discussed. —H- - interactions are considered to be present when the hydrogen atom of a —H bond interacts with an aromatic ring in a nearly normal fashion and with H centroid distances shorter than about 3 A. Interactions between clouds of electron density of neighboring nearly parallel aromatic rings that lead to intercentroid distances below 3.5 A are considered as effective - interactions. Although these contacts are not reported very often, they may play an important role in the packing forces of metal complexes. 

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