Active coordination

A model involving that variation of the catalyst active perimeter across the knee will first be considered. Afterwards, a model involving the variation of the number of active coordination sites at a constant catalyst surface will be suggested. 

Model based on the variation of the number of active" coordination sites at the catalyst surface. The growth of tubules during the decomposition of acetylene can be explained in three steps, which are the decomposition of acetylene, the initiation reaction and the propagation reaction. This is illustrated in Fig. 14 by the model of a (5,5) tubule growing on a catalyst particle ... 

Employees outside immediate release area who assist on-scene incident commander All activities coordinated through individual in charge of the incident command system Examples industrial hygienists or health physicists providing guidance on PPL selection Individuals likely to witness or discover a release and who are trained to initiate emergency response sequence... 

Alfred Werner s research on optically active coordination compounds. G. B. Kauffman, Coord. Chem. Rev., 1974,12, 105-149 (376). 

Optically active coordination compounds. S. Kirschner, Prep. Inorg. React., 1964,1,29-57 (140). 

Mechanisms in the racemization of optically active coordination complexes in the solid state. P. O Brien, Polyhedron, 1983, 2,233-243 (54). 

Fig. 2. Schematic representation of the most common architectures of metalated container molecules. The open boxes represent active coordination sites at the metal ions. The bowl-shaped representation of the host molecules should not be confused with the one used for the cyclodextrins.

The cobalt(III) ion has a small size-to-charge ratio and is therefore a strong Lewis acid which strongly activates coordinated phosphate ester... 

Zerovalent transition metal carbonyl moieties may act as electron acceptors, and thus activate coordinated polyene ligands toward nucleophilic attack. Reaction of (C.411<5 )-Fe(CO)3 with KBHEt3 (—80 °C) proceeds via attack at a coordinated carbon monoxide to generate the anionic iron-formyl species 185 (Scheme 47)184. Upon warming to... 

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The Cossee-Arlman mechanism as originally proposed has a weakness—the back-flip is required to explain isoselective placement since the two active (coordination) sites are assumed to be enantiotopic. However, the structure of the traditional Ziegler-Natta heterogeneous initiators is not sufficently understood to either support or reject the assumption of enantiotopic sites. Further, even if the sites are enantiotopic, there is no overwhelming reason why the polymer chain is more stable at one site than the other—which is the rationale for the back-flip. The mechanism of isoselectivity with various metallocene initiators is much better understood since these are initiators whose molecular structures are well-established [Busico and Cipullo, 2001 Busico et al., 1997, 1999 Cavallo et al., 1998 Ewen, 1999 Rappe et al., 2000 Resconi et al., 2000], Considerable advancements in understanding heterogeneous Ziegler-Natta initiators occur if one assumes that the active sites in these initiators mimic those in metallocene initiators. Two types of metallocene initiators offer possible models... 

Crown-ether containing triarylphosphines have been studied in the Shell laboratories in an attempt to find a favorable effect of metal ion complexation in rhodium-catalyzed hydroformylation [68] no effect was found, although precedents exist in which the metal ion acts as a Lewis add that activates coordinated carbon monoxide towards methyl migration [69]. 

In aqueous solution, the equilibrium between the cis- and franj-diaqua complexes lies almost completely toward the cis isomer881 (K 0.17, pH 3-4). The sparingly soluble potassium salt of the tram isomer may, however, be prepared by the slow evaporation of a saturated solution at room temperature,878,880 and the cis isomer by cooling a hot solution or by allowing potassium dichromate and oxalic acid to react in the presence of a minimal quantity of water.878,879,882 The tris complex was resolved by Werner in 1912,883 providing the first example of an anionic optically active coordination complex. 

The resolution of optically active coordination compounds, a feat which shook chemistry to its innermost foundations ,72 gained for the coordination theory the widespread recognition for which Werner had been striving for so long. Nor was the theory s founder neglected, for two years later, largely in recognition of the most brilliant confirmation of [his] stereochemical views ,73 Werner was awarded the Nobel Prize in chemistry for 1913.74... 

Recently, Kobayashi found that both Diels-Alder adduct enantiomers were accessible with Yb(OTf)3 by using a single chiral source, (R)-binaphthol, and a choice of achiral ligands [45] (Eq. 8A.24). In the presence of 3-phenylacetylacetone (PAA) as the achiral ligand, one of the two active coordination sites in the chiral Yb catalyst is occupied, hence a dienophile coordinates to the remaining active sites, giving the other enantiomer selectively. 

Unlike apoptosis, which involves the scheduled and active coordination of metabolic processes, necrosis is a passive response to a toxic or injurious environment. Whereas in apoptosis the cell membrane remains intact until very late in the game, permeabilization of the cell membrane is an early event in necrosis. Because propidium iodide... 

Level 3 laboratories actively coordinate with hospitals in clinical-specimen collection, storage, and shipment and work to develop an appropriate response plan (CDC 2006a). 

The catalytic activity of hematin on the surface of alumina gel and graphitized carbon black has already been comprehensively studied by the adsorption method [93, 94], It is shown that at low filling degrees hematin molecules are placed flatly, and the iron atom contacts with the surface playing the role of the sixth ligand in this case. The deactivation process is associated with reorientation of the surface hematin molecule which induces iron bond break with the carrier and simultaneous formation of hematin dimer by sixth catalytically active coordinate position. 

In a similar manner, CDER s International Activities Coordinating Committee (IACC), chaired by the Center Director, was established to lead the Center s participation in international initiatives and to coordinate and discuss these activities. IACC s activities include... 

The prochiral face selectivity of the catalytic sites is due to the unique spatial arrangement of the stereorigid Me2C(Cp)(Flu) ligand encompassing the Zr atom and its active coordination positions [Figure 3.42 (R = H)]. 

The simple molecular models of catalytic sites, which consider the specific site enantioselectivity via non-bonded contacts of cation-like species with the monomer, are insufficient for explaining all the experimental data obtained. These data could not be interpreted in terms of steric effects only and suggest that the electronic factors are also involved. Apart from the discussed steric factors, such as the size of the substituent of the ancillary ligand, the ionic radii of the transition metal (Ti, Zr, Hf) and the degree of coverage of the active coordination positions by the substituents ought to be taken into consideration [30,112,127,418],... 

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