Chelate closed ring

Chelate (cyclic complex) An inorganic metal complex having a close ring of atoms formed by attachment of ligands to a metal atom at two points, e.g.,... 

In addition to the Si NMR evidence presented in Fig. 3, supporting the nonionic dissociation, the C NMR also supports this reaction the C chemical shift of the chelate-ring carbon has an average value between those of the open and closed ring forms. 

Such compounds containing closed rings in which one or both of the linkages with the metal atom are coordinate links are known as chelate compounds, from the Greek chela, a crab s claw. Chelate compounds are nmnerous and are important in analytical chemistry. The famihar red pre-cipitate obtained from dimethylglyoxime and an ammoni-acal solution of a nickel salt is a chelate compound having the structure... 

Of this sub-class of reactions, one is most Interesting. This is the formation of a closed ring chelate of clsplatln with the N-7 and 0-6 nucleophilic sites of guanine. It had been shown previously that the clsplatln reacted primarily with the GC rich regions of DNA. It has also been suggested that the tertiary structure of DNA is probably too plastic to exhibit the necessary stereoselectivity. It is also known that the clsplatln does not... 

Chelation itself is sometimes useful in directing the course of synthesis. This is called the template effect (37). The presence of a suitable metal ion facihtates the preparation of the crown ethers, porphyrins, and similar heteroatom macrocycHc compounds. Coordination of the heteroatoms about the metal orients the end groups of the reactants for ring closure. The product is the chelate from which the metal may be removed by a suitable method. In other catalytic effects, reactive centers may be brought into close proximity, charge or bond strain effects may be created, or electron transfers may be made possible. 

The solid state structure of complex 7b is shown in Figure 25.1. Similar to parent chelating ether complex [9b], the solid-state structure of 7b shows a distorted square-pyramidal structure with the benzylidene moiety at the apical position. The N-aryl ring is located above the benzylidene moiety resulting in the relatively close contact of the benzylidene proton with the 7r-aromatic system of the mesityl group. 

Again a distorted tetrahedral coordination of Hg is achieved in the l,l-bis(diphenylphos-phino)ferrocene (dppf) complex with Hgl2. Both Hg—P in the dppf chelate ring and terminal Hg—I bonds (averages 256.5 pm and 277.6 pm, respectively) are clearly longer than in the previous examples all angles around Hg are close to the value for a tetrahedron. Nonlinear optical properties of the material have been studied.241... 

As already stated, DIOP led the way for a number of ligand systems that were built on a carbon framework containing stereogenic centers. Some of these ligands followed closely on the heels of DIOP, such as ChiraPhos (16) where the chelate ring is five-membered [82, 83]. Even one stereogenic center in the backbone, as in ProPhos (17), provides reasonable selectivity [83, 84]. The main problem with these systems is that of slow reactions. 

DR. MICHAEL BERGKAMP (Brookhaven National Lab) Do you have any direct evidence which indicates that when you irradiate these compounds the chelate ring actually opens up and closes ... 

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