Chelate, definition

Recently a definitive study of several isoxazol-5-ones using infrared and ultraviolet spectroscopy (Table I) has shown that the balance between the various tautomers is a delicate one and that all three of the structural types can predominate depending upon the nature of the substituents and the conditions of the experiment. However, the hydroxy form is only found when it is stabilized by chelation (i.e., a carbonyl substituent in the 4-position). The other compounds exist in the CH form in nonpolar media increasing polarity of the solvent stabilizes increasing amounts of the more polar NH forms. 

Cone angle concepts, 2,1012-1028 chelate effects, 2,1012 definitions, 2,1015 models, 2,1015 phosphines chiral, 2,1014 polydentate, 2,1013 solubility effects, 2,1014 use, 2,1028... 

Water soluble protein with a relative molecular mass of ca. 32600, which particularly contains copper and zinc bound like chelate (ca. 4 gram atoms) and has superoxide-dismutase-activity. It is isolated from bovine liver or from hemolyzed, plasma free erythrocytes obtained from bovine blood. Purification by manyfold fractionated precipitation and solvolyse methods and definitive separation of the residual foreign proteins by denaturizing heating of the orgotein concentrate in buffer solution to ca. 65-70 C and gel filtration and/or dialysis. 

The Lewis definition covers all AB cements, including the metal oxide/metal oxysalt systems, because the theory recognizes bare cations as aprotic acids. It is also particularly appropriate to the chelate cements, where it is more natural to regard the product of the reaction as a coordination complex rather than a salt. Its disadvantages are that the definition is really too broad and that despite this it accommodates protonic acids only with difficulty. 

Before studying some examples more closely, let us consider some cases which are not listed in Table 13. There are numerous compounds SnX2 which are definitely monomeric but are nevertheless no carbene analogs since their valence electron number at the tin atom is at least eight. These compounds contain chelating ligands which can stabilize the carbenoid tin atom due to intramolecular Lewis acid-base interactions as shown by structure A and B (see also Chapter 3). 

Excellent reviews have been written on the topic of chelating agents, radiolabeling, and receptor targeting. Because this research area, by definition, is a multi-disciplinary area, these topics are usually treated to some degree in any of these review articles.33-36... 

Shuman and Michael [10] applied a rotating disk electrode to the measurement of copper complex dissociation rate constants in marine coastal waters. An operational definition for labile and non-labile metal complexes was established on kinetic criteria. Samples collected off the mid-Atlantic coast of USA showed varying degrees of copper chelation. It is suggested that the technique should be useful for metal toxicity studies because of its ability to measure both equilibrium concentrations and kinetic availability of soluble metal. 

Much attention has been devoted to the examination of chiral enolate systems in which metal ion chelation may play an important role in establishing a fixed stereochemical relationship between the resident chirality and the enolate moiety. This has resulted in the conclusion that enolate geometry is critical in the definition of 7r-l acial selection. The following sections discuss this effort in several different chemical systems. 

There are problems of definition with the term mordant dye (section 1.6.8) and it is often more precise to refer to those chelatable dyes, mostly o.o -dihydroxyazo ligands, that are applied to wool at low pH and fixed by dichromate aftertreatment as chrome dyes. Nevertheless, mordant dyeing is a convenient way to describe this two-stage process that has become the focus of substantial development work in recent years because of increasing concern about the environmental hazards associated with residual chromium in dyehouse effluent. 

R3 R2 and R2 Ri gauche interactions however, for the same set of substituents, an increase in the steric requirements of either Rj or R3 will influence only one set of vicinal steric interactions (Rj R2 or R3 R2). Some support for these conclusions has been cited (eqs. [6] and [7]). These qualitative arguments may also be relevant to the observed populations of hydrogen- and nonhydrogen-bonded populations of the aldol adducts as well (see Table 1, entries K, L). Unfortunately, little detailed information exists on the solution geometries of these metal chelates. Furthermore, in many studies it is impossible to ascertain whether the aldol condensations between metal enolates and aldehydes were carried out under kinetic or thermodynamic conditions. Consequently, the importance of metal structure and enolate geometry in the definition of product stereochemistry remains ill defined. This is particularly true in the numerous studies reported on the Reformatsky reaction (20) and related variants (21). 

It was a hue shock to Astrid and myself when we learned last Sunday night that Hitoshi has passed away. This is a tremendous loss for our whole community. As yon correctly state, we had seen him in Tokyo twice, namely at October 12 and 13, when he attended both of my lectures. He was in good mood and spirit and we had in fact intense scientific discnssions about the chelate effect and this continued until November 3, when he sent me his last mail. We all definitely lost a veiy dear friend. Of conrse, Astrid and I have immediately written to his wife Ei... and as you say, at this time we can only pray for his sonl. 

Additional types of k C-k E chelating ylide complexes merit mentioning here, in addition to the species already presented, and some of them are shown in Scheme 24. The first is formed by ylides containing a pyridine functionality as substituent of the ylidic carbon. This type of ligand has allowed the S3mthesis of many loose clusters (82) of Cu, Ag, and Au which show weak metal( / °)-metal( / °) interactions. These facts have prompted the definition of a new phenomenon numismophilicity) to account for these weak interactions, uniquely shared by the three coinage metals [164]. Nitrogen and sulfur keto-stabilized ylides have been reacted with Pt and... 

In the case of ligands E, F and H the chelate/cryptate nature of the complexes will depend on whether or not the cation is contained between a branch and a ring or between two rings, or included inside a ring. Of course, in such cases, the above definitions, which concern the limiting cases, are less clear and classification may have to await a crystal structure determination. Finally, complexes formed by inclusion of a cation in a cavity delimited by a monocyclic, bicyclic or tricyclic structure may... 

With these definitions one may call complexes of ligands of type A [l]-chelates (or simply chelates), of types B, C [3]-chelates, of type B [l]-cryptates, of types F, E, H [3]-chelates or [l]-cryptates, of type O [2]-cryptates and of types I, J [3]-cryptates. 

La consideration de ces composes chelates an moyen d un metal, engage a notre sens a elargir la definition de la liaison hydrogene 5, puisque des ph io-menes analogues existent sans hydrogene. 

Edmondson et al (1971), who studied the enrichment of whole milk with iron, found that ferrous compounds normally caused a definite oxidized flavor when added before pasteurization. Aeration before addition of the iron reduced the off-flavor. The authors recommended the addition of ferric ammonium citrate followed by pasteurization at 81 °C. Kurtz et al. (1973) reported that iron salts can be added in amounts equivalent to 20 mg iron per liter of skim milk with no adverse flavor effects when iron-fortified dry milk is reconstituted to skim milk or used in the preparation of 2% milk. Hegenauer et al. (1979A) reported that emulsification of milk fat prior to fortification greatly reduced lipid peroxidation by all metal complexes. These researchers (Hegenauer et al. 1979B) concluded that chelated iron and copper should be added after homogenization but before pasteurization by a high-temperature-short-time process. 

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