Chelate complex, definition

When K[PtX(acac)2] (X = Cl, Br) is treated with a strong proton add the uncoordinated 0,0 site is protonated and complex (170) is formed.1606,1607 If the alkyl groups on the 0,0 -bonded acac ligand are non-equivalent, exchange can be observed. Although the mechanism has not been definitively proven, a dissociative mechanism is favored.160 This proposal correlates with the observations that the 0,0 -bonded chelate complex Pt(acac)2 will react with tertiary phosphines and nitrogen bases with substitution of one of the oxygen-bonded chelate arms.1609,1610 A variety of products are formed as outlined in Scheme 17. 

The a-substituted tropolonates 2 which are tris chelate complexes with a M06 coordination core have received considerable study by DNMR.27, 4S 46) The complexes are of the M(A-B)3 and M(A-B )3 types and therefore the averaging sets of Eaton efa/.26, 27 in Table 1 are applicable. The metal ions and a-R-substituents used in these studies include M = Al(III), Ga(III), Co(III), V(III), Mn(III), Ru(III), Rh(III) and Ge(IV) R = isopropyl (C3H7) and isopropenyl (C3HS), however, only complexes of Al(III), Ga(III), and Co(III) have yielded definitive mechanistic information.27, 45 46> On the basis of line shape changes of the methyl resonances these complexes can be classed kinetically as follows stereochemically nonrigid complexes which attain the fast-exchange limit of inversion and/or isomerization... 

Fig. 12. Definition of solid state structural parameters for tris chelate complexes with D3 symmetry 0 is the twist angle and is the projection of the bite angle a onto the plane which is perpendicular to the C3 symmetry axis, 0 is the pitch angle and is the angle subtended by the plane of the chelate ring and the C3 symmetry axis r, d, s and h are the metal ligand distance, bite distance, triangle edge, and the distance between the triangles, respectively. From Ref.82 ...

FIGURE 5.6 Definition of the skew-line convention used to define the absolute configuration of tris- or bis(chelate) complexes and conformation of diamine chelates. 

In order to determine the ethylene insertion starting from the chelated complexes, the slow growth MD simulations were performed, with the distance between the a-carbon of the chain and an olefin carbon chosen as a reaction coordinate.. The activation barriers obtained from the simulations are presented in Table 4-1. The results clearly show that in each case the barriers are substantially lower for the Ni-than for the Pd-catalyst. For all the systems, the ethylene insertion reactions starting from the most stable chelate structures 12, 13, and 14 have very high barriers (38-53 kcal/mol and 32 11 kcal/mol, for Pd and Ni, respectively). These values are much higher than the standard ethylene insertion barriers into the metal-alkyl bond (AE = 16.8 kcal/mol and 14.2 kcal/mol for Ni and Pd, respectively). These high barriers demonstrate that the ethylene insertion definitely cannot proceed from the most stable ethylene-chelate structure. 

The remaining examples were obtained in a more serendipitous fashion, while investigating the Tp Ir mediated C-H aetivation of substituted thiophenes. Thus, the reaction of Tp Ir( / -C2H4)2 (194) with 2,5-dimethylthiophene (3,5-Me2C4H2S) afforded the anticipated C(sp )-R activation product 252, in admixture with the chelate complex 253 (Scheme 16). Though no definitive evidence was found, 253... 

The second very important entropy-induced effect is the great stability of metal chelates (see definition of chelate in Section 1.3). Both ammonia and ethylenediamine (en) coordinate with metals through amine nitrogens in terms of the heat evolved in the complexation reaction two molecules of NH3 have been shown to be about equivalent to one en molecule. However, en eomplexes are considerably more stable than their NH3 eounterparts for example, [Ni(NH3)6], KiK2 = 6x 10 K3Ki = 5 x 10 KsK = 3. [Ni(en)3f, Ki = 5x 10 K2 = 2.2 X 10 K2 = 3.6x 10 ). It has been experimentally demonstrated that the unusual stability of the en compounds is due to a more favorable entropy associated with their formation. 

Although as a class the chelates are definitely bacteriostatic, the relative contributions of uptake and dissociation for both inert and labile species need to be more clearly defined before a full picture is obtained. The results suggest that kinetic reactivity may be more important than thermodynamic stability [30]. In the case of copper(II) complexes with 1,10-phenanthroline and 2,9-dimethyl-1,10-phenanthrolines, more recent studies showed that the major mode of action on P. denitrificans in vitro was in fact inhibition of respiratory electron transport in the cytoplasmic membrane, no correlation with inhibition of macromolecular synthesis being apparent [31]. These results are relevant in view of the discrepancy in the reported antitumour activity of these species (see Section 6.2). 

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. 

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. 

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. 

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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