Metal-Chelate Complexes

Lewis acid electron pair acceptor Lewis base electron pair donor 

The drug desferrioxamine mentioned at the opening of this chapter is produced by the microbe Streptomyces pilosus. Ferrioxamines are found in the ocean at 

What is the difference between a monodentate and a multidentate ligand Is a chelating ligand monodentate or multidentate How many and what kind of ligand atoms are in desferrioxamine B at the opening of this chapter  

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On refluxing a toluene solution of benzoisotellurazole and Fe3(CO)L2, cleavage of the Te—N bond occurs, resulting in formation of the metal chelate complex 11 whose structure was determined by X-ray (97MI1). 

The solvent extraction of a neutral metal chelate complex formed from a chelating agent HR according to the equation... 

Multi-element analyses involving solvent extraction and high performance liquid chromatography (HPLC) have also been described. The extracts, containing metal-chelate complexes with sulphur-containing reagents, such as dithizone and diethyldithiocarbamate, were used directly for determination of the metals by HPLC.14... 

Stereoselectivity in the binding of transition metal chelate complexes to nucleic acid constituents bonding and non-bonding effects. L. G. Marzilli and T. J. Kistenmacher, Acc. Chem. Res., 1977, 10,146-152 (29). 

Steric aspects of the formation of metal chelate complexes. V. G. Dashevskii, A. P. Baranov and M. I. Kabachnik, Russ. Chem. Rev. (Engl Transl), 1983, 52,152-167 (105). 

The given structure shows two molecules of TTA to have reacted with a cobalt ion to form the cobalt-TTA complex, in which the cobalt atom forms a valence bond solid lines) with one, and a coordinate bond (broken lines) with the other, oxygen atom of each TTA molecule. Thus, in the cobalt-TTA complex there is a six-membered ring formed by each TTA molecule with the cobalt atom. Metal chelate complexes of this type have good stability, they are nonpolar and soluble in the organic phase. The usefulness of the chelating extractants in solvent extraction is therefore obvious. 

Blake, D.A., Chakrabarti, P., Khosraviani, M., Hatcher, F.M., Westhoff, C.M., Goebel, R, Wylie, D.E., and Blake, R.C., Metal binding properties of a monoclonal antibody directed toward metal-chelate complexes, 7 Biol Chem, 111 (44), 27677-27685, 1996. 

Numerous commercial dyes are metal chelate complexes. These metals form pollutants which must be eliminated. One of the strongest points in favour of electrochemical reduction/removal of metal ions and metal complexes - the metal ions and weakly complexed ions form the toxic species - and of the metals from the metal-complex dye is that they are eliminated from the solution into the most favorable form as pure metal, either as films or powders. Polyvalent metals and metalloids can be transferred by reduction or oxidation treatment to one valency, or regenerated to the state before use, e.g. Ti(III)/Ti(IV), Sn(II)/Sn(IV), Ce(III)/Ce(IV), Cr(III)/Cr(VI), and can be recycled to the chemical process. Finally, they can be changed to a valence state better suited for separation, for instance, for accumulation on ion exchangers, etc. Parallel to the... 

S. Tokito, K. Noda, H. Tanaka, Y. Taga, and T. Tsutsui, Organic light-emitting diodes using novel metal-chelate complexes, Synth. Met., 111-112 393-396 (2000). 

Dendrimer-metal chelate complexes offer the possibility of delivering high concentrations of Gd(III) effectively. Thus adducts of the... 

In other white light devices, blue, green and red emitters are combined. Kido et al. [169, 170] designed multilayer systems using 6 (TPD) for blue, metal-chelate complexes for green and red emission, respectively. Similar devices have been developed by other groups, using Forster transfer or exciton confinement for the creation of the three primary colors [171, 172]. Exciplex emission was... 

Fig. 4.3 Extraction curves for various types of metal chelate complexes, when log Du is plotted against free ligand ion concentration, pA = -log[A ], or against [HA][H ]. From such plots, the general type of metal chelate complex may be identified (a) type MA , (see also Fig. 4.10) (b) type MA (OH)p(HA) (see also Figs. 4.14 and 4.30) (c) type MA (OH)p, (see also Fig. 4.19). (From Refs. 3a and 3b.)...

The extractants TBOA, TBMA, and TBSA are very similar, but their structural differences (see formulas in Table 4.16) allow the formation of only one type of metal chelate complex 5-, 6-, and 7-membered rings, respectively. Similar-ily, the reactants DMDOMA and DMDOSA form only 6- and 7-membered chelates. Table 4.16 shows that extraction (i.e., largest Ke value) is favored by 6-membered rings. This is not unexpected as the values in this case reflect the stability constants (3 acc. to Eq. (4.74). 

Eichhom and his co-workers have thoroughly studied the kinetics of the formation and hydrolysis of polydentate Schiff bases in the presence of various cations (9, 10, 25). The reactions are complicated by a factor not found in the absence of metal ions, i.e, the formation of metal chelate complexes stabilizes the Schiff bases thermodynamically but this factor is determined by, and varies with, the central metal ion involved. In the case of bis(2-thiophenyl)-ethylenediamine, both copper (II) and nickel(II) catalyze the hydrolytic decomposition via complex formation. The nickel (I I) is the more effective catalyst from the viewpoint of the actual rate constants. However, it requires an activation energy cf 12.5 kcal., while the corresponding reaction in the copper(II) case requires only 11.3 kcal. The values for the entropies of activation were found to be —30.0 e.u. for the nickel(II) system and — 34.7 e.u. for the copper(II) system. Studies of the rate of formation of the Schiff bases and their metal complexes (25) showed that prior coordination of one of the reactants slowed down the rate of formation of the Schiff base when the other reactant was added. Although copper (more than nickel) favored the production of the Schiff bases from the viewpoint of the thermodynamics of the overall reaction, the formation reactions were slower with copper than with nickel. The rate of hydrolysis of Schiff bases with or/Zw-aminophenols is so fast that the corresponding metal complexes cannot be isolated from solutions containing water (4). 

Chemical structures of several chelators. Ferroxamine (ferrioxamine) without the chelated iron is deferoxamine. It is represented here to show the functional groups the iron is actually held in a caged system. The structures of the in vivo metal-chelator complexes for dimercaprol, succimer, penicillamine, and unithiol (see text) are not known and may involve the formation of mixed disulfides with amino acids. 

That a number of metal chelate complexes are intensely fluorescent did not escape attention and the use of morin by E. B. Sandell (1940) for the quantitative determination of traces of beryllium in the presence of aluminum is a noteworthy landmark.5... 

Dyes (acridines, triphenylmethane) Metal chelate complexes Antisepsis Antisepsis Now little used... 

The stability of the metal chelate complex depends on the number of atoms that form a bond with the metal ion, the number of rings that are formed, the nature and concentration of the metal ion, and the pH (Stevenson, 1994). The stability order of the complexes formed between metals and humic acids has been determined through potentiometric titration and follows the Irwing-Williams series Pb2+ > Cu2+ > Ni2+ > Co2+ > Zn2+ > Cd2+ > Fe2+ > Mn2+ > Mg2+. 

Metal Chelator Metal-chelator complex (b) Immunoassay for heavy metals, oS +X oS 4- uu Metal-chelator-HRP Chromogenic substrate... 

A plot of the temperatures required for clouding versus surfactant concentration typically exhibits a minimum in the case of nonionic surfactants (or a maximum in the case of zwitterionics) in its coexistence curve, with the temperature and surfactant concentration at which the minimum (or maximum) occurs being referred to as the critical temperature and concentration, respectively. This type of behavior is also exhibited by other nonionic surfactants, that is, nonionic polymers, // - a I k y I s u I Any lalcoh o I s, hydroxymethyl or ethyl celluloses, dimethylalkylphosphine oxides, or, most commonly, alkyl (or aryl) polyoxyethylene ethers. Likewise, certain zwitterionic surfactant solutions can also exhibit critical behavior in which an upper rather than a lower consolute boundary is present. Previously, metal ions (in the form of metal chelate complexes) were extracted and enriched from aqueous media using such a cloud point extraction approach with nonionic surfactants. Extraction efficiencies in excess of 98% for such metal ion extraction techniques were achieved with enrichment factors in the range of 45-200. In addition to metal ion enrichments, this type of micellar cloud point extraction approach has been reported to be useful for the separation of hydrophobic from hydrophilic proteins, both originally present in an aqueous solution, and also for the preconcentration of the former type of proteins. 

These methods are based on the formation of a metal chelate complex, and then either directly or indirectly measuring its amount. Most of these methods do not differentiate between complexing agents, and are subject to interferences, and are not sufficiently sensitive for trace analysis. On the other hand, their rapidity and simplicity are unquestionable advantages. 

Figure 2.7 Structure of Ci2-dien-Zn(II) metal chelate complex.

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