Strongly chelated metal

Diastereoface selection has been investigated in the addition of enolates to a-alkoxy aldehydes (93). In the absence of chelation phenomena, transition states A and B (Scheme 19), with the OR substituent aligned perpendicular to the carbonyl a plane (Rl = OR), are considered (Oc-or c-r transition state R2 Nu steric parameters dictate that predoniinant diastereoface selection from A will occur. In the presence of strongly chelating metals, the cyclic transition states C and D can be invoked (85), and the same R2 Nu control element predicts the opposite diastereoface selection via transition state D (98). The aldol diastereoface selection that has been observed for aldehydes 111 and 112 with lithium enolates 99, 100, and 101 (eqs. [81-84]) (93) can generally be rationalized by a consideration of the Felkin transition states A and B (88) illustrated in Scheme 19, where A is preferred on steric grounds. 

The tetracyclines are strong chelating agents. Both the A-ring and 11,12 P-diketone systems are active sites for chelation (16). This abiUty to chelate to metals, such as calcium, results in tooth discoloration when tetracycline is administered to children (17). 

Upon strong chelation, aU. four protons are displaced and base titration resembles that of a typical strong acid at four times the equivalent concentration. This statement is in agreement with equation 19, which shows that pM can be large (low concentration of free metal) at low pH if iC is large (strong chelation). 

Titration of the hydrogen ion Hberated from a strong chelating agent is used to determine the concentration of metal ions in solution. The strength of chelation can also be deterrnined from these data. 

The presence of a sufficientiy strong chelating agent, ie, one where K in equation 26 is large, keeps the concentration of free metal ion suppressed so that pM is larger than the saturation pM given by the solubiUty product relation (eq. 29) and no soHd phase of MX can form even in the presence of relatively high anion concentrations. The metal is thus sequestered with respect to precipitation by the anion, such as in the prevention of the formation of insoluble soaps in hard water. 

The first two terms of the right-hand side of the equation are sometimes combined and expressed as E which is called the standard oxidation potential for the chelate system. If the chelation is strong and the ligand is in excess, the metal would be almost entirely in the chelated forms, and [M L] and [M g L] would essentially be equal to the total concentrations of the oxidized and reduced forms of the metal. If, as is usual, the oxidized form is the more strongly chelated K > ), the oxidation potential of a system is increased by the addition of the chelant. 

The complexers maybe tartrate, ethylenediaminetetraacetic acid (EDTA), tetrakis(2-hydroxypropyl)ethylenediamine, nittilotriacetic acid (NTA), or some other strong chelate. Numerous proprietary stabilizers, eg, sulfur compounds, nitrogen heterocycles, and cyanides (qv) are used (2,44). These formulated baths differ ia deposition rate, ease of waste treatment, stabiHty, bath life, copper color and ductiHty, operating temperature, and component concentration. Most have been developed for specific processes all deposit nearly pure copper metal. 

Protection from any poisonous metal ions liberated from their sulfides by oxidation by 02 was secured by the use of strong chelating agents in the cytoplasm, most of which are proteins, or small molecules, thiolates, which were connected to exit pumps or to chemical metabolic tricks for metal ion neutralisation (sequestration). The genes that code for these proteins are usually to be found on plasmids in the cytoplasm of the bacterial cells (Section 5.15). Bacteria adapt very quickly to... 

This shows that the strong chelating effect of the bidentate ligand efficiently retains the metal attached to the support. 

The major anions and cations in seawater have a significant influence on most analytical protocols used to determine trace metals at low concentrations, so production of reference materials in seawater is absolutely essential. The major ions interfere strongly with metal analysis using graphite furnace atomic absorption spectroscopy (GFAAS) and inductively coupled plasma mass spectroscopy (ICP-MS) and must be eliminated. Consequently, preconcentration techniques used to lower detection limits must also exclude these elements. Techniques based on solvent extraction of hydrophobic chelates and column preconcentration using Chelex 100 achieve these objectives and have been widely used with GFAAS. 

These catalytic effects are usually signaled by irreproducible behavior. If it is suspected that traces of metal ions may be causing peculiar rate effects, a strong ligand may be added to sequester the metal ion. The spontaneous decomposition of H2O2 has been reported as 4.7 X 10 M s at pH 11.6 and 35°C. This is the lowest recorded value and is obtained in the presence of strong chelators. In a similar way the decomposition of permanganate in alkaline solution (3.6) is markedly slowed when the reactants are extensively purified... 

In 17, X may be POj (but COCH3, SO3 and other groups have also been examined by this means). In the type of structure shown in 18 we have already encountered the 2-nitrile hydrolyses. With X = POf in 18, divalent metal ions show a pronounced catalysis of the hydrolysis of the dianionic species. The metal is strongly chelated to the phenanthroline but in the product it is unlikely that the 0 is coordinated since a four-membered ring would result (see Sec. 6.8). The monoanionic form (X = POjH ) is the reactive species (Prob. 3). Reaction of the dianion in the absence of metal ion cannot be observed and with Cu +, for example, accelerating effects of >10 are estimated. ... 

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