The Simple Chelate

The process of chelation. A single concerted step (solid arrow) from reactants to products is not favoured, but rather a stepwise process (hollow arrows) is involved. 

The step-by-step process of chelation described above is believed to be the usual way all polydentate ligands go about coordinating to metal ions in general - knitting themselves onto the metal ion stitch by stitch (or, more correctly, undergoing stepwise coordination). It also provides a way whereby compounds can achieve only partial coordination of a potential donor set, when there are insufficient coordination sites for the full suite of potential donor groups available on a ligand. 

Simple chelate rings with common donor atoms but different chain atoms. 

There are a range of ligand systems without carbon chains known, but they need no special consideration. They can be treated at an elementary level just like the more common carbon-backboned ligands. Herein, it is this latter and most common type upon which we shall concentrate. 

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All metal ions in water are hydrated, and at higher pH most of them also hydrolyze. It can be difficult to distinguish between the hydrolyzed and the complexed species, as well as their self-adducts. For such systems, plots of against [A ] at various pH and total concentrations of [HA] show three types of curves (a) for the simple chelate MA , (b) for the self-adduct MA (HA), and... 

Carey (1969) utilized the fluorescence of ternary complexes of Eu(III) with various -diketones such as thenoyltrifluoroacetic acid, TTA, and various neutral donors such as TBP, TOPO and MHDPO to study the stoichiometry of these species. The intensity of the red fluorescence band by these ternary complexes is much greater than that exhibited by the simple chelates of Eu(III) with B-diketonates and can be used for the determination of Eu(III) at the ultra trace levels. The ternary complexes can be conveniently prepared by the synergic extraction of the Eu(III) chelate into an organic solvent containing a neutral donor such as TBP, TOPO or MHDPO. 

If the metals or ligands involved in a displacement reaction form chelates where type formulas are different, the exchange equiUbrium constant is the simple ratio of the formation constants of the chelates. Rather, for the reaction... 

Oxo esters are accessible via the diastereoselective 1,4-addition of chiral lithium enamine 11 as Michael donor. The terr-butyl ester of L-valine reacts with a / -oxo ester to form a chiral enamine which on deprotonation with lithium diisopropylamide results in the highly chelated enolate 11. Subsequent 1,4-addition to 2-(arylmethylene) or 2-alkylidene-l,3-propanedioates at — 78 °C, followed by removal of the auxiliary by hydrolysis and decarboxylation of the Michael adducts, affords optically active -substituted <5-oxo esters232 (for a related synthesis of 1,5-diesters, see Section 1.5.2.4.2.2.1.). In the same manner, <5-oxo esters with contiguous quaternary and tertiary carbon centers with virtually complete induced (> 99%) and excellent simple diastereoselectivities (d.r. 93 7 to 99.5 0.5) may be obtained 233 234. 

The rate of peroxide decomposition and the resultant rate of oxidation are markedly increased by the presence of ions of metals such as iron, copper, manganese, and cobalt [13]. This catalytic decomposition is based on a redox mechanism, as in Figure 15.2. Consequently, it is important to control and limit the amounts of metal impurities in raw rubber. The influence of antioxidants against these rubber poisons depends at least partially on a complex formation (chelation) of the damaging ion. In favor of this theory is the fact that simple chelating agents that have no aging-protective activity, like ethylene diamine tetracetic acid (EDTA), act as copper protectors. 

The variety and extent of research devoted to ligands carrying both O- and N-donors is simply immense. The type of cobalt(III) systems extant include amino acids, amino alcohols, amino ethers, amino phosphates, amino phenolates, as well as amide and imine analogs of these. These are met as simple chelates or more elaborate polydentates. Here, we highlight a strictly limited selection of examples to illustrate the type of systems reported no attempt at exhaustive review has been made. 

Chelation of ixo-maleonitriledithiolate (imdt) has been structurally characterized in the octahedral cobalt(III) complex trmw-[Co(imdt)2(P(ra-Bu)3)2], formed via reaction of cobalt(II) ion with K2(imdt) in the presence of the phosphine.1037 Simple chelating thiolates such as SCH2CH2S not only form mononuclear compounds, but participate in bridging in clusters such as [Co3(SCH2CH2S)3(PEt3)3]3+ (243).1038... 

The N,S-chelate 2-aminoethanethiolate (aet-) forms stable cobalt(III) complexes, including clusters where the S can take on a bridging role. Reaction of [Co(NH3)5C1]2+ with Ni(aet)2 in water for several hours affords the black tetranuclear compound Co4(aet)8, which features act in simple chelate and bridging roles (245).1083 The simple monomeric complex [Co(aet)2(en)]+ has been reported 1084 when heated in water at 50 °C, the trimer Co Co(aet)3 2 is one product, with the central Co surrounded by six bridging S atoms. 

Little more need be said here about the simple ion-exchange reactions such as that between sodium hexametaphosphate and calcium ions (Scheme 10.7). It is useful, however, to consider in more detail those reactions involving chelation (Scheme 10.8). This is a reversible reaction, the equilibrium being dependent on the process pH and the concentrations of the reacting species (Equation 10.2). While chelated complexes are less stable at higher temperatures, this effect can be ignored in practice. The factors involved have been discussed in some considerable detail by Engbers and Dierkes [20,23]. 

Diastereoselective addition of a wide range of Grignard reagents to C -alkyl and C-aryl-A-[a.-phenyl- or u-methyl-j3-(benzyloxy)ethyl nitrones is determined by the presence of a stereogenic A -substituent (136, 137). High diastereoselectiv-ity in the addition of organometalic compounds to A-(( i-methoxyalkyl) nitrones can be explained by a simple chelation model (Scheme 2.132) (136). 

Taylor There s no EGTA here for the simple reason that it would chelate some of the inhibitors we use, such as Gd3+. But with that said, we don t need to chelate extracellular Ca2+ with EGTA to prevent Ca2+ entry we get no detectable Ca2+ -entry whether we simply omit Ca2+ or replace it with EGTA. 

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