EDTA ligand

A number of ligands based on the encapsulating hexadentate edta ligand have been structurally characterized with zinc, including the 3,4-toluenediamine-/V, V, /V",/V" -tetraacetate.897... 

The basis for the toxicological activity of this substance is the reaction of cobalt ion with cyanide ion to form a relatively nontoxic and stable ion complex. The hexacyanocobaltate ion contains a Co2+ central metal ion with six cyanide ions as ligands. This coordination complex involves six coordinate covalent bonds whereby each cyanide ion supplies a pair of electrons to form each covalent bond with the central cobalt ion. The formation constant for the hexacyanocobaltate ion is even larger than for dicobalt EDTA,3 and thus the cobalt ion preferentially exchanges an EDTA ligand for six cyano ligands ... 

FIGURE 5.18 The structure of the EDTA ligand. The bonding sites are pointed out with arrows. 

A very important ligand (or chelating agent) for titrimetric analysis is the ethylenediaminetetraacetate (EDTA) ligand. It is especially useful in reacting with calcium and magnesium ions in hard water such that water hardness can be determined. The next section is devoted to this subject. 

Structure of the EDTA ligand and its complex with the nickel(ll) ion. 

The La—0(nitrate) distances in M(dip)2( N03)3 compare reasonably well with other decacoordinated nitrate complexes cited above. The decrease in Tb(III) complex is in accord with the lanthanide contraction (Table 1). The M—N(dip) distances in both La(III) and Tb(III) complexes are shorter than in the decacoordinated [La(EDTA H)] 7H2O complex (La—N(EDTA) =2.865 A) [211) or nonacoordinated [M(EDTA) (0112)3] complexes [209, 210). It is the steric constraint in the EDTA ligand, that is responsible for the weak interaction of the nitrogen atoms of EDTA in the La(III) complexes. 

The influence of Na+, K+, calcite, and EDTA on complete hydroxyapatite dissolution and hydroxypyromorphite formation was also examined (Ma 1996). Only high EDTA concentrations (EDTA/Pb molar ratios up to 5) significantly affected the ability of dissolving hydroxyapatite from sequestering hydroxypyromorphite the strong bond between the EDTA ligand and the Pb2+ was fairly competitive with the tendency to precipitate out as hydroxypyromorphite. 

Starting with [Cr(Hedta)(OH2)], alkalimetric titration gives pK as 2.10 (97), and from [Cr(edta)], a similar titration gives pK = 7.42 (98). The questions under debate are (i) what is the coordination number of the edta ligand bound to Cr(III) in the pH range 4-7, and (ii) if the ligand is hexadentate, can a seventh position in the inner coordination sphere be occupied by a water molecule The latter proposal is not unreasonable, as 7-coordination is observed in the solid-state hexaden-... 

The consensus at the moment is that the edta ligand remains hexadentate when bound to Cr(III) in the pH range 3.5-6.5, and a seventh donor site occupied by water is not involved. Increased solution stability of the hexadentate can be achieved by increasing the size of the E ring, for example, in [Cr(l,3-pdta)] (propanediaminetetraace-tate) and [Cr(l,4-bdta)] (butanediaminetetraacetate), and there is no suggestion of 7-coordination in these complexes. 

As in the case of the Mo u-edta complex the edta ligand in [Mo204(edta)]2 is coordinated to the binuclear core in a basket like conformation. Studies on complexes of Mo204+ having optically active amine-carboxylate ligands such as propylenediaminetetraacetate have been reported.121... 

A germanium complex with ethylenediaminetetraacetic acid (Hedta) 532 has a pen-tadentate edta ligand with one protonated acetate group which does not coordinate, forming a six-coordinate complex1150. 

The recent flash photolysis study has revealed, that the [Femedta]- excited state is quenched by electron transfer processes by both electron donors and electron acceptors (see Figure 9.12). The latter cause oxidation of the EDTA ligand and the acceptor reduction, whereas the Fem centre does not change its oxidation state. One of the acceptors is molecular oxygen, which results in direct EDTA oxidation at the expense of 02 reduction [21, 22] ... 

The complex H[La(EDTA)(H20)4] 3H2O involves severe restrictions placed on the polyhedron by the EDTA ligand, and overall the polyhedron may be considered to be approaching a bicapped square antiprism [174]. This is shown in Fig. 5.28. The coordination polyhedron in La(NC>3)3 - (bipy)2 has features belonging to all three geometries D4d, D2 and C2V- Based on general shape criteria and adherence to various least squares best planes, the description would be in terms of D2 geometry [173]. 

Polycarboxylic Acids. Carboxylic acids have been found to bind strongly to actinide ions. The primary binding mode for the carboxyhc acids is bidentate. The affinity of the low-valent actinides with these ligands increases with the density of the hgand, for example, ethy lenediaminetetraacetate (EDTA) > acetate. For An" +, the EDTA ligand is hexadentate with a twist conformation. Diethylenetriaminepentaacetate (DTPA) has an even higher affinity for both An + and An + ions. 

Quenching [Fe°(edta )(H20)] by electron donors leads in a deoxygenated medium to the regeneration of the edta ligand from the edta radical and formation of the reduced Fe(II) complex and oxidized donor molecule (D = aliphatic alcohol, sulfate (IV), nitrate(III) or even extraneous edta ligand) ... 

It should be noted that most structurally characterized dinuclear complexes are products of adventitious lanthanide hydrolysis, and are not reproducible. With the ligand-controlled hydrolytic approach, they can now be rationally synthesized starting from a mononuclear aqua complex. Shown in Figure 6.30 is the synthesis of [(EDTA)Er (ti-OH)2Er(EDTA)] (EDTA=ethylenediaminetetraacetate) the dinuclear core doubly bridged by the fi-OH groups is encapsulated by two hexadentate EDTA ligands, one on each metal atom [69],... 

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