Chelating ligands metallic complexes

Figure 12-4 Structures of analytically useful chelating agents. Nitrilotriacetic acid (NTA) tends to form 2 1 (ligand metal) complexes with metal ions, whereas the others form 1 1 complexes.

Not mentioned in Table 2 (and often not in the original papers ) is the optical form (chirality) of the amino acids used. All the amino acids, except for glycine (R = H), contain an asymmetric carbon atom (the C atom). In the majority of cases the optical form used, whether l, d or racemic dl, makes little difference to the stability constants, but there are some notable exceptions (vide infra). Examination of the data in Table 2 reveals (i) that the order of stability constants for the divalent transition metal ions follows the Irving-Williams series (ii) that for the divalent transition metal ions, with excess amino acid present at neutral pH, the predominant spedes is the neutral chelated M(aa)2 complex (iii) that the species formed reflect the stereochemical preferences of the metal ions, e.g. for Cu 1 a 2 1 complex readily forms but not a 3 1 ligand metal complex (see Volume 5, Chapter 53). Confirmation of the species proposed from analysis of potentiometric data and information on the mode of bonding in solution has involved the use of an impressive array of spectroscopic techniques, e.g. UV/visible, IR, ESR, NMR, CD and MCD (magnetic circular dichroism). 

The stability of metal complex is also given by the number of chelate rings formed in the resultant ligand-metal complex. For example, desfer-rioxamine, the most widely used iron chelator, minimizes OH production by acting as a hexadentate ligand [Liu and Hider, 2002]. Unfortunately, there is not enough information on the denticity of polyphenols as metal chelators to assess the relevance of the stability of the flavonoid-metal complex formed. 

A variety of new ligand designs and ligand combinations were used in attempts to mimic some properties of the ubiquitous bent metallocene environment at the early metal centers consequently, some of these systems were used in the further development of butadiene zirconium chemistry. The pyridine based chelate zirconium dichloride complex 43 cleanly formed the butadiene complex 44 upon treatment with butadiene-magnesium. Its structure shows that the C4H6 is arranged perpendicular to the chelate ligand plane. Complex 44 inserts one equivalent of an alkene or alkyne to form the metallacyclic 7i-allyl system 4545 (Scheme 13). 

Chelates and metal complexes with heterocyclic ligands as modifiers for epoxy resins 02CCR(224)67. 

Chelation equilibria involving five different tetracyclines with cupric copper have been examined by Benet and Goyan. These interactions appear to form essentially 2 1 (ligand metal) complexes, the reaction showing large positive entropy changes. 

Type II The ligand of a metal complex or metal chelate is part of a linear or crosslinked macromolecule (Figure 2). Either a multifunctional ligand/metal complex or a multifunctional ligand metal complex precursor are converted in polyreactions to type II macromolecular metal complexes. 

To utilize poly(acrylic acid) (represented as HL) as a chelating polymer in polymer-assisted ultrafiltration based removal of a divalent heavy metal (M", n = 2) present in wastewater, it is useful to consider the following polymer ligand-metal complexation equilibria ... 

When a metal ion is chelated by a ligand such as citric acid, it is no longer free to undergo many of its chemical reactions. A metal ion that is normally colored may, in the presence of citrate, have Httie or no color. Under pH conditions that may precipitate a metal hydroxide, the citrate complex may be soluble. Organic molecules that are catalyticaHy decomposed in the presence of metal ions can be made stable by chelating the metal ions with citric acid. 

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