Ligand metal ion

If solution pH pKa(,v) of the ligand, significant complexation of the metal ion will tend to not occur if pH pK (.v). then extensive complexation will occur, and pM will not be strongly dependent on pH in this pH region. The behavior of pM at pH values close to the pK (.v) of the metal-ligand reaction system needs to be understood for each proposed metal ion-ligand system. 

Figure 12 [115] shows a series of complex formation titration curves, each of which represents a metal ion-ligand reaction that has an overall equilibrium constant of 1020. Curve A is associated with a reaction in which Mz+ with a coordination number of 4 reacts with a tetradentate ligand to form an ML type complex. Curve B relates to a reaction in which Mz+ reacts with bidentate ligands in two steps, first to give ML complexes, and finally close to 100% ML2 complexes in the final stages of the titration. The formation constant for the first step is 1012, and for the second 108. Curve C refers to a unidentate ligand that forms a series of complexes, ML, ML2. .. as the titration proceeds, until ultimately virtually 100% of Mz+ is in the ML4 complex form. The successive formation constants are 108 for ML, 106 for ML2, 104 for ML3, and 102 for ML4 complexes. 

While sharp changes in pM are desirable for complexation titrations, they can be undesirable for electroless solutions. Thus an electroless solution that involves a metal ion-ligand system with the titration characteristics of curve (a) in Fig. 12 would... 

Usually there is no ambiguity about the choice of components to form the species often they are metal ions, ligands, solvent molecules, protons etc. 

Metal ion Ligand and adduct former HA Organic solvent Log... 

Metal ion Ligand Nonionic surfactant Experimental conditions... 

LAMBDA (or A-j ISOMERS OE METAL ION-NUCLEOTIDE COMPLEXES Metal ion-ligand complex,... 

Measurements of metal ion-liganding atom distances in the crystal structures of many inorganic salts led to the derivations of atomic or ionic radii for these atoms or ions. These radii are based on the idea that each atom or ion behaves as a solid sphere of defined size (Wasastjerna, 1923 Zachariasen, 1931). The shortest distance between adjacent ions of opposite sign is then presumed to be the sum of the radii of the respective ions the cation and the anion. 

The contoured scatterplots showed that the most likely arrangements of metal cations are those designated syn, anti, and direct. The percentages of directionalities of metal liganding for a total of 1558 metal-carboxylate interactions are 62.9% syn, 22.7% anti, and 14.4% direct. Minimum metal ion-ligand distances are given in Table IX(a). syn is generally preferred, except when the distances are short... 

Metal Ion Ligand Log Rate Constant E Method Reference... 

Compounds with heterocyclic rings are inextricably woven into the most basic biochemical processes of life. If one were to choose a step in a biochemical pathway at random, there would be a very good chance that one of the reactants or products would be a heterocyclic compound. Even if this was not true, participation of heterocyclics in the reaction in question would almost be certain as all biochemical transformations are catalyzed by enzymes, and three of the twenty amino acids found in enzymes contain heterocyclic rings. Of these, the imidazole ring of histidine in particular would be likely to be involved histidine is present at the active sites of many enzymes and usually functions as a general acid-base or as a metal ion ligand. Furthermore, many enzymes function only in the presence of certain small non-amino acid molecules called coenzymes (or cofactors) which more often than not are heterocyclic compounds. But even if the enzyme in question contained none of these coenzymes or the three amino acids referred to above, an essential role would still be played by heterocycles as all enzymes are synthesized according to the code in DNA, which of course is defined by the sequence of the heterocyclic bases found in DNA. 

CrOl ) reasonably stable the Cr(I) oxidation stale is practically unknown. For both Cu2 and Cr3 (as well as many other transition metal ions) ligand field effects in their complexes (see Chapter II) are much more important in determining stable oxidation states than are electron configurations. 

Probably the most important effect contributed by metal coordination to ligands is stereochemical in nature. Because of the rather strict coordination geometry imposed by metal ions, ligands can be held in suitable juxtaposition for reactions to take place between them. This phenomenon is the hallmark of metal template reactions and is also a crucial feature of metal enzyme reactions, where high specificity occurs. 

Ligand exchange has proved to be very successful in the separation of several enantiomers. Davankov and Rogozhin (41) used chiral copper complexes bonded to silica. The enantiomeric separation is based essentially on the formation of diastereomeric mixed complexes with different thermodynamic stabilities. It is generally accepted that chiral discrimination proceeds via the substitution of one ligand in the coordination sphere of the metal ion. Ligand exchange technique is especially effective for the enantiomeric resolution of aminoacids, aminoacids derivatives, and hydroxy acids (42). 

The selectivity of peptide motifs for certain metals comes from the coordinating contribution from amino acid side chains, the common coordination number of the metal, hardness/softness of the metal ion, ligand field stabilisation effects and the hardness/softness of any coordinating side chains of the amino acid sequence. An example of the influence of side chains and the importance of the position of the side chain comes from the tripeptides Gly-Gly-His, also known as copper binding peptide. The side chain imidazole ring of the His residue has a very efficient nitrogen donor (the imidazole N), which can form a tetradentate chelate ring for coordination as in Scheme 10.3. 

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