Coordination of metal ions

The chelate ring size principle can have structural effects as well as effects on thermodynamic stability in aqueous solution. An example is coordination of metal ions by sugars (44). The cyclic polyol cts-inositol can coordinate metal ions in two distinct ways (Fig. 14) (45). In ax-ax-ax bonding (Fig. 14), the metal ion is part of three fused six-membered chelate rings. Alternatively, in ax-eq-ax coordination, the metal ion is part of two fused five-membered and one six-membered chelate rings. Angyal has noted that metal ions of radius more than 0.8 A adopt the ax-eq-ax structure (44), whereas with an ionic radius... 

In these discussions, we have focused attention on details of the physical measurements which confer understanding of structure in pyridine based compounds bond lengths, bond angles, electronic levels in both ground and excited states, and the involvement of solvation, complexation and other environmental features. In the latter, we have, out of necessity, restricted ourselves generally to complexes in which the role of the pyridine structure is of paramount interest, for the whole of the available space could have been filled with lists of complexes incorporating these structures, but where they were of subordinate interest to some other feature, typically the mode of coordination of metallic ions. 

We have briefly covered some of the important developments in structural characterization techniques. There are many other techniques such as Mossbauer spectroscopy, positron annihilation and Rutherford backscattering which have wide applications. Mossbauer spectroscopy is specially useful to investigate different oxidation states, spin-states and coordinations of metal ions, phase transitions, magnetic ordering. 

When cyanide was added to the solutions, complete coordination of metal ions occurred, and the reaction rate and product spectrum... 

This chapter will carefully differentiate situations in which coordination of metal ions assists in the achievement of specific electrochemical aims from experiments designed to study the electrochemistry of coordination compounds. (For information on the latter topic, see, particularly, Chapters 8.1-8.3). Two major areas have been selected for consideration one almost classical, namely the electrodeposition of metals, the other of more recent origin, namely the modification of electrode surfaces. 

Coordination of metal ions often has a dramatic effect on the n delocalization in porphyrins and porphyrinoids. It has particularly conspicuous influence on the electronic spectra of metalloporphyrins, which show a dependence on the identity of the metal ion, axial ligation, oxidation level, and spin state. In regular porphyrins, metal coordination reduces the number of observed Q bands from four to two, reflecting the higher symmetry of the chromophore relative to the free base. However, detailed quantitative information on the Jt-electron delocalization is more easily accessible from other physical methods. 

Stabilization of Peptide Microstructures by Coordination of Metal Ions... 

However, herein we focus on the discrete compounds, especially the three-dimensional cages or container molecules [6-10], formed mainly by coordination of metal ions with organic ligands. On the other hand, there also exist three-dimensional compounds where the stability is partly due to hydrogen bonds, 7t-7i interactions, Van der Waals forces or dipole-dipole interactions. Beside those three-dimensional compounds, one-dimensional structures like, for instance, helicates [26-30] and two-dimensional ones, the polygons, are also known [31-36]. 

CDC also encompasses dynamic coordination chemistry [35, 38, 40], whereby the coordination of metal ions induces the preferential formation of specific ligand molecules and/or induces reversible changes in them. Such processes may be traced back to early work on coordination reactions of imine-based macrocyclic ligands, when now revisited in the light of constitutional dynamics [52],... 

Coordination of metal ions with neighbors through phosphate tetrahedra in heavy metal phosphate minerals. 

As demonstrated in this chapter, the binding of metal ions to maclocyclic ligands (e.g., porphyrins) results in the change in both the thermodynamic and dynamic properties of ET reactions of metalloporphyrins. Excellent models of the photosynthetic reaction center were developed by the appropriate choice of combination of metal ions and macrocyclic ligands. The lifetimes of the CS states in models of photosynthetic reaction center composed of electron donors and acceptors also were controlled by binding of metal ions to radical anions of electron acceptor moieties in the electron donor-acceptor hnked molecules. The control of ET processes by coordination of metal ions to the dyads led us to develop a unique fluorescence sensor for the ion. The binding of metal ions to radical anions of electron acceptors results in acceleration of thermal ET reactions, which would otherwise be impossible to occur. Such effects of metal ions to enhance the ET... 

Originally, photoluminescence spectroscopy was applied to characterize the local coordination of metal ions as well as to probe structural perturbations that occur due to alkaline earth and rare earth metal ions in oxides such as silica and alumina. Emphasis has turned to elucidating the mechanisms of catalytic and photocataljTic reactivity, i.e., the characterization, at the molecular level, of the active surface sites as well as the significant role of these sites in catalysis and photocatalysis. 

The single-crystal X-ray structure of (L = 2, 5, 9, 12-tetrathia[13](2, 5)thiophenophane (see Fig. 2) shows all the S-donors to lie in exocyclic positions. Thus, although the cavity of is relatively large (ca. 4.8 x 8.0 A), coordination of metal ions in an endo manner appears to be inhibited (139). Complexation of metal ions by even in an exo manner occurs via conformational change of the ligand 139). 

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