Complex ions factors affecting stability

Another factor that affects trends in the stability constants of complexes formed by a series of metal ions is the crystal field stabilization energy. As was shown in Chapter 17, the aqua complexes for +2 ions of first-row transition metals reflect this effect by giving higher heats of hydration than would be expected on the basis of sizes and charges of the ions. Crystal field stabilization, as discussed in Section 17.4, would also lead to increased stability for complexes containing ligands other than water. It is a pervasive factor in the stability of many types of complexes. Because ligands that form tt bonds... 

The unique ability of crown ethers to form stable complexes with various cations has been used to advantage in such diverse processes as isotope separations (Jepson and De Witt, 1976), the transport of ions through artificial and natural membranes (Tosteson, 1968) and the construction of ion-selective electrodes (Ryba and Petranek, 1973). On account of their lipophilic exterior, crown ether complexes are often soluble even in apolar solvents. This property has been successfully exploited in liquid-liquid and solid-liquid phase-transfer reactions. Extensive reviews deal with the synthetic aspects of the use of crown ethers as phase-transfer catalysts (Gokel and Dupont Durst, 1976 Liotta, 1978 Weber and Gokel, 1977 Starks and Liotta, 1978). Several studies have been devoted to the identification of the factors affecting the formation and stability of crown-ether complexes, and many aspects of this subject have been discussed in reviews (Christensen et al., 1971, 1974 Pedersen and Frensdorf, 1972 Izatt et al., 1973 Kappenstein, 1974). 

Because of the greater flexibility imparted them by the lack of head-to-tail covalent linkage, the carboxylic ionophores respond much more strongly to environmental forces such as local polarity than do the neutral macrocyclic ionophores. Upon leaving a membrane interface during the course of a catalytic transport cycle, an ionophore does not experience an abrupt change from a polar aqueous environment to an apolar hydrocarbon-like environment. The polarity boundary is rather diffuse. In order to properly evaluate the factors affecting carboxylic ionophore mediated transport, it is necessary to determine the effects of each of the microenvironments encountered within the membrane on the conformation of the ionophore and the stability of the ionophore-ion inclusion complex. 

The use of ceria and ceria-based materials in catalytic science has been well established in recent decades. Several investigations have reported that different kinds of interaction between ceria and various metal oxides influence catalytic activity and facilitate an improvement in the dynamic performance in the removal of VOCs. Catalytic activity is a complex function of several factors, such as oxidation state, nature of the support, catalyst composition, preparation method, crystal structure, particle size and so on. Doping of ceria creates an additional structural defect that influences the oxygen storage/release properties. Further, oxygen ion mobility affects phase stability and the aging properties of CeOg, and increases catalytic activity. ... 

The factors which influence the stability of metal ion complexes have been discussed in Section 2.23, but it is appropriate to emphasise here the significance of the chelate effect and to list the features of the ligand which affect chelate formation ... 

The formation of a coordinate bond is the result of the donation and acceptance of a pair of electrons. This in itself suggests that if a specific electron donor interacts with a series of metal ions (electron acceptors) there will be some variation in the stability of the coordinate bonds depending on the acidity of the metal ion. Conversely, if a specific metal ion is considered, there will be a difference in stability of the complexes formed with a series of electron pair donors (ligands). In fact, there are several factors that affect the stability of complexes formed between metal ions and ligands, and some of them will now be described. 

Fig. 5.20. Modes of coordination of transition metal ions with /3-lactam antibiotics. Complex A In penicillins, the metal ion coordinates with the carboxylate group and the /3-lactam N-atom. This complex stabilizes the tetrahedral intermediate and facilitates the attack of HO-ions from the bulk solution. Complex B In benzylpenicillin Cu11 binds to the deprotonated N-atom of the amide side chain. The hydrolysis involves an intramolecular attack by a Cu-coordinated HO- species on the carbonyl group. Complex C In cephalosporins, coordination of the metal ion is by the carbonyl O-atom and the carboxylate group. Because the transition state is less stabilized than in A, the acceleration factor of metal ions for the hydrolysis of cephalosporins is lower than for penicillins. Complex D /3-Lactams with a basic side chain bind the metal ion to the carbonyl and the amino group in their side chain. This binding mode does not stabilize the tetrahedral transition complex and, therefore, does not affect the rate of... 

Amylose-iodine complexes have a deep blue color, which is a result of an electron relay on the polyiodide ions.196 The helix of amylose provides a tunnel for iodine molecules to align. Stability of the amylose-iodine complex has been studied.197 Iodine has been widely used for quantification of amylose contents despite the fact that the blue color development is affected by many factors, including temperature, pH and mechanical mixing. Several improved methods have been reported (see Section 6.III.1). 

When use of the ion-exchange separation method developed in this study was considered it was determined from the separation factor calculated from the stability constants 4] of their complexes with EDTA that a column ratio greater than 40 would be needed to separate them. Experiments showed that a column ratio nearly 10 times larger would be needed to affect their separation with CIEC. Theoretically based studies [7-9] led to success in separation Eu and Gd by the use of HPIEC and a binary displacer. This technique has made the separation of Gd and Eu simple and useful enough to warrant its use as an industrial process. 

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