Complex ions, lattice-stabilized

As a rule, the distortion of the water lattice that is found in water without a solute (1) can easily take place in cooperation with the accompanying cation except in the cases of potassium, rubidium, and cesium. These ions are large enough to fill the cavities of the water lattice and to attenuate the lattice vibrations, thus preventing a local collapse of the structure and an increase in the number of interstitial water molecules. The normal water structure is essentially retained, and the lattice, stabilized by cations of the proper size, rejects the complex nonfitting ion (2). 

However, it is sometimes profitable to compare the relative stabilities of ions differing by unit charge when surrounded by similar ligands with similar stereochemistry, as in the case of the Fe3+—Fe2+ potentials (Table 17-1), or with different anions. In these cases, as elsewhere, many factors are usually involved some of these have already been discussed, but they include (a) ionization enthalpies of the metal atoms, (b) ionic radii of the metal ions, (c) electronic structure of the metal ions, (d) the nature of the anions or ligands involved with respect to their polarizability, donor pir- or acceptor d77-bonding capacities, (e) the stereochemistry either in a complex ion or a crystalline lattice, and (f) nature of solvents or other media. In spite of the complexities there are a few trends to be found, namely ... 

Zirconium has a high corrosion resistance and low cross-section for neutron capture (see Section 3.4) and is used for cladding fuel rods in water-cooled nuclear reactors. For this application, Zr must be free of Hf, which is a very good neutron absorber. The main use of pure Hf is in nuclear reactor control rods. Zirconium and hafnium compounds possess similar lattice energies and solubilities, and their complexes have similar stabilities. This means that separation techniques (e.g. ion exchange, solvent... 

The spin-lattice relaxation time Tx has been used to measure the exchange rate of water molecules between the inner hydration sphere of Ni + ions and the bulk in concentrated solutions of Ni(N03)2. (Previous n.m.r. methods have involved the determination of the spin-spin relaxation time, Tg.) An activation energy of 10.5 1.0 kcalmol" is reported and the results indicate the existence of an inner-sphere nitrate complex with a stability constant of about 0.21 mol". ... 

One of the most important parameters that defines the structure and stability of inorganic crystals is their stoichiometry - the quantitative relationship between the anions and the cations [134]. Oxygen and fluorine ions, O2 and F, have very similar ionic radii of 1.36 and 1.33 A, respectively. The steric similarity enables isomorphic substitution of oxygen and fluorine ions in the anionic sub-lattice as well as the combination of complex fluoride, oxyfluoride and some oxide compounds in the same system. On the other hand, tantalum or niobium, which are the central atoms in the fluoride and oxyfluoride complexes, have identical ionic radii equal to 0.66 A. Several other cations of transition metals are also sterically similar or even identical to tantalum and niobium, which allows for certain isomorphic substitutions in the cation sublattice. 

The extent of the ionization produced by a Lewis acid is dependent on the nature of the more inert solvent component as well as on the Lewis acid. A trityl bromide-stannic bromide complex of one to one stoichiometry exists in the form of orange-red crystals, obviously ionic. But as is. always the case with crystalline substances, lattice energy is a very important factor in determining the stability and no quantitative predictions can be made about the behaviour of the same substance in solution. Thus the trityl bromide-stannic bromide system dilute in benzene solution seems to consist largely of free trityl bromide, free stannic bromide, and only a small amount of ion pairs.187 There is not even any very considerable fraction of covalent tfityl bromide-stannic bromide complex in solution. The extent of ion pair and ion formation roughly parallels the dielectric constant of the solvents used (Table V). The more polar solvent either provides a... 

From the previous paragraphes it follows that a substantial amount of experimental data exist that illustrate the oxygenation properties of zeolite catalysts. In very general terms zeolites are used to heterogenize transition metal ions in ion exchange or lattice positions, to stabilize transition metal oxide dispersions and to prepare ship-in-bottle complexes. 

These comparatively lipophilic ligands have been conceived as ion carriers for systems such as ion-selective electrodes, 29 therefore they do not have high stability constant values, but require fast complexation kinetics in order to achieve rapid equilibration. Nevertheless, it has been possible to recover crystalline species which often have present additional water molecules to help stabilize the crystal lattice. Coordinative participation of the carbonyl oxygen... 

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