Trivalent cations, structure

Cations other than lithium (such as the divalent or trivalent cations Mg, Zn, Co, Ni, and Cr) can also be substituted for Mn to stabilize the Li[Mn2]04 structure [107,... 

Finally, a new water-soluble polyphosphazene was recently synthesized that has the structure shown in 36 (46). This polymer has two attributes as a biomedical macromolecule. First, the pendent carboxylic acid groups are potential sites for condensation reactions with amines, alcohols, phenols, or other carboxylic acid units to generate amide, ester, or anhydride links to polypeptides or bioactive small molecules. Second, polymer forms ionic crosslinks when brought into contact with di- or trivalent cations such as Ca or Ai3+. The crosslinking process converts the water-soluble polymer to a hydrogel, a process that can be reversed when the system... 

Trivalent cations are present in most of niobium oxychloride cluster compounds known to date and play an important role in their structural chemistry. The series AxNb6Cli202 (A = K, Rb, Cs, In), on the other hand, represents an example of an oxychloride structure stabilized without a trivalent counter-ion (Fig. 6.12). 

Even greater disruption is encountered in the case of trivalent cations (Figures 4.9,4.10). They completely penetrate both hydration regions and destroy the structure of water around the polyion. This amounts to complete desolvation. The same is true of bound hydrogen ions which are localized. 

A trivalent cation, for example AF, has the potential to link three chains. Sterically, this is improbable Mehrotra Bohra (1983) assert that simple aluminium tricarboxylates are not known in solution. Nevertheless we consider it probable that a small proportion of AF ions link three chains, in which all three charged ligands are COO". More probable molecular structures would contain one or two F" ions with a single F", an [A1F(H20)3] unit could bridge two polyanion chains, while an [A1F2(H20)2] would have no crosslinking ability. 

The basic structure of an LDH may be derived by substitution of a fraction of the divalent cations in a brucite lattice by trivalent cations such that the layers acquire a positive charge, which is balanced by intercalation of anions (and, usually, water) between the layers. It is the possibility of varying the identity and relative proportions of the di- and trivalent cations as well as the identity of the interlayer ions that gives rise to the large variety of materials having the general formula [M S xM x(OH)2] [A" ] c/ yH20, which... 

LDHs containing nickel with trivalent cations such as aluminum, chro-miiun, manganese and iron, have been synthesized electrochemically [173]. The electrosynthesized LDHs have similar structural features to both the chemically synthesized LDHs and a-nickel hydroxide. A new Fe(II)/Fe(III)-... 

The next landmark was the synthesis of the germylium, 22, and the stannylium ion, 23, by one-electron oxidations from the corresponding stable radicals with trityl TPFPB by Sekiguchi and co-workers. As in the case of the allyl cleavage to generate the mesityl-substituted cations, the reaction, in this case the oxidation, occurs at the periphery of the molecule and gives the possibility for efficient steric protection of the incipient cation. Both trivalent cations were obtained as their TPFBP salts and the crystal structure show well separated anions and cations. 

Ferrite compounds with the inverse spinel structure are similar to magnetite, with different ions substituting for the iron atoms. As with FeO (cf. Figure 6.62), the oxygen ions have no permanent magnetic moment. Tetrahedral sites in the FCC oxygen array are occupied by half of the trivalent cations, and octahedral sites are occupied equally by divalent cations and the remaining trivalent cations. 

Molybdate, MoO2-, is isolated in the form of salts of monovalent, divalent and trivalent cations. The salts of the simple monovalent cations are usually water soluble while salts with larger cations, e.g. N-propylammonium, jV-ethylpyridinium and tetra-n-butyl ammonium may also have solubility in non-aqueous solvents.9 The salts of di- and tri-valent cations are generally insoluble and form three-dimensional structures in the solid state. As discussed below, although many of these maintain the MoO2- structural unit, some salts which stoichiometrically contain Mo04 have octahedral six-coordinate MoVI. 

Choosing divalent and trivalent cations and determining the composition is the most important in designing the multicomponent bismuth molybdate catalyst system. Catalytic activities of typical tri- and tetracomponent bismuth molybdate catalysts having multiphase structure were reported for the oxidation of propylene to form acrolein (35, 36, 40-43, 97, 98). A typical example of the activity test is shown in Fig. 6. Summarizing the results shown in Fig. 6 and reported previously (30, 43, 44), the following trends are generally found. 

In the discussion of fatty acids given so far it has been assumed that the subphase is sufficiently acid so that these materials are not ionised. However, if the pH is increased so as to be comparable to or larger than the pA"a of the fatty acid, then the situation will obviously be different. If now a divalent or trivalent cation is introduced into the subphase, the structure of the film and the resulting isotherms will be substantially changed. Wostenholme and Schulman [77] were amongst the first workers to make a systematic study of this effect. Binks [78] has given a recent review of the very extensive literature devoted to this phenomenon. The matter will be returned to in Section 3.3. 

However, it is possible to introduce two new factors into the problem of multilayer structure, either of which can influence structure in an interesting way. It is possible to replace the ordinary hydrocarbon chain by a perfluorinated chain and it is possible to dip over a subphase containing a trivalent cation. The perfluorinated carbon chain has a helical structure which produces a chain which appears to be approximately cylindrical. The handedness of a chain is determined randomly so that an equal number of right handed and left handed molecules exist in a given batch. 

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