Complex cationic oxidants, trivalent

Most zeolites have an intrinsic ability to exchange cations [1], This exchange ability is a result of isomorphous substitution of a cation of trivalent (mostly Al) or lower charges for Si as a tetravalent framework cation. As a consequence of this substitution, a net negative charge develops on the framework of the zeolite, which is to be neutralized by cations present within the channels or cages that constitute the microporous part of the crystalline zeolite. These cations may be any of the metals, metal complexes or alkylammonium cations. If these cations are transition metals with redox properties they can act as active sites for oxidation reactions. 

The sesquioxide, Cr Oa, containing trivalent chromium, is an amphoteric oxide. It yields chromic salts, such as chromic chloride, CrCla, and sulphate, Cr2(S04)a, which are very stable and show great similarity to the ferric salts and to salts of aluminium as, for example, in the formation of alums. Since, however, chromic oxide functions as a weaker base than chromous oxide, the latter having a lower oxygen content, the chromic salts are more liable to hydrolysis than the chromous salts. This is well marked in the case of the chlorides. Again, in spite of the stability of chromic salts, only a slight tendency to form simple Cr " ions is exhibited, whilst complex ions are formed much more readily, not only complex anions, as in the case of iron and aluminium, but also complex cations, as in the extensive chromammine series. In this respect chromium resembles cobalt and platinum. 

The importance of the size of the solute relative to that of the solvent mentioned above is evident also from experimental determinations of the extent of solid solubility in complex oxides and from theoretical evaluations of the enthalpy of solution of large ranges of solutes in a given solvent (e.g. a mineral). The enthalpy of solution for mono-, di- and trivalent trace elements in pyrope and similar systems shows an approximately parabolic variation with ionic radius [44], For the pure mineral, the calculated solution energies always show a minimum at a radius close to that of the host cation. 

The sluggish substitution properties of copper(III) and nickel(III) peptide complexes have permitted the isolation of complexes with these oxidation states (14, 15). Thus, the tri-valent peptide complexes pass through a cation exchange resin which readily strips copper(II) or nickel(II) from the corresponding complexes. We now have a little more information about the substitution characteristics of the trivalent metal complexes. 

The first few pentacoordinate cationic silicon complexes (153-155) were discussed in an earlier review2. The first two were described as trivalent silicenium ions stabilized by mtermolecularcoordination183 184, while 155 is an intramolecular silicenium ion21a. Since then, a few other compounds were reported. Oxidation of the dihydrido-hexacoordinate chelate 156 by the addition of excess iodine produced 157185. The X-ray structure of 157 showed a slightly distorted TBP for the cation, with a well separated (Si—I distance 5.036 A) I8 2 anion for every two cations. The nitrogens are in the apical positions, and both Si-N distances (2.08 and 2.06 A) are longer than covalent bonds but well within the coordination distance. 

Scheme I further indicates the tendency of the Ln(III) cations to form the mofe unusual oxidation states in solution [73]. Hitherto, organometallic compounds of Ce(IV), Eu(II), Yb(II) and Sm(II) have been isolated. Charge-dependent properties, such as cation radii and Lewis acidity, significantly differ from those of the trivalent species (Table 4). Ln(II) and Ce(IV) ions show very intense and ligand-dependent colors which is attributed to Laporte-allowed 4/-+ 5d transitions [65b]. Complexes of Ce(IV) and Sm(II) have acquired considerable importance in organic synthesis due to their strong oxidizing and reducing behavior, respectively their reaction patterns have been reviewed in detail [40, 44-47, 74], Catalytic amounts of compounds containing the hot oxidation states also initiate substrate transformations as a rule this implies switch to the more stable, catalytically-acting Ln(III) species [75],...

The divalent iron is oxidized by atmospheric oxygen into trivalent iron, but only 13% of this iron occurs as free Fe3+ cation. The rest is bound in hydroxo and sulfur complexes (Fig. 62). 

A further enhancement of the hardness of donating atoms by introduction of phosphine oxide ligands 20 did not give more stable Na" " complexes but resulted in selectivity for di- and trivalent cations. 

The first Ce(IV) complex stabilized by an alkoxo functionalized Ai-heterocyclic carbene anion has recently been reported. The complex is made by oxidation of the corresponding trivalent cerium complex using benzoquinone (Figure 8.36). The cerium cation is coordinated by two bidentate ligands and two monodentate ligands, in which the NHC groups are unbound [123],... 

All early actinides from thorium to plutonium possess a stable +4 ion in aqueous solution this is the most stable oxidation state for thorium and generally for plutonium. The high charge on tetravalent actinide ions renders them susceptible to solvation, hydrolysis, and polymerization reactions. The ions are readily hydrolyzed, and therefore act as Bronsted acids in aqueous media, and as potent Lewis acids in much of their coordination chemistry (both aqueous and nonaqu-eous). Ionic radii are in general smaller than that for comparable trivalent metal cations (effective ionic radii = 0.96-1.06 A in eight-coordinate metal complexes), but are still sufficiently large to routinely support high coordination numbers. 

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