Tetravalent metals, substitution

The Electron Transfer Step. Inner-sphere and outer-sphere mechanisms of reductive dissolution are, in practice, difficult to distinguish. Rates of ligand substitution at tervalent and tetravalent metal oxide surface sites, which could be used to estimate upward limits on rates of inner-sphere reaction, are not known to any level of certainty. 

One such problem is the possibility of the existence and aromatic character of analogues of five-membered heteroaromatics with one heteroatom and those of azoles in which one or several ring C atoms are substituted by a metal atom, e.g. structures (289)-(305) (M = Ge, Sn, Pb X = O, S, NR). The other specific problems involve antiaromaticity of 871-electron carbenoids (306) and homoaromaticity of 67t-electron systems with a tetravalent metal (307). 

In contrast to the conventional approach whereby various organic groups are subsequently bound to a previously prepared surface, we have been synthesizing a broad series of anchored, layered-structure solids by precipitating the pre-derived phosphonate salts with tetravalent metal ions. The two-dimensional backbone has the zirconium phosphate structure however, substituted for hydroxylic groups are the desired organics, oriented away from the basal surfaces in a bilayered fashion in the interlayer region. 

At one time the preferred catalyst for propylene ammoxidation was a uranium-antimony oxide composition whose active phase was USb3O2 Q. We have found that the partial substitution of certain tetravalent metals for the pentavalent antimony in this phase greatly increases catalytic activity. 

The literature on metallo-de-hydrogenations of aliphatic diazo compounds is, in contrast to that on substitutions with classical electrophilic reagents, fairly large. It includes the monovalent metals Li, Na, and Ag, the divalent metals Mg, Zn, Cd, and Hg, the trivalent metal Tl, and the tetravalent metals Ge, Sn, and Pb. In addition, substitutions with the hetero atoms B, As, Sb, Bi, and Si, as well as metallo-de-metallations are reported in the literature. Although the synthesis of diethyl mer-cury(bisdiazoacetate) (9.4) was performed quite early (Buchner, 1895), the majority of papers on metallation was published in the 1960 s and 1970 s. ... 

Tables from 5 to 14 show, that the substitution of Al + and/or P + by divalent metal ions or tetravalent silicon in aluminophosphate structures creates both the Br0nsted and Lewis acid sites. These acid sites differ mutually in their donor-acceptor ability. The first one can transfer protons from the catalyst to the adsorbed molecules, whereas the latter can accept an electron pair from the adsorbed molecules. The strength and concentration of both types of acid sites determine the activity, selectivity, and lifetime of catalysts in acid-catalyzed reactions. The acid strength varies among aluminophosphates, and it is mainly dependent on the type of metal substituted in the framework. Also the catalytic performance is affected by structural characteristics of the framework such as the pore size, pore shape, or geometry.

There has recently been much interest around proton conduction in condensed phosphates. Lanthanum metaphosphate (La(P03)3) exhibits a modest proton conductivity [59] whereas diphosphates of tetravalent metals, e.g., SnP207 and TiP207, appear to exhibit a high proton conduction peak at intermediate temperatures (around 200°Q. The effect is reportedly enhanced by substituting In for the tetravalent cation, and the conductivity can exceed 0.1 S/cm [60]. It is uncertain what is the defect or doping mechanism behind these behaviors. The same materials exhibit a lower, temperature-dependent conductivity above 400°C, tentatively attributed to protons from hydrolysis of the diphosphate groups [61]. 

Tin amidinates display a rich coordination chemistry with the metal in both the di- and tetravalent oxidation states. The first results in this area were mainly obtained with N-silylated benzamidinate ligands. Typical reactions are summarized in Scheme 48. A stannylene containing unsymmetrically substituted amidinate ligands, [o-MeC6H4C(NSiMe3)(NPh)]2Sn, has been prepared accordingly and isolated in the form of colorless crystals in 75% yield. ... 

In terms of the development of an understanding of the reactivity patterns of inorganic complexes, the two metals which have been pivotal are platinum and cobalt. This importance is to a large part a consequence of each metal having available one or more oxidation states which are kinetically inert. Platinum is a particularly useful element of this pair because it has two kinetically inert sets of complexes (divalent and tetravalent) in addition to the complexes of platinum(O), which is a kinetically labile center. The complexes of divalent and tetravalent platinum show significant differences. Divalent platinum forms four-coordinate planar complexes which have a coordinately unsaturated 16-electron d8 platinum center, whereas tetravalent platinum is an 18-electron d6 center which is coordinately saturated in its usual hexacoordination. In terms of mechanistic interpretation one must therefore consider both associative and dissociative substitution pathways, in addition to mechanisms involving electron transfer or inner-sphere atom transfer redox processes. A number of books and articles have been written about replacement reactions in platinum complexes, and a number of these are summarized in Table 13. 

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 layered oxides AMO2 (A = Li, Na M = V, Cr, Mn, Fe, Co, Ni) comprise alternating layers of edge-shared MOe octahedra and layers of alkali metal cations, and are structurally similar to the alkali-transition metal dichalcogenides. Nonstoichiometric phases A M02 can be synthesized at high temperatures by substitution of a tetravalent cation of a different metal or partial oxidation of the trivalent M cation, hi general, stable phases cannot be synthesized at high temperature for X < 0.5. 

Zeolites and zeotypes can be defined as microporous crystalline structures (Figure 1 and Table 1) in which the framework is formed by tetrahedral of silica, in which there is isomorphic substitution with trivalent or tetravalent elements such as for instance Al, Ge, B, Fe, Cr, Ge, Ti, etc. [1]. Similar types of structure can also be achieved with the framework formed by Al and P, with or without other transition metal elements [2]. These types of structure are denoted as AlPOs, SAPOs, and MEAPOs, depending on the composition of the framework [3]. 

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