Tetravalent states

Although rare-earth ions are mosdy trivalent, lanthanides can exist in the divalent or tetravalent state when the electronic configuration is close to the stable empty, half-fUed, or completely fiUed sheUs. Thus samarium, europium, thuUum, and ytterbium can exist as divalent cations in certain environments. On the other hand, tetravalent cerium, praseodymium, and terbium are found, even as oxides where trivalent and tetravalent states often coexist. The stabili2ation of the different valence states for particular rare earths is sometimes used for separation from the other trivalent lanthanides. The chemicals properties of the di- and tetravalent ions are significantly different. 

In aqueous solutions, trivalent lanthanides ate very stable whereas only a limited number of lanthanides exhibit a stable divalent or tetravalent state. Practically, only Ce and Eu " exist in aqueous solutions. The properties of these cations ate very different from the properties of the trivalent lanthanides. For example, Ce" " is mote acidic and cetium(IV) hydroxide precipitates at pH 1. Eu " is less acidic and eutopium(II) hydroxide does not precipitate at pH 7—8.5, whereas trivalent lanthanide hydroxides do. Some industrial separations ate based on these phenomena. 

Chlorine may initially convert the selenium in solution to the hexavalent state, but as the hydrochloric acidity increases, reduction to the tetravalent state occurs spontaneously. 

In dilute HE solutions and higher anodic voltages, the siUcon is dissolved in the tetravalent state ... 

However, it has been concluded from sorption and diffusion experiments that plutonium exists largely in the tetravalent state (53) and clearly not as Pu(V), in the intermediate pH-range under oxic conditions and at low carbonate concentration. This would be representative of many groundwaters and also in agreement with the calculated curves of Figure 2. 

For plutonium in the tri- and tetravalent state, when hydrolysis would dominate the solution chemistry, most sorption phenomena in geologic systems can be looked upon largely as physical adsorption processes. Ion exchange processes, as defined above, would be... 

The extractant is octyl pyrophosphoric acid (OPPA process). The stripping is by concentrated hydrofluoric acid. Yields UF4. Extracts uranium in tetravalent state. It is, therefore, necessary to use metallic iron as a reducing agent. 

The extractant is a commercial mixture of mono- and dioctyl phenyl phosphoric acid (OPPA). It is used in conjuction with tributyl phosphate (TBP). Stripping is by ammonium carbonate solution. The mixture shows synergism. Uranium is extracted in the tetravalent state. The process is much less expensive and possesses a higher extracting power than D2EHPA-TOPO combination. 

Physical properties of binary or ternary Ru/Ir based mixed oxides with valve metal additions is still a field which deserves further research. The complexity of this matter has been demonstrated by Triggs [49] on (Ru,Ti)Ox who has shown, using XPS and other techniques (UPS, Mossbauer, Absorption, Conductivity), that Ru in TiOz (Ti rich phase) adopts different valence states depending on the environment. Possible donors or acceptors are compensated by Ru in the respective valence state. Trivalent donors are compensated by Ru5+, pentavalent acceptors will be compensated by Ru3+ or even Ru2+. In pure TiOz ruthenium adopts the tetravalent state. The surface composition of the titanium rich phase (2% Ru) was found to be identical to the nominal composition. 

The reduction of pertechnetate with concentrated hydrochloric acid finally yields the tetravalent state, and no further reduction to the tervalent state takes place. Therefore, the tervalent technetium complex has usually been synthesized by the reduction of pertechnetate with an appropriate reductant in the presence of the desired ligand. Recently, the synthesis of tervalent technetium complexes with a new starting complex, hexakis(thiourea)technetium(III) chloride or chloropentakis(thiourea)technetium(III) chloride, has been developed. Thus, tris(P-diketonato)technetium(III) complexes (P-diketone acetylacetone, benzoyl-acetone, and 2-thenoyltrifluoroacetone) were synthesized by the ligand substitution reaction on refluxing [TcCl(tu)5]Cl2 with the desired P-diketone in methanol [28]. 

This view is supported by the types of compounds that can be prepared. Group IVa metals in the tetravalent state have no d electrons and tetra-valent vanadium has one. Compounds with a large number of d electrons, e.g., nickel, do not form benzyl compounds readily and attempts to synthesize Ni (benzyl) 2 have not succeeded. 

Rare earth elements are the general term for 15 kinds of lanthanide elements (La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Py, Ho, Er, Tm, Yb, Lu) together with Sc and Y elements. They prefer trivalent states in the complex formation, though three elements (Eu, Sm, Yb) can assume tri- and divalent stateos and Ce a tri- or tetravalent state. Their ionic radii are fairly large (1.0-1.17 A) and their electronegativities are low (1.1-1.2). In fact, the former are much larger than those of... 

Selenium (IV) adsorbed as selenotrisulfate was then eluted from the column with either 0.1 M penicillamine or 0.1M cysteine. The eluate was then subjected to an acid digestion procedure to reduce selenium to the tetravalent state with diaminonaphthalene for fluorometric determination. Approximate agreement with the tellurium coprecipitation method was obtained. The application of both methods to the analysis of estuarine waters permitted the separate determination of both selenium (IV) and selenium (VI), since the tellurium coprecipitation methods did not differentiate between the two species. 

The enthalpies of formation of selected perovskite-type oxides are given as a function of the tolerance factor in Figure 7.17. Perovskites where the A atom is a Group 2 element and B is a d or / element that readily takes a tetravalent state [19, 20] show a regular variation with the tolerance factor. Empirically, it is suggested that the cations that give t close to 1 have the most exothermic enthalpies of formation. When t is reduced, the crystal structure becomes distorted from cubic symmetry and this also appears to reduce the thermodynamic stability of the... 

In the trivalent state the stereochemistry of the actinides is similar to the lanthanides, that is, eight coordinate. However, higher coordination states are known for some trivalent actinides, for instance, UC13 exists in the nine coordinate state. In the tetravalent state the species normally encountered are eight coordinate. 

An examination of the chemistry of americium indicates that it is intermediate between that of plutonium and that of curium. The chemistry of curium bears a strong resemblance to that of the lanthanides in solution curium exists in the + 3 oxi dation state, although the tetravalent state has been observed as Cm F4 in a solid matrix. 

Organotellurium compounds are readily oxidized from the divalent to tetravalent state. Consequently, this property makes tellurides attractive as scavengers of reactive oxidizing agents such as hydrogen peroxide, hypochloride and peroxyl radicals. 

The titanium carbide spectrum shows strong absorption at 0 ev., which may be attributed to the metallic nature of this carbide or to the tetravalent state of titanium. This ambiguity is not appreciably resolved by the discussion of Bundle 18), who explains how the tetravalent nature of the titanium in TiC leads to its metallic character. The TiO spectrum does not show this low-lying absorption peak TiO is not metallic and does not contain tetravalent titanium. 

The chemical properties of hafnium are very much similar to those of zirconium. In aqueous solutions, the metal exists in tetravalent state. The elec-... 

The valence state of vanadium in as synthesized VAPO -S was mainly the tetravalent state, in which... 

Commercial separations involving the oxidation to the tetra-valent state are limited to the removal of cerium after oxidation. These separations, which are based upon reduced basicity in the tetravalent state, include ... 

One major difficulty of this attribution lies in the assigmnent of a tetravalent state for Am, Cm, and Bk metals. This is not consistent with other physical properties of these metals, as we will see later. 

Nevertheless, the inspection of other transition metal series shows that, just as atomic volumes, there are regular variations of cohesive energies when the metal valence changes. Thus, a general increase of about 45 Kcal/mol is found when a metal transforms from a trivalent to a tetravalent state. 

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