Tetravalent chemistry cerium

Even flien well-characterized coordination compoimds are limited to only a few classes of compounds. Notable are, e.g., halogeno complexes and complexes of oxo acids, p-diketonates and related SchifF-base complexes, as well as porphyrinates and related complexes. Two other important classes of cerium(lV) compounds, the alkoxides and amides of Ce" +, can be regarded as pjewfifo-organometallics and are discnssed together with the organocerium(IV) complexes in Tetravalent Chemistry OrganometalUc. 

Cerium (IV) in solution is obtained by treatment of Ce111 solutions with very powerful oxidizing agents, for example, peroxodisulfate or bismuthate in nitric acid. The aqueous chemistry of CeIV is similar to that of Zr, Hf, and, particularly, tetravalent actinides. Thus Ce gives a phosphate insoluble in 4 M HN03 and an iodate insoluble in 6 M HN03, as well as an insoluble oxalate. The phosphate and iodate precipitations can be used to separate Ce from the trivalent lanthanides. Ce is also much more readily extracted into organic solvents by tributyl phosphate and similar extractants than are the Lnm lanthanide ions. 

Many of the actinoids are also separated by exploiting their redox behavior. Thorium is exclusively tetravalent and berkelium is chemically similar to cerium, so iodate precipitation of Th and extraction of Bk(IV) with bis(2-ethylhexyl)orthophos-phoric acid (HDEHP) are used to isolated these elements. The differing stabilities of the (III), (IV), (V), and (VI) states of U, Np, and Pu have be exploited in precipitation and solvent extraction separations of these elements from each other and from fission product and other impurities with which they are found. Because of its technical importance, the process chemistry to separate U and Pu in nuclear materials has been highly developed. Extraction of Bk(IV) with HDEHP is used to separate Bk from neighbouring elements. 

Fossil bone chemistry as a paleoredox indicator. Uniquely among the REE, cerium has the capability to adopt a tetravalent ion under the appropriate oxidizing conditions. The oxidized form of Ce (Ce" ) is relatively insoluble compared to Ce (de Baar et al. 1985), so that cerium takes part in active redox cycling. Under oxic conditions, Ce exists in the tetravalent state, and is readily removed from solution either onto particle surface coatings, or into authigenic minerals (Sholkovitz et al. 1993, Koeppenkastrop and... 

Because the tripositive ions are the most stable for all the rare earth elements in almost all compounds, the thermochemistry of the solid (crystalline) rare earth sesquioxides dominates this chapter. Some rare earths have divalent or tetravalent states, so the chemistry of solid monoxides and dioxides are included. There are also many nonstoichiometric binary oxides of cerium, praseodymium, and terbium. As much as possible, the thermochemistry of these nonstoichiometric binary oxides is included. The stability, phase diagrams, and structures of ternary and polynary... 

This chapter gives an overview on the chemistry of tetravalent lanthanide compounds, especially those of tetravalent cerium. Following a brief introduction, it covers the tetrahalides, dioxides, and other lanthanides(IV) salts. Coordination compounds of cerium in the oxidation state +4 include halogeno complexes and complexes of oxo acids, /3-diketonates and related Schiff-base complexes, as well as porphyrinates and related complexes. 

The readily occurring transition from colorless Ce + to bright yellow or orange Ce" + forms the basis for the use of cerium(IV) sulfate solutions in redox titrations ( cerimetric analysis). The ease of access to various tetravalent cerium compounds makes cerium(IV) most valuable in research as well as in various practical applications. Important fields of application for cerium(IV) compounds include organic syntheses, bioinorganic chemistry, materials science, and industrial catalysis (e.g., vehicle emissions control, oxygen storage). ... 

With only a few exceptions, the coordination chemistiy of rare earth elements in the oxidation state is basically the coordination chemistry of tetravalent cerium. 

The synthesis and full characterization of organolan-thanide(IV) complexes remain a very difficult and often unpredictable task. Thus far, organolanthanide chemistry in the oxidation state +4 remains entirely limited to cerium. This can be traced back to the very highly positive normal potentials of the other tetravalent lanthanide ions Nd" +, Tb +, and... 

The characteristic oxidation state for the rare earth elements is +3 (Nash and Sullivan, 1991). The chemistry of the rare earths is largely determined by this oxidation state. There are only few examples of stable rare-earth ions in other oxidation states. One example is tetravalent cerium, cerium(IV). The stability of the +4 oxidation state can be attributed to the empty 4f-shell in the [Xe]4f electronic configuration of... 

The coordination chemistry of tetravalent cerium is in many aspects very similar to the coordination chemistry of tetravalent plutonium. The ionic radius of Ce" " (0.94 A) is within the experimental error identical to the ionic radius of Pu + (Shannon and Prewitt, 1969). Due to the similarity in the charge-to-ionic size ratio, the complex formation constants of tetravalent cerium are essentially the same as those of tetravalent plutonium. Complex formation causes for the two metal systems the same shift of the redox potential. 

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