Thorium lower oxidation states

Much new information about compounds of the lower oxidation states of uranium and thorium has accumulated during the last decade, and the rapid development of the organometallic chemistry of these elements has enhanced interest in this topic. 

However, all of the emf results by Chiotti and Dock [1975CH1/DOC], Yoshida et al. [1967YOS/OYA], and Srinivasan and Flengas [1964SR1/FLE] failed to identify lower oxidation states of thorium in the Th-ThCLt system. The voltametric determina-tiorrs by Martinot [1986MAR4] also showed no evidence of the existence of Th(ll) in the eutectic system, even as an intermediate species. 

Uranium differs considerably from tungsten and molybdenum in the chemistry of the lower oxidation states. Uranium (III) has great similarity to the tripositive rare-earth elements and actinium, and uranium (IV) resembles thorium and cerium (IV). Thus uranium (III) and uranium (IV) are not acidic in character they do not tend, like tungsten... 

Manning, Ball, and Menis (162) have carried out polarographic and coulometric reductions of molybdenum (VI) in a nitrilotriacetic acid medium and have applied their findings to the analysis of thorium-uranium oxide mixtures. The determination of molybdenum in steel using coulometric techniques has been reported by Ibrahim and Nair (163) who reduced molybdenum at —0.40 V vs. SCE in a sodium acetate buffered chloride medium. Chromium interference can be removed by pre-reduction with alcohol. The catalytic effect of lower oxidation states of molybdenum in the reduction of perchlorate has been used as an indirect electro-analytical method for the determination of perchlorate (159, 164). 

Coordination Complexes. The coordination and organometaHic chemistry of thorium is dominated by the extremely stable tetravalent ion. Except in a few cases where large and stericaHy demanding ligands are used, lower thorium oxidation states are generally unstable. An example is the isolation of a molecular Th(III) complex [107040-62-0] Th[Tj-C H2(Si(CH2)3)2]3 (25). Reports (26) on the synthesis of soluble Th(II) complexes, such as... 

On the basis of these facts, it was speculated that plutonium in its highest oxidation state is similar to uranium (VI) and in a lower state is similar to thorium (IV) and uranium (IV). It was reasoned that if plutonium existed normally as a stable plutonium (IV) ion, it would probably form insoluble compounds or stable complex ions analogous to those of similar ions, and that it would be desirable (as soon as sufficient plutonium became available) to determine the solubilities of such compounds as the fluoride, oxalate, phosphate, iodate, and peroxide. Such data were needed to confirm deductions based on the tracer experiments. 

Some representative thorium compounds are given in Table 20-8 only few compounds in oxidation states lower than +4 are known. 

Stoichiometry. At present the observed limit of two halide ions per metal does not seem particularly important as a necessity for close approach of the cations and hence suitable band formation rather, it more probably results from other characteristics of this formal oxidation state for these elements. One possible fact to the contrary is that thorium (III) iodide is evidently not metallic (4), though it would probably meet the second criterion below. The general electronic conduction in sulfide vs. chloride melts in the metal-rich region (as well as in the solid state) may be attributed to the lower anion to cation ratio and therefore closer approach of the cations (5), although covalency as discussed below may be more significant. 

Little need be said about the halides of actinium. All of the binary halides of this element have been prepared by conventional procedures, and the straightforward, uncomplicated chemical behavior that actinium exhibits generally is also evident here. Thorium, however, presents quite a different problem. Thorium gives no indication of existence in aqueous solution in an oxidation state lower than +4. From considerations of the... 

Semiempirical calculations of free energies and enthalpies of hydration derived from an electrostatic model of ions with a noble gas structure have been applied to the ter-valent actinide ions. A primary hydration number for the actinides was determined by correlating the experimental enthalpy data for plutonium(iii) with the model. The thermodynamic data for actinide metals and their oxides from thorium to curium has been assessed. The thermodynamic data for the substoicheiometric dioxides at high temperatures has been used to consider the relative stabilities of valence states lower than four and subsequently examine the stability requirements for the sesquioxides and monoxides. Sequential thermodynamic trends in the gaseous metals, monoxides, and dioxides were examined and compared with those of the lanthanides. A study of the rates of actinide oxidation-reduction reactions showed that, contrary to previous reports, the Marcus equation ... 

Thorium is somewhat more common in nature than uranium, with an average content in the earth s crust of 10 ppm (by comparison the average abundance of lead is about 16 ppm in the earth s crust). In minerals it occurs only as oxide. The content of thorium in sea water is < 0.5 x 10" g/m, which is lower than that of uranium because of the lower solubility of Th compounds (the most stable valency state of Th). 

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