Trivalent thorium

For the thorium cyclopentadienyls with R = CH3, photolysis in aromatic solvents produces 1 1 mixtures of alkene and alkanes and the trivalent thorium complex Th(C5E5)s. It was proposed that the reaction takes place by a photoinduced beta-elimination to give a hydrido complex and olefin followed by a bimolecular reductive elimination of the alkane. Schematically it can be represented by the following equations ... 

The trivalent thorium complex Th(CsH5)3 has been prepared by sodium naphthalide reduction of (C5H5)3ThCl in tetrahydrofuran [33] ... 

In the area of superconductivity, tetravalent thorium is used to replace trivalent lanthanides in n-ty e doped superconductors, R2 Th Cu0 g, where R = Pr, Nd, or Sm, producing a higher T superconductor. Thorium also forms alloys with a wide variety of metals. In particular, thorium is used in magnesium alloys to extend the temperature range over which stmctural properties are exhibited that are useful for the aircraft industry. More detailed discussions on thorium alloys are available (8,19). 

The apparent failure of trivalent and tetravalent cations to enter plants could result from the interaction of the cations with the phospholipids of the cell membranes. Evidence for such interactions is provided by the use of lanthanum nitrate as a stain for cell membranes (143) while thorium (IV) has been shown to form stable complexes with phospholipid micelles (144). However, it is possible that some plant species may possess ionophores specific to trivalent cations. Thomas (145) has shown that trees such as mockernut hickory can accumulate lanthanides. The proof of the existence of such ionophores in these trees may facilitate the development of safeguards to ensure that the actinides are not readily transported from soil to plants. 

After filtering off the solid residue, the cold solution is treated with sodium sulphate. The precipitated double sulphate of the rare earths is washed with 0.3N sodium sulphate, and then boiled with excess (10%) sodium hydroxide to convert the double sulphate into a hydroxide. The rare earths hydroxides are filtered off and dried at 120° C to oxidize trivalent cerium into the tetravalent state. The dried material contains about half of the original thorium coprecipitated with rare earth double sulphates. 

ACTINIDE CONTRACTION. An effect analogous to the Lanthanide contraction, which lias been found in certain elements of the Actinide series. Those elements from thorium (atomic number 90) to curium (atomic number 96) exhibit a decreasing molecular volume in certain compounds, such as those which the actinide tetrafluoiides form with alkali metal fluorides, plotted in Eig. 1. The effect here is due to the decreasing crystal radius of the tetrapositive actinide ions as the atomic number increases. Note that in the Actinides the tetravalent ions are compared instead of the trivalent ones as in the case of the Lanthanides, in which the trivalent state is by far the most common. 

For the trivalent lanthanides99-100 and actinides,99 as well as for yttrium and scandium,75 the equilibrium constant for the extraction reaction has been shown to vary inversely with the ionic radius of the metal ion. It has therefore been concluded that the extracted complexes are all of the M(HA2)3 type, involving predominantly ionic metal—ligand bonds.75 The similarity of the IR spectra of the scandium(III) and thorium(IV) complexes of D2EHPA to those of the alkali metals is also indicative of the importance of ionic bonding.102... 

In spite of considerable similarities between the chemical properties of lanthanides and actinides, the trivalent oxidation state is not stable for the early members of the actinide series. Due to larger ionic radii and the presence of shielding electrons, the 5f electrons of actinides are subjected to a weaker attraction from the nuclear charge than the corresponding 4f electrons of lanthanides. The greater stability of tetrapositive ions of actinides such as Th and Pu is attributed to the smaller values of fourth ionization potential for 5f electrons compared to 4f electrons of lanthanides, an effect that has been observed in aqueous solution of Th and Ce (2). Thus, thorium... 

In the Strasbourg laboratory, which cannot handle radioactive substances, trivalent and tetravalent transuranic elements were simulated by lanthanides (generally europium) and thorium, respectively. 

As expected, tetravalent thorium is better extracted than trivalent europium. All calixarenes are stronger extractants than TOPO or OOCMPO. The dealkylated series is better than the alkylated one. For the dealkylated series and alkylated series, the sequence of increasing efficiency toward two cations is tetramer < octamer < hexamer. 

In the hexavalent state these elements are similar to uranium in the quadrivalent state however they have, like uranium and protactinium in this valency state, externally the inert gas configuration of the thorium ion in the trivalent state that of the trivalent actinium ion. The name 5f series may perhaps be the best in place of actinides, thorides or uranides. 

Americium. The low solubilities and high sorption affinity of thorium and americium severely limit their mobility under environmental conditions. However, because each exists in a single oxidation state—Th(IV) and Am(III)— under environmentally relevant conditions, they are relatively easy to study. In addition, their chemical behaviors provide valuable information about the thermodynamic properties of trivalent and tetravalent species of uranium, neptunium, and plutonium. 

With the exception of thorium and protactinium, all of the early actinides possess a stable +3 ion in aqueous solution, although higher oxidation states are more stable under aerobic conditions. Trivalent compounds of the early actinides are structurally similar to those of their trivalent lanthanide counterparts, but their reaction chemistry can differ significantly, due to the enhanced ability of the actinides to act as reductants. Examples of trivalent coordination compounds of thorium and protactinium are rare. The early actinides possess large ionic radii (effective ionic radii = 1.00-1.06 A in six-coordinate metal complexes),and can therefore support large coordination numbers in chemical compounds 12-coordinate metal centers are common, and coordination numbers as high as 14 have been observed. 

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. 

Dissolution of the calcium fluoride in aluminum nitrate-nitric acid oxidizes the plutonium to the tetravalent hexanitrate complex (3), while the transplutonium nuclides remain in the trivalent state. The only actinides retained by a nitrate-form anion-exchange column are thorium, neptunium, and plutonium. The uranium distribution coeflBcient under these conditions is about ten, but uranium should not be present at this point since hexavalent uranium does not carry on calcium fluoride (4). 

The chemistry of actinide ions is generally determined by their oxidation states. The trivalent, tetravalent and hexavalent oxidation states are strongly complexed by numerous naturally occurring ligands (carbonates, humates, hydroxide) and man-made complexants (like EDTA), moderately complexed by sulfate and fluoride, and weakly complexed by chloride (7). Under environmental conditions, most uncomplexed metal ions are sorbed on surfaces (2), but the formation of soluble complexes can impede this process. With the exception of thorium, which exists exclusively in the tetravalent oxidation state under relevant conditions, the dominant solution phase species for the early actinides are the pentavalent and hexavalent oxidation states. The transplutonium actinides exist only in the trivalent state under environmentally relevant conditions. 

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