Actinides stability

Reed D. T., Wygmans D. G., and Richman M. K. (1996) Actinide Stability/Solubility in Simulated WIPP Brines Interim Report under SNL WIPP Contract AP-2267. Sandia National Laboratories. 

Table 6. Stability of Actinide Ions in Aqueous Solution...

The redox behaviour of Th, Pa and U is of the kind expected for d-transition elements which is why, prior to the 1940s, these elements were commonly placed respectively in groups 4, 5 and 6 of the periodic table. Behaviour obviously like that of the lanthanides is not evident until the second half of the series. However, even the early actinides resemble the lanthanides in showing close similarities with each other and gradual variations in properties, providing comparisons are restricted to those properties which do not entail a change in oxidation state. The smooth variation with atomic number found for stability constants, for instance, is like that of the lanthanides rather than the d-transition elements, as is the smooth variation in ionic radii noted in Fig. 31.4. This last factor is responsible for the close similarity in the structures of many actinide and lanthanide compounds especially noticeable in the 4-3 oxidation state for which... 

From Pu onwards, sesquioxides become increasingly stable with structures analogous to those of Lu203 (p. 1238) Bk02 is out-of-sequence but this is presumably due to the stability of the f configuration in Bk. For each actinide the C-type M2O3 structure (metal CN = 6) is the most common but A and B types (metal CN = 7) are often also obtainable. 

In view of the magnitude of crystal-field effects it is not surprising that the spectra of actinide ions are sensitive to the latter s environment and, in contrast to the lanthanides, may change drastically from one compound to another. Unfortunately, because of the complexity of the spectra and the low symmetry of many of the complexes, spectra are not easily used as a means of deducing stereochemistry except when used as fingerprints for comparison with spectra of previously characterized compounds. However, the dependence on ligand concentration of the positions and intensities, especially of the charge-transfer bands, can profitably be used to estimate stability constants. 

This is the only oxidation state which, with the possible exception of Pa, is displayed by all actinides. From U onwards, its resistance to oxidation in aqueous solution increases progressively with increase in atomic number and it becomes the most stable oxidation state for Am and subsequent actinides (except No for which the f " configuration confers greater stability on the +2 state). 

This state is found for the six elements Am and Cf No, though in aqueous solution only for Fm, Md and No. However, for No, alone amongst all the f-series elements, it is the normal oxidation state in aqueous solution. The greater stabilization of the +2 state at the end of the actinides as compared to that at the end of the lanthanides which this implies, has been taken " to indicate a greater separation between the 5f and 6d than between the 4f and 5d orbitals at the ends of the two series. This is the reverse of the situation found at the beginnings of the series (p. 1266). 

The other actinides have been synthesized in the laboratory by nuclear reactions. Their stability decreases rapidly with increasing atomic number. The longest lived isotope of nobelium (102N0) has a half-life of about 3 minutes that is, in 3 minutes half of the sample decomposes. Nobelium and the preceding element, mendelevium (ioiMd), were identified in samples containing one to three atoms of No or Md. 

The outstanding characteristic of the actinide elements is that their nuclei decay at a measurable rate into simpler fragments. Let us examine the general problem of nuclear stability. In Chapter 6 we mentioned that nuclei are made up of protons and neutrons, and that each type of nucleus can be described by two numbers its atomic number (the number of protons), and its mass number (the sum of the number of neutrons and protons). A certain type of nucleus is represented by the chemical symbol of the element, with the atomic number written at its lower left and the mass number written at its upper left. Thus the symbol... 

There are three common ways by which nuclei can approach the region of stability (1) loss of alpha particles (a-decay) (2) loss of beta particles (/3-decay) (3) capture of an orbital electron. We have already encountered the first type of radioactivity, a-decay, in equation (/0). Emission of a helium nucleus, or alpha particle, is a common form of radioactivity among nuclei with charge greater than 82, since it provides a mechanism by which these nuclei can be converted to new nuclei of lower charge and mass which lie in the belt of stability. The actinides, in particular, are very likely to decay in this way. 

Despite the problems of direct experimental evaluation of plutonium stability constants, they are needed in modeling of the behavior of plutonium in reprocessing systems in waste repositories and in geological and environmental media. Actinide analogs such as Am+3, Th+, NpOj and UOj2 can be used with caution for plutonium in the corresponding oxidation states and values for stability constants of these analogues are to be found also in reference 20. 

DeCarvalho and Choppin (10, 11) previously have reported the stability constants, complexation enthalpies, and entropies for a series of trivalent lanthanide and actinide sulfates. As their work was conducted a pH 3, the dominant sulfate species was S0 and the measured reaction was as in equation 12. 

In contrast to the situation observed in the trivalent lanthanide and actinide sulfates, the enthalpies and entropies of complexation for the 1 1 complexes are not constant across this series of tetravalent actinide sulfates. In order to compare these results, the thermodynamic parameters for the reaction between the tetravalent actinide ions and HSOIJ were corrected for the ionization of HSOi as was done above in the discussion of the trivalent complexes. The corrected results are tabulated in Table V. The enthalpies are found to vary from +9.8 to+41.7 kj/m and the entropies from +101 to +213 J/m°K. Both the enthalpy and entropy increase from ll1 "1" to Pu1 with the ThSOfj parameters being similar to those of NpS0 +. Complex stability is derived from a very favorable entropy contribution implying (not surprisingly) that these complexes are inner sphere in nature. 

Most of the known borides are compounds of the rare-earth metals. In these metals magnetic criteria are used to decide how many electrons from each rare-earth atom contribute to the bonding (usually three), and this metallic valence is also reflected in the value of the metallic radius, r, (metallic radii for 12 coordination). Similar behavior appears in the borides of the rare-earth metals and r, becomes a useful indicator for the properties and the relative stabilities of these compounds (Fig. 1). The use of r, as a correlation parameter in discussing the higher borides of other metals is consistent with the observed distribution of these compounds among the five structural types pointed out above the borides of the actinides metals, U, Pu and Am lead to complications that require special comment. 

It is likely that identification of lanthanide(III)-protein complexes would be even more difficult than for the actinide-protein complexes because of their lower stability and greater tendency to dissociate during separation processes. 

In hydrogenation, early transition-metal catalysts are mainly based on metallocene complexes, and particularly the Group IV metallocenes. Nonetheless, Group III, lanthanide and even actinide complexes as well as later metals (Groups V-VII) have also been used. The active species can be stabilized by other bulky ligands such as those derived from 2,6-disubstituted phenols (aryl-oxy) or silica (siloxy) (vide infra). Moreover, the catalytic activity of these systems is not limited to the hydrogenation of alkenes, but can be used for the hydrogenation of aromatics, alkynes and imines. These systems have also been developed very successfully into their enantioselective versions. 

With the Pa2 dimer, we have reached the maximum bonding power among the actinide dimers. In U2 the bond energy decreases and the bond length increases, which is from the increased stabilization of the 5f orbitals and the corresponding destabilization of 6d. Large transfer of electrons from... 

The Van Arkel process can also be used to prepare actinide metals if the starting compound reacts easily with the transporting agent (I2). The thorium and protactinium carbides react with I2 to give volatile iodides above 350°C these are unstable above 1200°C and decompose into the actinide metals and iodine. Attempts to prepare other actinides, such as U and Pu, through the process were not successful, because from Th to Pu along the actinide series, the vapour pressure of the iodide decreases and the thermal stability increases. 

An interesting, peculiar laboratory preparative reaction may finally be mentioned. This is based on the very high stability of the intermetallic compounds of actinides (and lanthanides) with the platinum family metals. The combined reduction capability of Pt with H2 (coupled reduction, see 6.7.2 fi) can be used to obtain, from its oxide, the platinide of the actinide metal. The An-Pt intermetallic compound can then be decomposed by heating in vacuum and the actinide can be obtained by distillation. 

Figure 5.9. Lanthanide and actinide chemical properties. A scheme is shown of the oxidation states they present in their various classes of compounds. A rough indication of a greater frequency and a higher relative stability of each state is given by the darker blackening of each box. Notice the overwhelming presence of oxidation state 3, in the lanthanides and heavy actinides, oxidation state 2 in Eu andYb and of several higher oxidation states in U and nearby elements.

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