Chemistry of the Later Actinides

The succeeding actinides (Cm, Bk, Cf, Es, Em, Md, No, Lr) mark the point where the list of isolated compounds tends to involve binary compounds (oxides, halides and halide complexes, chalcogenides, and pnictides) rather than complexes. Those studies of complexes that have been made are usually carried out in solution and, from Em, onwards, have been tracer studies. 

Eermium coprecipitates with lanthanide fluorides and hydroxides, showing it to form lanthanide-like Em + ions. These elute from cation-exchange resins slightly before Es +, whilst its chloride and thiocyanate complexes are eluted from anionic exchange resins just 

Question 11.1 Use the reduction potentials to show why the U02 (aq) tends to disproportionate. 

Since E is positive, the reaction is energetically feasible although this only predicts that the UOj (aq) ion is thermodynamically unstable with respect to disproportionation and says nothing about kinetic stability, the fact is that the uranium(v) aqua ion is very short-lived. 

Question 11.2 Write the expression for K and K2 for complex formation between Th + and F ions. 

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Recently the fsr-fds separation has been established for Cm I (19) and furnishes a third point which should be added to Figure 11. The Cm separation is of interest because instead of lying 12,000 cm. below the corresponding separation in Gd, it is found to be about 8,000 above. This reversal presumably corresponds to the change in sign for the coefficients of the f-d parameters, which are smaller in the actinides. It also corresponds with the chemistry of the later actinides, which more resemble the lanthanides than the earlier actinides do. 

A second reason for the wealth of chemical investigations of the early actinide elements is the relative diversity of their chemistry. While the chemistry of the later actinides is most often restricted to that of the tri- and tetravalent oxidation states, compounds of the early actinides can be isolated in all oxidation states from +3 to +7. The accessibility of a range of oxidation states is the impetus for signficant chemical interest in the early actinides, but also vastly complicates investigation of these elements under some circumstances, such as aqueous redox behavior. In the case of plutonium, ions in four different oxidation states (+3, +4, +5, and - -6) can exist simultaneously in comparable concentrations in the same solution. 

Studies of the coordination chemistry of the actinides have been limited by a number of factors - the care needed in handling radioactive materials and the possibility of damage to human tissue from the radiation toxicity (especially Pu) the very small quantities available and very short half-lives of the later actinides radiation and heating damage to solutions and radiation damage (defects and dislocations) to crystals. 

The first computer-controlled automated system for performing very rapid solution chemistry experiments on an atom-at-a-time basis was used in later pioneering experiments (Hulet et al., 1980). Results showed that the anionic-chloride complexes of Rf were clearly similar to Hf and much stronger than those of the trivalent actinides, again confirming the position of Rf in group 4 of the periodic table. 

It has been more than fifty years since the discovery of the transuranium elements. The initial activities in this field established the fundamental solution and solid-state chemistry of the first two of these elements and their compounds under the auspices of the Manhattan Project. New separation methods including solvent extraction techniques and uranium isotope separation played a leading role in these programs. Tracer techniques were widely used to determine solubilities (or solubility liinits) of transuranium compounds as well as to obtain information about the coorination chemistry in aqueous solution. A little later, special solvent extraction and ion-exchange techniques were developed to isolate pure transplutonium elements on the milligram and smaller scale. The second edition of The Chemistry of the Actinide Elements, published in 1986 (i), covers most of these topics. A detailed overview of the history of transuranium chemistry is given in Transuranium Elements A Half Century (2). 

When applying the principles of basis set design, the characteristics of the elements for which the basis sets are developed must be taken into account. The f series start out like the d-block transition metals, with the 6d orbital occupied for several of the early actinides, and the 5d occupied for La and Ce. These early elements even have some low-lying states in which the d orbital is multiply occupied. Further along the series, the f orbital is the dominant occupied open-shell orbital and the d is unoccupied, except in the middle of the block. The outer s orbital is doubly occupied in all of these elements. The chemistry of the lanthanides is largely (but by no means solely) that of the +3 oxidation state, whereas higher oxidation states are of importance in the early part of the actinide series, and the -h3 oxidation state becomes dominant later in the series. 

The majority of the chemistry that has been investigated for the actinide elements has been in aqueous solutions. For the light actinides in acidic solutions, four types of cations persist trivalent, tetravalent, pentavalent, and hexavalent. The later two ions are always found to have trans oxo ligands, making up a linear dioxo unit. Actinide ions of this type are typically referred to yls and have the names, uranyl (1, U02+/ +), neptunyl (2, Np02+ +), plutonyl (3, Pu02+ +), (4, Am02+/ ) and so on. 

Similarities exist between the chemical characteristics of the actinides and those of the lanthanides. The metal ions are generally considered to be relatively hard Lewis acids, susceptible to complexation by hard (i.e., first row donor atom) ligands and to hydrolysis. Both actinide and lanthanide ions are affected by the lanthanide contraction, resulting in a contraction of ionic radius and an increasing reluctance to exhibit higher oxidation states later in the series. Most species are paramagnetic, although the electron spin-nuclear spin relaxation times often permit observation of NMR spectra, and disfavor observation of ESR spectra except at low temperatures. The elements display more than one accessible oxidation state, and one-electron redox chemistry is common. 

Chemistry used in the recovery of plutonium from irradiated fuel must provide a separation from all these elements, other fission and activation products, and the actinides (including a large amount of unburned uranium), and still provide a complete recovery of plutonium. The same issues apply to the recovery of uranium from spent thorium fuel. Most of the processes must be performed remotely due to the intense radiation field associated with the spent fuel. As in the enrichment of uranium, the batch size in the later steps of the reprocessing procedure, where the fissile product has become more concentrated, is limited by the constraints of criticality safety. There is a balance between maximizing the yield of the precious fissile product and minimizing the concentrations of contaminant species left in the final product These residual contaminants, which can be detected at very small concentrations using standard radiochemical techniques, provide a fingerprint of the industrial process used to recover the material. 

Any discussion of uranium and its chemical properties should refer to the classic 1951 book by Katz and Rabinowitch that was the first publication that provided a comprehensive description of the chemical and physical properties of uranium the element and its binary and related compounds (Katz et al. 1951). More than 60 years later, and despite the considerable developments and extensive research on the chemistry of uranium, this tome is still an excellent primary source. The section on uranium in the series on actinide and trans-actinide chemistry (ATAC) was mentioned earlier as a superb source for understanding the behavior of uranium compounds (Grenthe 2006). A less-known volume that focused on the industrial and technological applications of uranium was translated from Russian in the 1960s and also is useful (although somewhat outdated in parts) for following the production processes of uranium (Galkin 1966). 

The next important fact of lanthanide and actinide chemistry is going to be one of the central themes of this chapter, namely, relativistic effects. Relativistic effects arise from the differences in the assumed infinite speed of light in non-relativistic mechanics (whether classical or quantum mechanical) and the true speed of light. Although the special theory of relativity was proposed by Einstein in the very early part of the century, its impact in chemistry came about much later. In fact, it is not an exaggeration... 

In contrast to the now relatively extensive metal-metal bonded chemistry of sub valent Mg, Al, and Zn, homomet2Jlic bonds involving the 6s and 7s orbitals of the lanthanide and actinides elements are rare (diatomic overlap between the 5f orbitals in Uj and other cases is discussed later). Their absence is largely a consequence of the rdatively low second and third ionization energies (compared to Al), which reduce the stability of the -i-l and +2 oxidation states. Examples... 

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