Complexes of the actinide elements

Table 2. Preparation of tt-Cyclopentadienyl Complexes of the Actinide Elements...

Treatment of UCI4 with the lithium complex obtained from dicyclohexylcar-bodiimide followed by crystallization from pyridine afforded a dinuclear uranium(rV) oxalamidinate complex in the form of dark green crystals in 94% yield (Scheme 191). The same compound could also be obtained by first reducing UCI4 to LiUCli (or UQs+LiCl) followed by reductive dimerization of di(cyclo-hexyl)carbodiimide as shown in Scheme 191. The molecular structure of this first oxalamidinato complex of an actinide element is depicted in Figure 31. ° ... 

The redox reactions of the actinide elements have been the subject of a recent and authoritative review by Newton and Baker . The net activation process concept is used to interpret the experimental data. Empirical correlations shown to exist include those between the entropies of the activated complexes and their charges, and, for a set of similar reactions, between AG and AG , and and A/f . The present state of the evidence for binuclear species is discussed. 

Studies of sulfoxide complexes of other actinide elements have appeared, (38, 40), but insufficient data are available to make any meaningful comparisons along the series. Work on the solvent extraction of actinide elements by sulfoxides has been reported (423). 

The first and thus far only silsesquioxane complex of an actinide element is [Cy7Si70i2]2U (100). This colorless, nicely crystalline uranium(VI) compound is formed upon reaction of 3 with any uranium precursor, e.g., UCI4 in the presence of NEt3. In all cases oxidation of uranium to the hexavalent oxidation state is observed. The best synthetic route leading to 100 in ca. 80% yield is the reaction of 3 with uranocene as outlined in Scheme 33. 

The measurement of stability constants of complexes of yttrium, lanthanide, and actinide ions with oxalate, citrate, edta, and 1,2-diaminocyclohexanetetra-acetate ligands has revealed that there is a slight increase in the stability of complexes of the /-electron elements, relative to the others. A series of citric acid (H cit) complexes of the lanthanides have been investigated by ion-exchange methods and the species [Ln(H2cit)]", [Ln(H2cit)2] , [Ln-(Hcit)], and [Ln(Hcit))2] were detected. Simple and mixed complexes of dl- and jeso-tartaric acid have been obtained with La " and Nd ions, and the stability constants of lactate, pyruvate, and x-alaninate complexes of Eu and Am " in water have been determined. 

The redox chemistry of the actinide elements, especially plutonium, is complex (Katz et al., 1980). Disproportionation reactions are especially important for the +4 and +5 oxidation states. Some of the equilibria are kinetically slow and irreversible. All transuranium elements undergo extensive hydrolysis with the +4 cations reacting most readily due to their large charge/radius ratio. Pu (IV) hydrolyzes extensively in acid solution and forms polymers. The polymers are of colloidal dimensions and are a serious problem in nuclear fuel reprocessing. 

Despite the extremely low concentrations of the transuranium elements in water, most of the environmental chemistry of these elements has been focused on their behavior in the aquatic environment. One notes that the neutrality of natural water (pH = 5-9) results in extensive hydrolysis of the highly charged ions except for Pu(V) and a very low solubility. In addition, natural waters contain organics as well as micro- and macroscopic concentrations of various inorganic species such as metals and anions that can compete with, complex, or react with the transuranium species. The final concentrations of the actinide elements in the environment are thus the result of a complex set of competing chemical reactions such as hydrolysis, complexation, redox reactions, and colloid formation. As a consequence, the aqueous environmental chemistry of the transuranium elements is significantly different from their ordinary solution chemistry in the laboratory. 

The most widely used neutral extractants, however, are the organophosphoms compounds, of which the ester, TBP, is the most important. TBP forms complexes with the actinide elements thorium, uranium, neptunium, and plutonium by bonding to the central metal atom via the phosphoryl oxygen in the structure... 

As a result of this disparity, many questions on the structure and bonding of the actinides center on the role of the 5f-electrons. The molecular orbital descriptions for the bonding of the actinide elements continue to evolve. One of the first general models used to describe the chemical bonding in d- and f-electron complexes is the FEUDAL model. FEUDAL is an acronym for f orbitals essentially unaffected d orbitals accommodate ligands . This model is represented in Figure 3, which depicts the molecular orbital... 

The logarithms of the stability constants fly for the formation of 1 1 complexes of the actinide ions M +, M " ", MOj and MO with various inorganic ligands are plotted in Fig. 21.1. Carbonato complexes of alkaline-earth elements, lanthanides, actinides and other transition elements play an important role in natural waters and may stabilize oxidation states. 

The fluorite-related oxide phases which are known in the lanthanide and actinide series are displayed in Table II for closer comparison. The most obvious feature is that the oxide systems of Ce, Pr, and Tb reveal greater complexity than any of the actinide elements so far studied. The dioxides of the actinide elements are more easily reduced as one goes from Th02 to Cm02 showing an approach to the behavior of the lanthanides. More complete measurements on CmOx and BkO may well show marked similarity to the rare earths. 

More rapid extracting reactions result from the formation of relatively loose nonchelating complexes with organic molecules. A widely used organic complexing agent for the extraction of the actinide elements thorium, uranium, neptunium, and plutonium is TBP, which probably forms bonds by the electron from the phosphoryl oxygen atom in the structure [S4]... 

The compounds formed are normally quite ionic. The ionic radii of the actinide elements of the different valency states decrease with increasing atomic number (the actinide contraction. Table 16.1). Consequently the charge density of the actinide ions increases with increasing atomic number and, therefore, the probability of formation of conq>lexes and of hydrolysis increases with atomic number. This is illustrated in Figure 16.7, where the heavier actinides are eluted before the lighter ones because the a-hydroxy-isobutyrate eluant forms stronger complexes as the cation radius decreases. 

The actinide elements begin with actinium. Actinium serves as a starting point for the other actinide series elements, which span 14 spaces across the periodic table from actinium to lawrencium. Actinides are more reactive than lanthanides. They form covalent compounds and organometallic complexes. Most of the actinide elements were discovered during development of the nuclear bomb. The set of actinides is represented by the symbol An. 

The bisd,3,5,7-tetramethyl[8]annulene)actinide(IV) complexes made from 1,3,5,7-tetramethy1 eye 1ooctatetraene, 2,i i have been synthes i zed for many of the actinide elements. 

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