Actinide metals early studies

The first actinide metals to be prepared were those of the three members of the actinide series present in nature in macro amounts, namely, thorium (Th), protactinium (Pa), and uranium (U). Until the discovery of neptunium (Np) and plutonium (Pu) and the subsequent manufacture of milligram amounts of these metals during the hectic World War II years (i.e., the early 1940s), no other actinide element was known. The demand for Pu metal for military purposes resulted in rapid development of preparative methods and considerable study of the chemical and physical properties of the other actinide metals in order to obtain basic knowledge of these unusual metallic elements. 

These early studies were carried out on metals of typically 90-99% purity, which sufficed to determine at least their gross properties. During the 1960s, interest diminished somewhat in actinide metallurgy due in part to the increasing use of ceramic rather than metallic fuel elements in nuclear reactors. The bulk of actinide metal research was for secret military purposes and only a fraction of the fundamental research was published. 

Early band structure calculations for the actinide metals were made both with and without relativistic effects. As explained above, at least the mass velocity and Darwin shifts should be included to produce a relativistic band structure. For this reason we shall discuss only the relativistic calculations. There were some difficulties with the f-band structure in these studies caused by the f-asymptote problem , which have since been elegantly solved by linear methods . Nevertheless the non-self-consistent RAPW calculations for Th through Bk indicated some interesting trends that have also been found in more recent self-consistent calculations ... 

The chemistry of the early actinide metals has been most extensively studied for many reasons. Chief among these is the availability of materials for study. Thorium and uranium obtained from ores as described above have been available for chemical investigations for well over 100 years. In fact, all early actinide elements may be found in nature, although only thorium, protactinium, and uranium are present in sufficient quantities to justify extraction. The remaining early actinide elements, neptunium and plutonium, are produced in large quantities in nuclear reactors. 

In summary, we have tried to describe the most recent state of understanding for the cohesive energies of the actinide metals. New results show there are still some surprises, even beyond the f-bonded early actinides. Vapor pressure measurements on Ra, Ac, and E are planned, along with completion of the Bk studies. The Pa and Pa-oxygen systems will obviously require extensive work. 

Regarding actinide compounds, the possible extent of f-electron involvement in chemictJ bonding is a very interesting aspect that is widely addressed [59]. In an early study, the metal-metal bond in [Cp 2Th(l)RuCp(CO)2] was probed using quasirelativistic Xa-SW calculations on the model complex [Cp2Th(I)RuCp(CO)2] [60]. The orbital contributions to metal-metal bonding were found to be 16%... 

Certainly the clearest conclusion from the examples of this chapter is the total absence of sharp features in the inelastic response function of anomalous lanthanide and metallic actinide materials. This contrasts strongly with the sharp dispersionless crystal-field excitations observed in most lanthanide compounds, in which the exchange interactions are weak (fig, 2), and with the sharp spin-wave excitations found in systems with strong exchange interactions. In many of the early studies with neutron inelastic scattering, for example of the heavy lanthanides or transition metals and their compounds, the width of the excitations was never an issue. It was almost always limited by the instrumental resolution, although it should be stressed that this resolution is relatively poor compared to that obtained by optical techniques. However, the situation is completely different in the materials discussed in this chapter. Now the dominant factor is often the width indeed in some materials the width of the over-damped response function is almost the only remaining parameter with which to characterize the response. 

The organoactinide surface complexes exhibited catalytic activities comparable to Pt supported on sihca [at 100% propylene conversion at —63°C, >0.47s (U) and >0.40 s (Th)], despite there being only a few active sites (circa 4% for Th, as determined by CO poisoning experiments and NMR spectroscopy) [92]. Cationic organoactinide surface complexes [Cp An(CH3 ) ] were proposed as catalytic sites. This hypothesis could be corroborated by the use of alkoxo/hydrido instead of alkyl/hydrido surface ligands, which led to a marked decrease of the catalytic activity, owing to the oxophilic nature of the early actinides [203, 204]. Thermal activation of the immobihzed complexes, support effects, different metal/ligand environments and different olefins were also studied. The initial rate of propylene conversion was increased two-fold when the activation temperature of the surface complexes under H2 was raised from 0 to 150°C (for Th 0.58 0.92 s" ). 

More recent studies have shown that a number of other mechanisms are operative in the hydrosilation process for different metals. Mechanistic proposals for early metals, lanthanides and actinides have been elaborated on. These involve a Chalk-Harrod like initial migratory insertion into a metal-hydride bond, followed by a a-bond metathesis step (Scheme 4). An alternative mechanism, however, was proposed for Group 4 metallocene catalysis, which involves a coordinated olefin, which undergoes a-bond metathesis with the hydrosilane. ... 

The various metallic glasses reported in the literature [4.9] fall into a few well-defined categories (i) late transition metal + metalloid (ii) early transition metal + late transition metal or group IB metal (iii) earth alkali metal + group IB metal (iv) early transition metal + alkali metal and, (v) Actinide + early transition metal. In catalysis research, exclusively metallic glasses of categories (i) and (ii) have been used so far. Table 4.1 lists glassy metals which have been used in catalytic studies. Note that metal-zirconium alloys and Ni, Fe, and mixed Ni-Fe alloys with P and/or B as metalloid have been used most frequently. 

Many of the enzymes involved in the cleavage of organic phosphorus compounds require metal ions for activation. However many metal ions can also facilitate the hydrolysis of organic phosphorus compounds in the absence of enzymes. As early as 1938 it was shown that lanthanide (lanthanum and cerium) and actinide (thorium) hydroxides could accelerate the hydrolysis of a-glycerol phosphate in alkaline solution (Bamann and Mersenheimer, 1938). A similar study by Butcher and Westheimer (1955) showed that lanthanum hydroxides could accelerate the hydrolysis of three simple phosphate esters (methoxyethyl phosphate, hyrdroxyethyl phosphate and aminoethyl phosphate) by a factor of about 1000. Using... 

One of the most exciting and active areas of actinide research involves the development of novel catalysts. Thoriiun and uranium metallocene complexes have been shown to react in highly specific manners that in some cases parallel those of early transition metals, and in others the reactions are unique to the actinides. M. Sharma and M.S. Eisen s chapter details metallocene organoac-tinide chemistry with a special focus on novel reaction pathways that have in some cases been deduced from thermochemical studies. 

To probe the bonding in the above types of compounds, Ciliberto, Condorelli, Fagan, Manriquez, Fragala, and Marks [45] carried out a comparative gas phase He I/He II photoelectron spectroscopic study of the series CP2MCI2 and Cp2M(CH3)2, M Zr,Th,U. It was found that the bonding in the early transition metal and actinide complexes is surprisingly similar. The major differences between zirconium and the actinides could be ascribed to the involvement of 5f orbitals in the latter. 

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