Actinide metal hydrides

Carbides of the Actinides, Uranium, and Thorium. The carbides of uranium and thorium are used as nuclear fuels and breeder materials for gas-cooled, graphite-moderated reactors (see Nuclearreactors). The actinide carbides are prepared by the reaction of metal or metal hydride powders with carbon or preferably by the reduction of the oxides uranium dioxide [1344-57-6] UO2 tduranium octaoxide [1344-59-8], U Og, or thorium... 

Metal hydrides and acyl-like CO insertion products are two types of species likely to be present in any homogeneous or heterogeneous process for the catalytic reduction of carbon monoxide. The discovery and understanding of new types of reactivity patterns between such species are of fundamental interest. As discussed elsewhere (11,22,54-57), bis(pentamethylcyclo-pentadienyl) actinide hydrides (58) are highly active catalysts for olefin hydrogenation as well as H-H and C-H activation. 

The actinides. The actinides metals are electropositive and very reactive they are pyrophoric in finely divided form. They tarnish rapidly in air forming an oxide protective coating in the case of Th, but more slowly for the other actinides. The metals react with most non-metals. With steam or boiling water, oxide is formed on the surface of the metal and H2 evolves in this way hydrides are produced that react rapidly with water and facilitate further attack on the metals. The oxidation states observed in the chemistry of lanthanides and actinides are shown in Fig. 5.9. Notice the predominant oxidation state III for the lanthanides... 

The chalcogenides are synthesized either by reaction of the chalcogen vapour with the actinide metal or the hydride (Faraday-method), or in reaction with compounds ... 

Due to the low penetration depth of X-rays in heavy element samples, XRD patterns are not always significant for the structure of the bulk. In analogy to observations made on rare earth metal foils, fee phases observed occasionally after heating and interpreted as high temperature phases of the actinide metals, might be the product of a reaction between the metal surface and residual gas leading, e.g. to hydride etc. 

Metallic hydrides are formed by reaction of the lanthanide and actinide metals and certain of the d-block transition metals with variable amounts of hydrogen. These hydrides have the general formula MHX, where the x subscript represents... 

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. ... 

Actinide metals react with hydrogen at about 300 "C to form non-stoichiometric metallic hydrides of composition MH2 to MH.n The foimation of uranium hydride is reversible at higher temperatures and can be used to store tritium. 

The hydrides formed in reaction (a) may be classified as (1) saline or ionic hydrides, (2) metallic hydrides and (3) covalent hydrides. The saline hydrides include the hydrides of the alkali and alkaline-earth metals, except BeHj, which is covalent. Transition metals form binary compounds with hydrogen that are classified as metallic hydrides including rare-earth and actinide hydrides. Intermetallic compound hydrides, such as TiFeHj and LaNijH, may be thought of as pseudobinary metallic hydrides. 

Various cyclopentadienyl-ligated early metal and actinide alkoxides were prepared by reactions of the alcohols with the metal hydride precursors to eliminate hydrogen gas,266,275,277,278 g j.jy metal alkoxides, as well as their high-valent late-metal analogs, can also be formed by attack of an alkyl group at an oxo ligand, where the alkyl could be added as a carbocation, a radical, or a carbanion (Equation 4.59). ... 

Thorocene, uranocene and plutonocene can also be synthesized by the reaction of cyclooctatetraene with the finely divided metals obtained by heating the metal hydrides.i7 in this reaction, a sealed Pyrex tube containing the actinide-cyclooctatetraene mixture was heated in a tube furnace at 150-160 C. The reaction product was isolated by sublimation. This preparative method provides demonstration of the thermodynamic stability of the bis([8]annulene)actinide complexes. 

In comparing and contrasting the rare-earth and late-actinide hydride systems, it should be noted that a shifted homologous relationship exists for the metals, as shown in fig. 1. As noted by Johansson (1975) and discussed by Ward (1985b), the heavy actinide metals relate to the beginning of the rare-earth series, rather than to the direct... 

The phase and structural relationships of the lanthanide- and actinide-hydride systems are outlined here with relevant corollary evidence to provide a suitable background and preparation for some of the more esoteric discussions to follow. Historically, the first realizations of latent structural complexities appeared with the advent of more precise neutron-diffraction measurements in conjunction with new low-temperature capabilities. X-ray studies have been of limited utility because only metal atom positions are defined and only the near-surface regions of opaque heavy-metal hydrides are sampled. However, the influence of the H sublattice ordering upon the metal lattice can be deduced. 

Each of the four kinetic regions observed in fig. 14 corresponds to control of the bulk hydriding rate by a separate process. The decrease in r as P approaches P is clearly thermodynamic in nature because r approaches zero as the thermochemical activity gradient driving the reaction approaches zero. As described by Haschke (1991), the approach to equilibrium gives rise to the rate decreases observed at high temperature in Arrhenius data for the U-l-H system under isobaric conditions. Unlike the U-UH3 system, equilibrium pressures for the metal-hydride systems of the lanthanides and transuranium actinides are not in the 0.1 to 1 bar range frequently employed in hydride... 

The application of powder metallurgical techniques using mixtures of the elements or the metal hydride and graphite have resulted in relatively pure carbides of various compositions. This is a particularly useful method when defective compounds in Groups 5 and 6 are needed. The powdered metals or hydrides in Group 4 and the actinide series are sufficiently reactive to air that the introduction of oxygen is hard to avoid. 

Representative actinide hydride compounds are represented in Table X. Actinide metals react readily with hydrogen when heated. The temperature needed for reaction depends on the state of the metal, the amount of surface oxidation on the metal, and the purity and pressure of the hydrogen used. The actinide hydrides are not very thermally stable and are very air and moisture sensitive. The thermal instability of these compounds has been used to obtain finely divided metal via thermal decomposition of the corresponding hydride. 

All the solid actinide monoxides which have been reported are now believed to have been oxynitrides, oxycarbides, or hydrides. The highest potential for existence would have the monoxides for the divalent actinide metals einsteinium through nobelium. Only the gaseous monoxides are well-established species. All actinides are known or expected to form gaseous monoxides. 

The actinide trichlorides are hygroscopic and water soluble and melt between 1030 and 1110 K. They can be obtained by reaction of the metal hydride with HCl at elevated temperatures or by the reaction of CCU with An(OH)3. With the larger actinide(ni) ions, the crystal structures of the trichlorides show nine chlorine atoms arranged in a tricapped trigonal prismatic geometry. As the atomic number increases, the three actinide to facecapping-chlorine distances increase relative to the other six chlorines. At caUfomium, a second form of CfCb has eight coordination. 

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