Actinide hydrides

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. 

Some properties of berkelium metal have been reported. Thus, its melting point is 986 + 25 °C and its volatility, relative to its congeners, is in the order Cm < Bk < Am < Cf. Its chemical behaviour is described as somewhat similar to Sm, and it does not correspond, as a metal, to Tb or Lu. It reacts with hydrogen at 225 °C to give BkH2, which is isomorphous with other lanthanide and actinide hydrides of the type MH2+ (x < 1). BkO may be formed as an impurity in the production of metallic Bk. 

The compositions of the most numerous actinide compounds with elements of the group V of the periodic table (X = N, P, As, Sb, Bi) belong to the types AnX2, An3Xt, AnX. These pnictides can be synthesized in solid-gas-reactions with actinide hydride, or with metal powder obtained by thermal dissociation of hydrides. 

The properties of the nonstoichiometric hydrides are discussed in groups the saline hydrides, the Group IVA hydrides, the rare earth hydrides, the actinide hydrides, the Group VA hydrides, and palladium hydride. 

Only three of the actinide hydrides have been studied to the point of providing sufficient information as to their deviations from stoichiometry thorium, uranium, and plutonium. 

The studies on the neptunium nitride system are somewhat limited with the mononitride, NpN, being the predominant phase. PuN is also the only known componnd inthe plutoninm series. The AnN (An = Np, Pu) has been synthesized by reaction of an actinide hydride with NH3 at high temperatures. 

Actinide hydrides are rather hydridic and nndergo rapid reactions typical of snch systems for example, reactions with ketones and halocarbons provide actinide alkoxides and halides, respectively. Rapid qnantitative addition of terminal alkenes (the reverse of /S-hydride elimination) is consistent with the known bond disruption enthalpies (eqnation 66). ... 

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. 

The metal lattices are expanded but otherwise little distorted the densities are below those of the parent metal. Many are interstitial compounds but with lanthanide and actinide hydrides the high heats of formation suggest that the bonding is to a certain degree ionic. 

Actinide Hydrides. Thorium and other actinides form complex systems with non-stoichiomelric and stoichiometric phases. Uranium hydride is of some importance chemically as it is often more suitable for the preparation of uranium compounds than is the massive metal. Uranium reacts rapidly and exothermically with hydrogen at 250-300° to give a pyrophoric black powder. The reaction is reversible ... 

Amido-substituted actinide hydrides [71,90] can be synthesized as shown in eqs.(76) and (77). These compounds are both highly reactive... 

The second review is due to Pepper and Bursten (1991). This review focussed on the electronic structure of actinide-containing molecules. Note that the present chapter complements this in that our chapter is mostly on lanthanide-containing species. Consequently, the reader is referred to the excellent review by Pepper and Bursten (1991) for a comprehensive summary of the electronic structure of actinide-containing species. The review by Pepper and Bursten (1991) contains the details of calculations on actinide hydrides, actinide halides, actinide oxides, cyclopentadienyl-actinide complexes, aetinocenes, metal-metal bonding in actinide systems and miscellaneous other actinide systems. This review also consists of descriptions of theoretical techniques employed to study the actinide-containing molecules. The reader is directed to this review for further details on such calculations on actinide-containing molecules. 

A combined review of the rare-earth and actinide hydrides is appropriate. A reasonable literature on actinide-hydrogen systems now exists with an attendant realization that the 5f metals form fully rare-earth-like hydrides as well as totally unique hydride phases. This chapter concentrates on correlating the 4f and 5f metal-hydrogen systems and relies heavily on the most recent interpretations of the solid-state physics and chemistry. 

The early-actinide hydrides are diverse and unlike those observed for their rare-earth analogs. Ward (1985a) has provided an extensive review of the properties of the actinide... 

In remarkable contrast to the behavior of the early-actinide hydrides, the physicochemical properties of the transuranium phases are suddenly more rare-earth-like. As discussed earlier, the progressive narrowing of the 5f bands favors the stabilization of the fee structure with the larger M-M distance attendant localization can then occur and the f bands become core-like and magnetic. However, in contrast to the rare earths, the 5f electrons remain near the Fermi level and one can expect that effects such as scattering will then influence resistivity, neutron scattering and other properties. 

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... 

Bearing this correlation in mind, one can immediately say that the hydrides of Np, Pu and Am are rare-earth-like with wide cubic solid solution ranges up to H/M = 2.7-2.S and somewhat smaller lattice parameters. The hexagonal phase appears with regularity, unlike the behavior encountered for Ce, Pr and Nd. P-C-T data are not available for the Cm system and beyond, but relatively normal behavior is expected. As pointed out above, the large and simple lattices of the early rare earth and late-actinide hydride systems preclude f-f overlap. Since the actinide contraction is delayed as compared to the rare-earth contraction, phase relationships such as those seen for Gd hydride and beyond are not expected for the actinides. 

It should be noted that the late-actinide hydrides react more easily and vigorously than any of the rare-earth counterparts. This effect, which is kinetic and not thermodynamic, is discussed in section 5. As an introduction to the new data and information to be presented, comparative trends for the rare-earth and late-actinide hydrides are summarized in table 1. It should be understood that MHj simply implies a value at or near the phase boundary and that the temperature of the measurement is not unambiguously defined. For most systems, the phase-boundary composition is near... 

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