Actinide elements hydrides

The conversion of CO + H2 (syn-gas) to hydrocarbons and oxygenates (Fischer-Tropsch chemistry)119 is of considerable industrial importance and recently the activation and fixation of carbon monoxide in homogeneous systems has been an active area for research.120,121 The early transition elements and the early actinide elements, in particular zirconium124 and thorium,125 126 supported by two pentamethylcyclopentadienyl ligands have provided a rich chemistry in the non-catalytic activation of CO. Reactions of alkyl and hydride ligands attached to the Cp2M centers with CO lead to formation of reactive tf2-acyl or -formyl compounds.125,126 These may be viewed in terms of the resonance forms (1) and (2) shown below. 

Bulk hydriding rates for rare earth and actinide elements at 0.10 bar hydrogen pressure and 25°C. 

The physicochemical properties of the actinide hydrides are as varied as any in the entire periodic table. Thorium forms a normal dihydride like those of Zr and Hf, but also forms Th4His, a unique superconductor. The hydrides of protactinium and uranium have cubic structures which have no counterparts in the periodic table. The transuranium element hydrides are more lanthanide like with wide cubic solid solution ranges. Hexagonal phases appear with regularity. 

Flotow, H. E., Haschke, J. M., and Yamauchi, S. (1984) The Chemical Thermodynamics of Actinide Elements and Compounds, part 9, The Actinide Hydrides, STI/PUB/424/9, International Atomic Energy Agency, Vienna. 

Hydrogen reacts at elevated temperatures with many transition metals and their alloys to form hydrides. The electropositive elements are the most reactive, that is, scandium, yttrium, the lanthanides, the actinides and members of the titanium and vanadium groups (Figure 5.20). 

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. 

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. 

The orgarometallic chemistry of actinides. ignored in the early development of the field, is currently receiving a great deal of attention.1 7 In many instances the chemistry of this group of elements is unlike that of the transition metals. For example, it has been shown that a thorium hydride, in contrast to the manganese hydride shown above, does undergo CO insertion.138... 

DV-Xa methods. It was found that relativistic effects become significant in chemical bonding of molecules containing heavy elements with Z larger than 50 and APb of the XFg (X=S to W) shows a Z -dependence similar to that of ARg for the hydrides [1]. On the other hand, the atomic-number dependence of APb for actinide compounds such as UFg was not well explained using the Z -dependence. However, the number of examples examined in the previous work are not satisfactory for discussing the reason for the dependence of APb-... 

The interstitial structures comprise the compounds of certain metallic elements, notably the transition metals and those of the lanthanide and actinide series, with the four non-metallic elements hydrogen, boron, carbon and nitrogen. In chapter 8 we discussed the structures of a number of hydrides, borides, carbides and nitrides of the most electropositive metals, and these we found to be typical salt-like compounds with a definite composition and with physical properties entirely different from those of the constituent elements they are generally transparent to light and poor conductors of electricity. The systems now to be considered are strikingly different. They resemble... 

The terminal hydride (1) is the most common type. In these species, the M-H distance is close to the sum of the Covalent Radii. Neutron diffraction gives this distance reliably, but X-ray methods, because they detect electron density and not the nuclear positions, underestimate the M-H distance by about 0.1 A. It is not uncommon for the H to escape detection entirely by X-ray methods as a result of the proximity of the metal, always a strong diffractor of X rays. Typical M-H distances are 1.45 to 1.6 A for first-row elements, 1.6 to 1.7 A for the second row, and 1.65 to 1.75 A for the third row actinides have M-H distances near 2 A. Early transition metals have M-H distances about 0.1A longer than do late metals. Electron diffraction methods have been used for volatile carbonyl hydrides such as HMn(CO)s, HCo(CO)4, and H2Fe(CO)4 for the Mn compound, the e-diffraction and n-diffraction Mn-H bond lengths are in good agreement. 

Superhydride, MHBEtj, has been widely used in the chemistry of 5/ elements, (actinides), [16], but for the harder Af elements, only one example of hydride... 

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