Binary Compounds of the Actinides

This chapter examines some of the most important binary compounds of the actinides, especially the halides. Despite the problems caused by their radioactivity, some binary compounds of most of these elements have been studied in considerable detail, and form a good vehicle for understanding trends in the actinide series. 

The compounds known are summarized in Table 10.1. The only compound of an early actinide in the -1-2 state is Thl2, a metallic conductor which is probably Th + (e )2 (D)2-Certain heavier actinides form MX2 (Am, Cf, Es), which usually have the structure of the corresponding EuX2 and are thus genuine M + compounds. All four trihalides exist for all the actinides as far as Es, except for thorium and protactinium. Tetrafluorides exist for Th-Cm and the other tetrahalides as far as NpX4 (and in the gas phase in the case of PuCE). Pentahalides are only known for Pa, U, and Np whilst there are a few MFe (M = U-Pu), uranium is the only actinide to form a hexachloride. The known actinide halides are generally stable compounds most are soluble in (and hydrolysed by) water. 

Lanthanide and Actinide Chemistry S. Cotton 2006 John Wiley Sons, Ltd. 

A considerable number of synthetic routes are available, many involving reaction of the oxides with HX or X2(according to the oxidation state desired) though sometimes more unusual halogenating agents like AIX3 or hexachloropropene have been used. A selection of methods follow. 

In some cases, where the +3 state is easily oxidized, a reductive route is used  

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Thousands of compounds of the actinide elements have been prepared, and the properties of some of the important binary compounds are summarized in Table 8 (13,17,18,22). The binary compounds with carbon, boron, nitrogen, siUcon, and sulfur are not included these are of interest, however, because of their stabiUty at high temperatures. A large number of ternary compounds, including numerous oxyhaUdes, and more compHcated compounds have been synthesized and characterized. These include many intermediate (nonstoichiometric) oxides, and besides the nitrates, sulfates, peroxides, and carbonates, compounds such as phosphates, arsenates, cyanides, cyanates, thiocyanates, selenocyanates, sulfites, selenates, selenites, teUurates, tellurites, selenides, and teUurides. 

Solid Compounds. Thousands of compounds of the actinide elements have been prepared, and the properties of some of the important binary compounds are summarized in Table 4. 

In addition to the binary oxides of the actinides there are many compounds that are made by reaction of these with oxides of alkali metals, transition metals, alkaline earths, and some other elements. Comprehensive surveys of these by Keller [38] and Morss [540] provide lists of known compounds and organize the available crystallographic data to show the isostructural series that exist. They... 

Figure 5.14. Compound formation capability in the binary alloys of Sc, Y, light trivalent lanthanides (as exemplified by La), heavy trivalent lanthanides (exemplified by Gd) and of the actinides (exemplified by Th, U and Pu). The different partners of the 3rd group metals are identified by their position in the Periodic Table. Notice that a sharper subdivision between compound-forming and not forming metals will result from a shifting of Be and Mg from their position in the 2nd group towards the 12th group (see 5.12.3). The behaviour of the divalent lanthanides Eu and Yb is shown in Fig. 5.7 where it is compared with that of the alkaline earth metals.

Table 7. The density of states at the Fermi level and the Stoner product in some NaCl-structure binary compounds of actinides...

Among the actinide compounds the interest is concentrating on binary compounds of simple structure (e.g. 1 1 compounds with elements of the groups V and VI of the periodic table) for which the theoretical treatment is rather advanced, and on intermetal-lic (e.g. Laves-) phases. 

In actinide binary compounds an equation of state can also be developed on the same lines. The difference in electronegativity of the actinide and the non-actinide element plays an important role, determining the degree of mixing between the actinide orbitals (5 f and 6 d) and the orbitals of the ligand. A mixture of metallic, ionic and covalent bond is then encountered. In the chapter, two classes of actinide compounds are reviewed NaCl-structure pnictides or chalcogenides, and oxides. 

A new field of coordination chemistry is that of polymetallic cage and cluster complexes [Mm(p-X)xLJz with molecular (i.e. discrete) structure. They contain at least three metal atoms, frequently with bridging ligands X and terminal ligands L. These compounds link the classical complexes (m = 1) and the non-molecular (m - oo) binary and ternary compounds of the metals.1 Molecular polymetallic clusters (with finite radius) also provide a link with the surfaces (infinite radius) of metals and their binary compounds.2"5 Polymetallic complexes are known for almost all metals except the actinides. 

Just as the lanthanides form a series of closely related elements following La in which the characteristic ions M have from 1 to 14 4f electrons, so the actinide series might be expected to include the 14 elements following the prototype Ac (which, like La, is a true member of Group III), with from 1 to 14 electrons entering the 5f in preference to the 6d shell. In fact there are probably no 5f electrons in Th and the number in Pa is uncertain, and these elements are much more characteristically members of Groups IV and V respectively than are the corresponding lanthanides Ce and Pr. Thus the chemistry of Th is essentially that of Th(iv), whereas there is an extensive chemistry of Ce(iii) but only two solid binary compounds of Ce(iv), namely, the oxide and fluoride. In contrast to Pa, the most stable oxidation state of which is v, there are no compounds of Pr(v). 

Binary intermetallic compounds of the light actinides display a wide variety of magnetic and electronic properties that are not well understood. Physical phenomena associated with... 

It was already mentioned that only one binary compound of actinide metal ThMni2 is known, but its magnetic properties have not been reported. The ternaries are known to exist in a bulk crystalline form with Fe, Co and Ni as transition element and with Si, Mo and Re as stabilizing components. 

All the actinide elements form +3 oxides similar to those of the lanthanides. The elements which have additional oxidation states in their binary compounds with oxygen are shown in Table 7.8, together with their +3 oxides. The oxides of the actinides show a pattern of oxidation states similar to those exhibited by the fluorides. The oxides of the ear-... 

Comparable recent detailed reviews of the actinide halides could not be found. The structures of actinide fluorides, both binary fluorides and combinations of these with main-group elements with emphasis on lattice parameters and coordination poly-hedra, were reviewed by Penneman et al. (1973). The chemical thermodynamics of actinide binary halides, oxide halides, and alkali-metal mixed salts were reviewed by Fuger et al. (1983), and while the preparation of high-purity actinide metals and compounds was discussed by Muller and Spirlet (1985), actinide-halide compounds were hardly mentioned. Raman and absorption spectroscopy of actinide tri- and tetrahalides are discussed in a review by Wilmarth and Peterson (1991). Actinide halides, reviewed by element, are considered in detail in the two volume treatise by Katzet al. (1986). The thermochemical and oxidation-reduction properties of lanthanides and actinides are discussed elsewhere in this volume [in the chapter by Morss (ch. 122)]. 

Element, its Binary and Related Compounds by J. L. Katz and E. Rabinowitch (McGraw-Hill 1951, Dover 1961), the Chemical Behaviour of Zirconium by W. B. Blumenthal (Van Nostrand, 1958), the Transuranium Elements by G. T. Seaborg (Yale, 1958) and the Chemistry of the Actinide Elements by J. J. Katz and G. T. Seaborg (Methuen, 1957). 

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