Californium halides

Table 11.6 Preparative reactions for californium halides. tetrafluoride...

There is evidence that californium halides may form so-called mixed-valence compounds (MsXu, M11X24, or M6X13, where M = metal ion and X = halide ion) like those reported for some of the lanthanide elements [142-144]. To avoid the necessity of carefully controlling reduction of californium to the proper stoichiometry, mixtures of gadolinium and californium (comparable radii, see Table 11.4) were used such that total reduction of the californium to Cf(ii) would produce the desired structures, Cf GdClu and Cf GdBru [145]. 

Except for the trifluoride, anhydrous californium trihalides are hygroscopic and must be protected from moisture. The dihalides are very sensitive to both moisture and oxygen. Normally, their syntheses are carried out in glass (except for fluorides) so that products can be flame-sealed in situ, avoiding subsequent transfers of the products. Some preparative and experimental techniques for studying californium halides have been reviewed [74,146]. 

Studied through the use of tracer quantities, the chemical properties of californium indicate that its chemical properties are analogous to those of the tripositive actinides and lanthanides, showing the fluoride and the oxalate to be insoluble in acid solution, and the halides, perchlorate, nitrate, sulfate and sulfide to be soluble. 

When these isotopes become available, chemical studies will be greatly simplified, and the complications introduced by the radioactivity of the actinide elements will be substantially minimized. The longest-lived isotopes of berkelium, californium, and einsteinium are still fairly short-lived substances, and macroscopic amounts have a tremendous associated radioactivity. Nevertheless, it should eventually be possible to prepare and study the solid halides of the actinide elements through the element einsteinium using weighable amounts of reactants. This remains for the future, however. The special experimental problems associated with highly radioactive substances are considered below. 

Truly divalent actinide halides are known only for americium and californium. AnX2 species for Es have been identified by their absorption spectra. For Fm, Md, and No, AnX2 halides should be possible if sufficient amounts of these metals could be obtained. Thl2 is also known, but crystallographic studies of this compound reveal the true formulation to be Th(IV), 21 , and 2e . This compound has some metallic character, including its luster and electrical conductivity. 

In view of the estimated iv-iii reduction potentials [87,127] for californium, it is reasonable that the only stable, bulk binary Cf(iv) halide would be Cfp4, which can be prepared by fluorinating (F2 or CIF3) CfF3, Cf203 (see Table 11.6), or possibly other materials [113,114,128,129] at 3(X)-400°C. The tetrafluoride has limited thermal stability, decomposing to form CfFj [113]. There has been some... 

In addition to the oxides and halides, several other compounds of californium have been prepared and their crystallographic data reported (see Table 11.5). Some of these data represent preliminary values or results from single experiments. In some cases (pnictides, chalcogenides, etc.), the limited supply of californium metal has precluded the preparation of specific compounds, especially where close control of the stoichiometries is required (for example, CfS). The general preparative techniques for pnictides and chalcogenides of the transuranium elements has been reviewed [147]. 

Some of the first data on californium complexes involved materials such as the halides, citrate, lactate, a-hydroxyisobutyrate, etc., as these materials played a role in early separation/purification schemes [7,23] (also see Section 11.4). In general, it is expected that only small differences in stability would exist between Cf(iii) and its trivalent neighbors Bk(iii) and Es(iii), with larger differences being found between Cf(iii) and Am(in). A general discussion of actinide complexes (including californium) in aqueous solution is available [198]. 

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