Protactinium halides compounds

During the past few years numerous new penta- and tetravalent protactinium halides and oxyhalides have been characterized, but of the. possible trivalent compounds only Pals been reported. The presently... 

Actinide halides and oxyhalides are known to form numerous complexes with oxygen and nitrogen donor ligands and the preparation and properties of such compounds have recently been reviewed (12, 13). Relatively few protactinium halide complexes are known, but this situation reflects the lack of research rather than a tendency not to form complexes. However, there is sufficient information available for certain ligands to permit a comparison with the behavior of other actinide halides, and to illustrate the similarities and differences observed with the tetrahalides of thorium to plutonium inclusive and, to a lesser extent, with the protactinium and uranium pentahalides. 

Protactinium halides. Presently known halides of protactinium ate Pap4, PajF9, PaFs, PaCU, Pads, PaBr4, PaBrs, Pals, Pa, and Pals. The pentafluoride is formed by the high-temperature reactions of fluorine ivith protactinium compounds. Hydrogen-HF mixtures stabilize the tetrafluoride PaF4 [B4]. 

The gaps in the list of protactinium halides are due not so much to the instability of the missing compounds but rather to difficulties in characterization. Until very recently, protactinium was the least studied of the actinide elements, primarily because of the great difficulties hitherto experienced in obtaining milligram amounts of pure protactinium-231. The lack of pure protactinium and the difficult chemistry of the element have made... 

The known halides of vanadium, niobium and tantalum, are listed in Table 22.6. These are illustrative of the trends within this group which have already been alluded to. Vanadium(V) is only represented at present by the fluoride, and even vanadium(IV) does not form the iodide, though all the halides of vanadium(III) and vanadium(II) are known. Niobium and tantalum, on the other hand, form all the halides in the high oxidation state, and are in fact unique (apart only from protactinium) in forming pentaiodides. However in the -t-4 state, tantalum fails to form a fluoride and neither metal produces a trifluoride. In still lower oxidation states, niobium and tantalum give a number of (frequently nonstoichiometric) cluster compounds which can be considered to involve fragments of the metal lattice. 

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. 

Tn reviewing the chemistry of the actinides as a group, the simplest approach is to consider each valence state separately. In the tervalent state, and such examples of the divalent state as are known, the actinides show similar chemical behavior to the lanthanides. Experimental diflB-culties with the terpositive actinides up to plutonium are considerable because of the ready oxidation of this state. Some correlation exists with the actinides in studies of the lanthanide tetrafluorides and fluoro complexes. For other compounds of the 4-valent actinides, protactinium shows almost as many similarities as dijSerences between thorium and the uranium-americium set thus investigating the complex forming properties of their halides has attracted attention. In the 5- and 6-valent states, the elements from uranium to americium show a considerable degree of chemical similarity. Protactinium (V) behaves in much the same way as these elements in the 5-valent state except for water, where its hydrolytic behavior is more reminiscent of niobium and tantalum. 

Tetrachloride and tetrabromide complexes are known for thorium, protactinium, uranium, neptunium, and plutonium. These are similarly produced by halide-based oxidation of metals or hydrides, or by halogenation of oxides. A common structural type is reported for most compounds. The reported structure of thorium tetrachloride reveals that the coordination geometry about the metal is dodecahedral.The compounds are generally volatile and can be sublimed. The gas-phase electron diffraction structure of suggests that the molecule is... 

The intermetallic compounds are synthesized by heating mixtures of actinide oxides or halides with finely divided noble metal powders in pure hydrogen. Protactinium metal was prepared in a modified version of the van Arkel-de Boer procedure protactinium iodide, formed by reaction between iodine and protactinium carbide, was thermally dissociated on a resistance heated tungsten wire (6,7) ... 

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