Protactinium iodides

The method of choice for the preparation of Pa metal is a somewhat modified van Arkel-De Boer process, which uses protactinium carbide (Section II,C) as the starting material. The carbide and iodine are heated to form protactinium iodide, which is thermally dissociated on a hot filament 12-15). An elegant variation is to replace the filament with an inductively heated W or Pa sphere 109). A photograph of a 1.4-g sample of Pa metal deposited on a radiofrequency-heated W sphere is shown in Fig. 6. From the analytical data presented in Table V, the impurities present before and after application of this modified iodide transport process (Sections II,D and III,C) can be compared. 

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) ... 

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 Van Arkel process can also be used to prepare actinide metals if the starting compound reacts easily with the transporting agent (I2). The thorium and protactinium carbides react with I2 to give volatile iodides above 350°C these are unstable above 1200°C and decompose into the actinide metals and iodine. Attempts to prepare other actinides, such as U and Pu, through the process were not successful, because from Th to Pu along the actinide series, the vapour pressure of the iodide decreases and the thermal stability increases. 

Protactinium metal was first prepared in 1934 by thermal decomposition of a pentahalide on a hot filament 50). It has since been prepared from PaF4 by metallothermic reduction (Section II,A) with barium 26, 27, 34,102), lithium 40), and calcium 73, 74). However, the highest purity metal is achieved using the iodide transport (van Arkel-De Boer) process (Section II,D). 

In the fall of 1934, Dr. Grosse reduced this pure oxide by two methods and obtained from it the metal protactinium, which is even rarer than radium, but much more permanent in air. In die first method, he bombarded the oxide on a copper target, in a high vacuum, with a stream of electrons. After a few hours, he obtained a shiny, partly sintered, metallic mass, stable in air. In his second method, he converted the oxide to the iodide (or chloride or bromide) and cracked it in a high vacuum on an electrically heated tungsten filament, according to the reaction ... 

Protactinium metal is prepared (1) by reducing the tetrafluoride witli metallic barium at about 1,500 0 (2) by heating the halide, usually the iodide, under a high vacuum and (3) by bombardment of the oxide under high vacuum with 35-keV electrons for hours at a current strength of 0.005 0.010 Amperes. 

Volatile protactinium pentaehloride has been prepared in a vacuum by reaction of the oxide with phosgene at 550° C or with carbon tetrachloride at 200°C. Reduction of this at 600°C with hydrogen leads to protactinium(IV) tetrachloride, Pad. which is isostructural with uranium(IV) tetrachloride, UCI4. The pentaehloride can be converted into the bromide or iodide by heating with the corresponding hydrogen halide or alkali halide... 

Unlike uranium pentaehloride, which is thermally unstable, protactinium pentachloride sublimes unchanged above 180°C in vacuo. It is a yellow, moisture-sensitive solid which is slightly soluble in benzene, tetrahydrofuran, and carbon tetrachloride. Visible absorption speetra have been reeorded for solutions in the last two solvents and in aleohol (110). Reactions with hydrogen, aluminum, oxygen, and silicon tetra-iodide are discussed below. It is unaffected by carbon monoxide at 350°C in a sealed tube. 

The iodide complexes are somewhat less stable, and well-characterized examples exist only for thorium, protactinium, and uranium. The thorium and uranium derivatives can be coveniently prepared by the reaction of iodine and metal, while protactinium tetraiodide is generated by reduction of Pals. The molecular structure of Thl4 has been examined the metal lies within a distorted square antiprism of iodide ions. 

Protactinium monocarbide was prepared by carbothermic reduction of the pent-oxide and treated with iodine (400 C), bromine (350 C), and thionyl chloride (200°C) in evacuated sealed tubes to give the appropriate pentahalide. The tetraiodide was obtained by interaction of the monocarbide with either protactinum pentaiodide (600 °C) or mercury(ii) iodide (500 °C) in vacuo. 

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