Protactinium tetrachloride

Protactinium tetrachloride was first prepared (125) on the microgram scale by hydrogen reduction of the pentachloride at 800°C, a method since employed at 400°C for the preparation of 50-mg hatches (44 , 46). It has also been prepared (125) by reacting the dioxide with carbon tetrachloride vapor, hut since the pentachloride is much easier to prepare than the dioxide reduction of the former with aluminum at 400°C appears to he the most attractive route to the tetrachloride (44). 

Like other actinide tetrahalides, protactinium tetrachloride and tetrabromide form stable complexes with phosphine oxides (48, 55) and iV jAT -dimethylacetamide (24), but the tetrachloride-dimethyl sulfoxide... 

The actinide tetrachloride-DMA complexes cover quite a range of stoichiometry (22,24,27) and the behavior of protactinium tetrachloride is intermediate between that observed for thorium and uranium tetrachloride (Table XI). PaCl4 2.5 DMA is isostructural with the y-modifica-tion of the other 1 2.5 complexes and with PaBr4 2.5 DMA (24). However, the uranium tetrabromide complex, UBi 4 2.5 DMA, possesses a different structure. The 1 5 tetrabromide-DMA complexes (Th to U inclusive) are isostructural (24). 

The actinide tetrachloride-DMSO complexes are particularly interesting (18). There is a pronounced change in stability proceeding along the actinide series with 1 5 complexes being the most stable for thorium and protactinium and the 1 3 complexes for the remaining actinides. The 1 7 complex could not be obtained pure with thorium tetrachloride and under the preparative conditions required, namely, recrystallization from hot dimethyl sulfoxide, protactinium(IV) was oxidized. The solid 1 5 and 1 3 protactinium tetrachloride compounds are, in fact, unstable in dry nitrogen, behavior which contrasts markedly with the stability of the tetrahalide-phosphine oxide and DMA complexes. 

Even fewer complexes with nitrogen donor ligands have been reported and all are methyl cyanide adducts (Tables X and XI). Protactinium pentabromide forms a soluble 1 3 complex in contrast to the 1 1 complexes formed by niobium and tantalum pentahalides (46). Other actinide pentahalide-methyl cyanide complexes are still unknown. Protactinium tetrachloride, tetrabromide, and tetraiodide react with anhydrous, oxygen-free methyl cyanide to form slightly soluble 1 4 complexes (44, 48) which are isostructural with their actinide tetrahalide analogs. 

The magnetic susceptibility of protactinium tetrachloride has been measured between 3.2 and 296 K [25], as shown in Table 18.3, and exhibits a ferromagnetic transition at 182 2 K. A high degree of co valency has been suggested to explain this relatively high transition temperature. The magnetic susceptibility of protactinium tetraformate [26] has been measured from 80 to 300 K and follows... 

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. 

Protactinium pentaiodide crystallizes with orthorhombic symmetry (104) (Table III). The pentaiodide is extremely moisture-sensitive and hydrolyzes immediately on contact with water. It is slightly soluble in methyl cyanide, but insoluble in isopentane and carbon tetrachloride. 

Table XIV). Gaseous ammonia reacts with both the penta- and the tetrachloride to yield a yellow solid believed to be a nitride and some evidence for the existence of a monocarbide has been observed during the reduction of protactinium tetrafluoride in a carbon crucible (125). Vapor pressure data for carbide produced by neutron irradiation... 

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 most utilized route for the synthesis of bis([8]annulene)-actinide(IV) complexes is the reaction of a tetrafiydrofuran (THF) solution of [8]annulene dianion with a stoichiometric amount of the actinide tetrachloride in THF. This method has been employed for the synthesis of bis([8]annulene)thorium(IV) (thorocene),a -protactinium(IV)... 

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