Protactinium pentachloride

Protactinium pentachloride possesses monoclinic symmetry (Table III), but is not isostructurEll with either niobium (138) or uranium pentachloride (127). The structure (67) comprises infinite chains of nonregular... 

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 pentachloride (42) and pentabromide (43) form both 1 1 and 1 2 complexes with phosphine oxides, the former being analogous to those formed by niobium, tantalum, and uranium pentahalides (26, 42, 43). Unlike niobium and tantalum pentachloride (42, 64) however, they do not react with excess triphenylphosphine oxide (TPPO) to form... 

Both protactinium pentachloride and pentabromide react with triphenylphosphine sulfide and selenide to form stable 1 1 complexes... 

No binary sulfide has been reported, but the bright yellow oxysulfide, PaOS, isostructural with other actinide oxysulfides, has been obtained (125) by heating protactinium pentachloride or pentoxide in a mixture of carbon disulfide and hydrogen sulfide at 900° and 1200°C, respectively... 

Fig. 2. The structure of protactinium pentachloride (67). (a) Portion of the infinite chains in Pads. Coordination of Cl around each Pa is pentagonal bipyra-midal. (b) Bond distances and angles within an isolated PaCl7 group. The four Cl(3) atoms are bridging.

The acetylacetonate PajacacjgClg has been prepared by direct reaction between protactinium pentachloride and acetylacetone in methylene dichloride. The addition of isopentane to this reaction mixture results in the formation of long, bright yellow needles of the complex, and single crystal studies have shown these to possess monoclinic symmetry, space group P2i/c with do = 8.01, = 23.42, Cq = 18.63 A, and... 

Protactinium (continued) pentachloride, 12 10, II complexes with phosphine oxides, 12 30, 31... 

The preparation of the metal was first reported by von Grosse (80) who obtained it by bombarding protactinium pentoxide with 35 keV electrons in a high vacuum and by decomposing the pentachloride on a hot wire. No properties were reported for these products and more recently the pure metal has been obtained by reduction of protactinium tetrafluoride with lithium (73) or barium (65,125) vapor at 1300°-1400°C using the double crucible technique and on a larger scale by reduction with barium (106) or 10% magnesium in zinc alloy (107). 

Protactinium pentafluoride, which is isostructural with -UFg (Table III). is best prepared (160) by fluorination of the tetrafluoride at 700°C (Fig. 1). It has been obtained in an amorphous state (38) by hydro-fluorination of the pentachloride at 200°C and, contaminated with... 

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). 

Although protactinium(V) chloro complexes can also be prepared using methyl cyanide as the solvent (32) (cf. PaBrg" and Pale ), the use of thionyl chloride has several advantages. Thus, it means that one can start with protactinium(V) hydroxide and not the pentachloride, thionyl chloride itself affords protection against atmospheric moisture and dry-atmosphere boxes are not necessary for the preparations, and, in addition, protactinium(V) concentrations up to 0.5 M have been obtained by dissolving the hydroxide in thionyl chloride (75). Such solutions are quite stable in contrast to the hydrolytic condensation reactions which occur in concentrated hydrochloric acid at Pa(V) concentrations as low as 10 M. 

Protactinium pentaiodide was first prepared by reacting the pentox-ide with aluminum triiodide at 400°C in a vacuum 104). For large scale preparations, however, direct union of the elements or metathesis of the pentachloride or pentabromide with an excess of silicon tetraiodide are best 40). The latter reactions take place rapidly at about 180°C in vacuo and the black crystalline product is then purified by vacuum sublimation at 400°-450°C. Silicon tetraiodide also reacts with protactinium pent-oxide, but temperatures in excess of 600°C are required with the reaction vessel completely enclosed in the furnace, and the yield is only about 70% 40). 

Penta(tropolonato)-protactinium and -uranium have been prepared using tropolone and the metal pentaethoxide in benzene. Uranium pentachloride reacts with an excess of tropolone in non-aqueous solvents to give chlorotetra(tropolonato)uranium. Enthalpies of formation of tri(tropolonato)- and tri(4-methyltropolonato)-aluminium (ill) have been determined at 298.15 K by solution calorimetry. The gas phase enthalpies of formation have been calculated and from these the A1—O bond energies derived. The crystal and molecular structure of a tetranuclear cobalt(ii)-tropolonate complex, [Co4(C7H502)s(H20)2] has been reported. Spectroscopic data (i.r., n.m.r.) of a mercury (ii) tropolonate indicates that it has an unsymmetrical structure with two approximately linear short Hg—O bonds and two longer ones. ... 

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