Protactinium pentaiodide

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 pentaohloride 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). 

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. 

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 oxytriiodide, PaOIs, and dioxymonoiodide, PaOgl, have both been prepared (40) by reacting the pentaiodide with the stoichiometric amount of antimony sesquioxide [Eqs. (9) and (10)]. PaOIs, a dark brown solid, is thermally unstable [Eq. (11)] in vacuo like certain of the oxychlorides and oxybromides. The positions of the protactinium-oxygen... 

Protactinium tetraiodide is a dark green solid which can be prepared by reducing the pentaiodide with aluminum at about 400°C (44). The structure is not known. On sublimation in silica above 500°C small amounts of the pink oxydiiodide, PaOIg, are formed. Structural information is not available for the oxydiiodide, but it is known to be iso-morphous with ThOl2-... 

The only mixed halide known is PaBrglg, a black, crystalline solid which is isostructural with the pentaiodide. It is formed (40) when equimolar amounts of the pentabromide and pentaiodide are heated together at 300°C. Numerous uranium(IV) mixed halides are known (34) and undoubtedly many protactinium(IV) and (V) mixed halides could be prepared, but this field has so far been neglected. 

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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