Protactinium pentabromide

Protactinium pentabromide is appreciably more stable than uranium pentabromide, which decomposes to the tetrabromide at 160°C (116). It is moisture-sensitive and hydrolyzes rapidly on the addition of water. The few known complexes are discussed later. 

A. A -Dimethylacetamide (DMA) forms a 1 3 complex with protactinium pentabromide (23), but its reaction with other pentahalides has not been studied. 

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. 

As mentioned above, protactinium(V) oxytribromide is invariably obtained as a by-product during the preparation of the pentabromide. It is also formed (32, 45) when stoichiometric amounts of the penfa-bromide and either oxygen [Eq. (6)] or antimony sesquioxide [Eq. (7)] are heated together in a sealed vessel at 350°C. Although it is found with... 

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). 

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 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... 

---