Protactinium IV

The longest living isotope of protactinium is with a half-life of 32 000 years. Typically, it occurs in aqueous solution in the pentavalent state, although the tetravalent state is also known. The aqueous form of protactiniumfV) is the PaO ion, and only monomeric hydrolysis species have been identified, PaOOH to PaO(OH)3(aq). There is some conjecture over whether the tetravalent ion occurs as Pa or PaO . No conclusive data have been published on protactinium(IV) hydrolysis species. 

In studying the complexation of protactinium(IV) with acetylacetone, Lundqvist (1974) indicated that in solution the metal ion was present as PaO (or equally PafOH) )- However, Lundqvist also noted that this behaviour was not present in other tetravalent metal ions such as Zr(IV), Hf(IV) or other actinide(IV) ions. As shown in Section 9.2.3, protactinium(V) exists as PaO , a behaviour that is also not followed by other actinide(V) ions. The observations of Lundqvist (1974) differ from those of Guillaumont (1965, 1968) who indicated the formation of three protactinium(IV) hydrolysis species, PaOH, Pa(OH)2 and PafOHlg, with respective stability constants of log = -0.14, log 2 = -0.52 and log = -1.77 in measurements carried out in 3.0 mol 1 LiClO. Given that there are no supporting data for either of these interpretations, no data are retained for protactinium(IV). 

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Axe, J. D. "Electronic Structure of Octahedrally Coordinated Protactinium(IV) in Cs2ZrCl6", UCRL-9293,... 

Table 17 Some Hydrates of Thorium(IV), Protactinium(IV), Uranium(IV), Neptunium(IV)...

Because of the ease of oxidation of protactinium(IV) and uranium(IV), peroxides and peroxo complexes are limited to their higher oxidation states. The compounds M04"JcH20 precipitated from dilute acid solutions of neptunium(IV) and plutonium(IV) by hydrogen peroxide appear to be actinide(IV) compounds. Soluble intermediates of the type [Pu( U-02)2Pu]4+ are formed at low hydrogen peroxide concentrations. 

Th(PhCOCHCOPh)4 has been reported as being isomorphous with the corresponding protactinium(IV), uraniumflV) and cerium(IV) complexes the coordination geometry in the last is a triangular faced dodecahedron, but a more recent publication73 reports the coordination geometry of the uranium(IV) compound as square antiprismatic. 

In the structures of compounds of the type M3UF7 the seven F atoms are statistically distributed over fluorite lattice sites.153 The nine-coordinate thorium atom in (NH ThFg is surrounded by a distorted tricapped trigonal prismatic array of fluorine atoms, with the prisms sharing edges to form chains, whereas the uranium(IV) compound contains discrete dodeca-hedrally coordinated [UF8]4 ions. The protactinium(IV), neptunium(IV) and plutonium(IV) analogues are isostructural with the uranium compound.154... 

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... 

HEXAHALOGENO SALTS AND ALKYL NITRILE COMPLEXES OF TITANIUM(IV), ZIRCONIUM(IV), NIOBIUM(V), TANTALUM(V), PROTACTINIUM (IV) AND -(V), THORIUM(IV), AND URANIUM(IV)... 

A 0.200-g sample of protactinium(IV) oxide is converted to another oxide of protactinium by heating in the presence of oxygen to give 0.2081 g of the new oxide, PaxOr Determine the values of x and y. 

Protactinium(IV) oxydifluoride, PaOF2, is the only tetravalent protactinium oxydihalide not yet characterized. This seems to be due solely to a lack of investigation since there are no negative reports concerning the most likely routes involving interaction of the tetrafluoride and either protactinium dioxide or antimony sesquioxide, both of which methods have been employed for the preparation of the other tetravalent oxydihalides. 

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. 

Since protactinium(IV) is readily oxidized in the atmosphere halo-genoprotactinates(IV) must be prepared in oxygen-free solvents or by heating the component halides together in an inert atmosphere. It is reported that solvent extraction studies have established the existence of the fluoro and chloro species PaXl" " and PaX + in aqueous acid solution (81)... 

The protactinium(IV) fluoro complexes have been prepared either by hydrogen reduction of a pentavalent complex at 400°C or by heating together appropriate amounts of MF and PaF4 in sealed vessels. The reaction between ammonium fluoride and protactinium tetrafluoride to yield (NH4)4PaFg, the only octafluoro complex known, takes place when the component halides are ground together at room temperature (4,114). 

It is noteworthy that complexes of the type MjPaFa are unknown this is all the more surprising since both thorium(IV) and uranium(IV) complexes of this type, where = Na, K, Rb, and Cs, have been recorded. Attempts to prepare the protactinium(IV) complexes by the hydrogen reduction technique have been unsuccessful for example, K7PaflF3i has been obtained from KgPaF (4, 114). Nevertheless, it is probable that with the right experimental conditions the tetravalent 2 1 complexes will be prepared. 

It is convenient to discuss protactinium(IV) and (V) bromo and iodo complexes together since they have only been prepared by reacting together the component halides in anhydrous methyl cyanide (40, 46, 48). Thus NMe4PaBrg, NEtiPaBrg, and PhgMeAsPala are isolated (40,46) by removal of solvent in vacuo at room temperature. Hexabromo-niobates(V) and tantalates(V) have been prepared in a similar manner (46). Attempts to obtain octabromo complexes for these three elements have been unsuccessful. The orange hexabromoprotactinates(V) are... 

DMSO) complexes (18) are relatively unstable toward oxidative decomposition. The known complexes are listed in Table X, together with infrared data, and are compared with the complexes formed by other actinide halides in Table XI. The protactinium(IV) complexes have been prepared by reacting the anhydrous halide with the appropriate ligand in nonaqueous, oxygen-free solvents such as methylene dichloride, chloroform, or methyl cyanide. 

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. 

The fiuorosulfate dihydrate, PaF2S04 2H2O, analogous to the known uranium(IV) compound (1S2), is precipitated on the addition of aqueous hydrofluoric acid to solutions of protactinium(IV) in dilute sulfuric acid (131). Others (71, 84) have reported the formation of a white, insoluble precipitate in hydrofluoric acid, believed to be a tetrafluoride hydrate, but the product has not been completely characterized. 

Hexaiodo-complexes of the type R2M l6 (R = NEt or NMejPh = Th, Pa, or U) have been prepared by reactions in oxygen-free anhydrous methyl cyanide, and the infrared, Raman, and electronic spectral data have been reported for the protactinium(iv) and uranium(iv) complexes. The ligand field parameters and spin-orbit coupling constants derived from the hexaiodoprotactinates(iv) were discussed in relation to values obtained from earlier studies of octahedral hexahalogenopro-tactinates(iv) of the type (NEtJjPaXg (X = F, Cl, or Br) and were shown to fit the trends previously identified. 

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