Protactinium fluorides

Crystals of Potassium Protactinium Fluoride—KsPaFT. Left Dark field illumination X 60. 

Table II. Absorption Spectrum of Tetravalent Protactinium Fluoride...

The extensive hydrolysis of protactinium in its V oxidation state makes the chemical investigation of protactinium extremely difficult. Ions of protactinium(V) must be held in solution as complexes, eg, with fluoride ion, to prevent hydrolysis. 

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 element is difficult to maintain in aqueous solution in the form of simple salts. Solubility data seem to indicate that such amounts as can be dissolved probably do so entirely by formation of complex ions. Fluoride ion strongly complexes protactinium, and it is due to this that protactinium compounds are in general soluble in hydrofluoric acid. 

Protactinium exists naturally in the pentavalent state and although it is possible, by employing strongly reducing conditions, to obtain the tetravalent state in solution and in solid compounds this state is, in general, unstable in the presence of oxygen, exceptions being the tetra-fluoride and dioxide, which are stable in the atmosphere. The potential... 

Protactinium tetrafluoride, like the other actinide tetrafluorides, possesses the 8-coordinate UF4-type of structure (Table III) but no bond distances are available. It is easily the most stable tetravalent halide of protactinium and can be handled in the atmosphere, at least for a limited period, without hydrolysis or oxidation occurring. As mentioned earlier it is the usual starting material for the preparation of protactinium metal. Tetrafluoride hydrates have not been fully characterized, but a mixed fluorosulfate, PaF2S04 2H20 can be precipitated from aqueous solution (131). Protactinium tetrafluoride is soluble in aqueous ammonium fluoride solutions, for which some spectral properties have been recorded (4, 83). 

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). 

A recent study of the alkali metal/uranium fluoride complex systems has shown that KriUFo, K3UF7, and K UFs are isomorphous (99), the crystal symmetry being unaffected by the fluoride ion absences. The structure of K PaFy has been determined only recently (39), and it has been shown that each protactinium atom is surrounded by nine fluorine atoms in what is effectively a trigonal prism with three added equatorial fluorine atoms the PaFo groups are linked in infinite chains by two fluorine bridges. 

Table I. Alkali Fluoride Complexes of Tetravalent Protactinium...

Complex fluorides containing tetravalent protactinium and the alkali fluorides (Li, Na, K, Rb, and NH4) were prepared from MF -j- PaF4 starting mixtures and by reducing MF-PaF compounds at 400°—500°C. using pure hydrogen, obtained from UH3. The x-ray results on compounds obtained by these methods are presented in Table I. 

Table IV. Some Alkali Fluoride Protactinium ( V ), Uranium ( V ), Neptunium(V), Plutonium(V) Fluoride Complexes"...

Binary halides. A number of homoleptic halides of pentavalent protactinium, uranium, and neptunium have been reported. In particular, the fluoride complexes AnFs are prepared by high... 

Protactinium(V) octafluoro complexes, MlPaF, have been prepared (5, 35, 37, 59) by precipitation from aqueous hydrofluoric acid (M = Li, Na, and Rb), by heating the alkali metal fluoride with the corresponding heptafluoro complex in argon (M = K and Cs) or by fluorinating the product obtained by evaporation of a hydrofluoric acid solution containing 3 1 mixtures of MF and Pa(V). 

---