Protactinium halides, 3-19,

During the past few years numerous new penta- and tetravalent protactinium halides and oxyhalides have been characterized, but of the. possible trivalent compounds only Pals been reported. The presently... 

Actinide halides and oxyhalides are known to form numerous complexes with oxygen and nitrogen donor ligands and the preparation and properties of such compounds have recently been reviewed (12, 13). Relatively few protactinium halide complexes are known, but this situation reflects the lack of research rather than a tendency not to form complexes. However, there is sufficient information available for certain ligands to permit a comparison with the behavior of other actinide halides, and to illustrate the similarities and differences observed with the tetrahalides of thorium to plutonium inclusive and, to a lesser extent, with the protactinium and uranium pentahalides. 

Protactinium(V) dioxymonofluoride, PaOgF, is obtained by thermal decomposition of Pa20Fg in air at 270°C (Bagnall et al.). It is a white, air-stable solid which decomposes to another, as yet unidentified, oxyfluoride at 450°-470°C which, in turn, decomposes to PagOg above 650°C. Other papers on the protactinium halides describe a new, im-... 

Protactinium halides. Presently known halides of protactinium ate Pap4, PajF9, PaFs, PaCU, Pads, PaBr4, PaBrs, Pals, Pa, and Pals. The pentafluoride is formed by the high-temperature reactions of fluorine ivith protactinium compounds. Hydrogen-HF mixtures stabilize the tetrafluoride PaF4 [B4]. 

The gaps in the list of protactinium halides are due not so much to the instability of the missing compounds but rather to difficulties in characterization. Until very recently, protactinium was the least studied of the actinide elements, primarily because of the great difficulties hitherto experienced in obtaining milligram amounts of pure protactinium-231. The lack of pure protactinium and the difficult chemistry of the element have made... 

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. 

Protactinium metal is prepared (1) by reducing the tetrafluoride witli metallic barium at about 1,500 0 (2) by heating the halide, usually the iodide, under a high vacuum and (3) by bombardment of the oxide under high vacuum with 35-keV electrons for hours at a current strength of 0.005 0.010 Amperes. 

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... 

The compounds known are summarized in Table 10.1. The only compound of an early actinide in the -1-2 state is Thl2, a metallic conductor which is probably Th + (e )2 (D)2-Certain heavier actinides form MX2 (Am, Cf, Es), which usually have the structure of the corresponding EuX2 and are thus genuine M + compounds. All four trihalides exist for all the actinides as far as Es, except for thorium and protactinium. Tetrafluorides exist for Th-Cm and the other tetrahalides as far as NpX4 (and in the gas phase in the case of PuCE). Pentahalides are only known for Pa, U, and Np whilst there are a few MFe (M = U-Pu), uranium is the only actinide to form a hexachloride. The known actinide halides are generally stable compounds most are soluble in (and hydrolysed by) water. 

Protactinium has a wealth of halides, with all PaX4 and PaXs known, also Pals. There are also several oxyhalides. 

Progress in the preparative and structural fields of protactinium chemistry has been rapid during the past 6 years and there is now sufficient information available, particularly in the halide and oxide fields, to permit a more balanced comparison than has previously been possible with the properties of the actinide elements, on the one-hand, and those of niobium and tantalum, on the other. In this connection one must, of course, bear in mind the fact that the ionic radii of protactinium in its various valence states [Pa(V), 0.90 A and Pa(IV), 0.96 A] are appreciably larger than those of niobium or tantalum and this itself will have a considerable influence on the chemical and crystallographic properties of the elements. 

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 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 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. 

Attempts to prepare protactinium pentanitrate by reacting penta-halides with liquid dinotrogen pentoxide have resulted in the formation of HPalNOglfl, possibly as a result of traces of anhydrous nitric acid present in the N Os 49). The presence of the jiroton has not been confirmed by electron spin resonance studies, but infrared results have shown that all the nitrate is covalently bound and vibrations associated with the nitronium and nitrosonium cations were not observed. Niobium and tantalum pentahalides react under similar conditions to form the anhydrous oxytrinitrates, M 0(N0g)3 20, 87). 

Tn reviewing the chemistry of the actinides as a group, the simplest approach is to consider each valence state separately. In the tervalent state, and such examples of the divalent state as are known, the actinides show similar chemical behavior to the lanthanides. Experimental diflB-culties with the terpositive actinides up to plutonium are considerable because of the ready oxidation of this state. Some correlation exists with the actinides in studies of the lanthanide tetrafluorides and fluoro complexes. For other compounds of the 4-valent actinides, protactinium shows almost as many similarities as dijSerences between thorium and the uranium-americium set thus investigating the complex forming properties of their halides has attracted attention. In the 5- and 6-valent states, the elements from uranium to americium show a considerable degree of chemical similarity. Protactinium (V) behaves in much the same way as these elements in the 5-valent state except for water, where its hydrolytic behavior is more reminiscent of niobium and tantalum. 

Tetrachloride and tetrabromide complexes are known for thorium, protactinium, uranium, neptunium, and plutonium. These are similarly produced by halide-based oxidation of metals or hydrides, or by halogenation of oxides. A common structural type is reported for most compounds. The reported structure of thorium tetrachloride reveals that the coordination geometry about the metal is dodecahedral.The compounds are generally volatile and can be sublimed. The gas-phase electron diffraction structure of suggests that the molecule is... 

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... 

---