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Home , Aluminium iodide, Caesium fluoride, Carbonyl bromide, Halogen, Molybdenum hexafluoride, Platinum hexafluoride

The Chemistry of the

K. W. BAGNALL*

Atomic Energy Research Establishment, Chemistry Division, Harwell, England

1. Introduction . . 304 2. The Trivalent Actinides 306 A. General Chemistry 306 B. Trifluorides 307 C. Trichlorides 309 D. Tribromides 311 E. Tri- 313 F. Mixed 314 G. Oxyhalides 314 3. The Tetravalent Actinides 315 A. General Chemistry 315 B. Tetrafluorides 316 C. Tetrachlorides 319 D. Tetrabromides 326 E. Tetraiodides 327 F. Mixed Halides 328 G. Halo Complexes 329 H. Oxyhalides 335 4. The Pentavalent Actinides 337 A. General Chemistry 337 B. Pentafluorides 338 C. Intermediate 339 D. Pentachlorides . . 340 E. Pentabromides . . 342 F. Pentaiodides 343 G. Mixed halides 343 H. Halo Complexes . . 343 I. Oxyhalides 347 5. The Hexavalent Actinides 351 A. General Chemistry 351 B. 352 C. Hexachloride 358 D. Oxyhalides 359 References 367

* Present address: Department of Chemistry, University of Manchester, England.

303 304 κ. w. BAGNALL

1. Introduction The classification of the heavy elements from (89) to (103) as a second/-transition series, the actinides, originally suggested by Seaborg, is now well established. The earlier members of the , up to (95) exist in a greater variety of valency states (Table I) than do the , largely because the 5/-electrons have relatively lower binding energies, and are less effectively shielded by the outer electrons, than are the 4/-electrons. The 4/-electrons are not accessible for bonding in the lanthanides, whereas the 5/-orbitals

TABLE I. Oxidation states of the Ughter actinides*

Element Ac Th Pa U Np Pu Am Atomic No. 89 90 91 92 93 94 95 3 3 3 3 3 4 4 4 4 4 4 5 5 5 5 5 6 6 6 6 a The most stable state in aqueous solution is underlined. extend spatially into the outer regions of the atom and are more accessible for bonding, which may involve 5/-, 6d', 7s- and 7^-orbitals. As a result, the actinides form a wide variety of complex species, in contrast to the lanthanides in which the bonding is largely ionic. The heavier actinides are predominantly tervalent in solution and in the solid state, but no definite compounds have been recorded for the elements from (99) to lawrencium which are at present only available in extremely minute quantities. The elements from uranium to americium are best regarded as an inner transition series, with the chemistry of the hexavalent elements characterized by the uniquely stable oxygenated MO both in solution and in solid compounds and, with the exception of americium, by volatile hexafluorides MF^, the chemistry of which shows some similarity to that of the group VI d-transition element hexafluorides WFg and MoFg. UClg, the only other known hexahalide, also resembles its analogue to some extent. In the pentavalent state the MOion predominates in aqueous media, in contrast to the behaviour of (V), with which hydrolysis and polymeriza­ tion to -bridged species occurs readily in the absence of com- plexing anions, a behaviour rather similar to that of and , and there is no satisfactory evidence for the protactinyl , PaOg"^. However, the complexes of pentavalent protactinium and uranium resemble those formed by niobium and tantalum except THE HALOGEN CHEMISTRY OF THE ACTINIDES 305

that the former can increase their apparent coordination number to 8 in both fluoro and chloro complexes whereas the latter exhibit the higher coordination only in the fluoro complexes, the chloro species being restricted to 6-coordination. In the quadrivalent state the halocomplexes show many resemblances to their uranium analogues, but there are some marked differences in the complexing behaviour of the tetrahalides with oxygen donor ligands. The commonest coordination numbers are 8 and 6, but 7 and 9 are also known. The tervalent actinides behave in much the same way as the lanthanides, but with more evidence of complexing in aqueous halogen at high halide ion concentrations. In view of these factors, the chemistry of the halides has been treated in four sections, by valency states rather than element by element. The available data on the complexing behaviour of the actinide halides indicates that in all valency states the actinides can be regarded as nearly pure Chatt-Ahrland A-type ions, fluoro complexing in every case being much stronger than with the more polarizable heavier ; as far as is known, oxygen donors, such as iV^iV^-dialkylamides, substituted phosphine and dimethylsulphoxide, form more stable complexes than simple donors. Studies of such com­ plexes have been largely restricted to the lighter actinides and little structural information is available. All of the actinides are radioactive to a greater or lesser degree and many of them are therefore extremely toxic; because of this, work with any of them, other than thorium and uranium, must be carried out under very carefully controlled conditions in glove-boxes or similar enclosures. The techniques used for macroscale (protactinium, neptu­ nium, and americium) and microscale (actinium, (96) and later actinides) studies of these elements have been adequately described in the reviews of the chemistry of the actinides (Katz and Seaborg, 1957) and of their halides (Katz and Sheft, 1960), which also include extremely useful compilations of physical and thermodynamic data, which are not discussed in this review. Rand and Kubaschewski (1963) have also published a critical compilation of the thermochemical properties of uranium compounds and other detailed reviews of the chemistry of the group (Haissinsky, 1962) and of the fluorides in particular (Hodge, 1961 ; Tananaev ei αί., 1961 ), as well as comprehensive reviews of the earlier work on the halides of thorium (Katzin, 1954), uranium (Katz and Rabinowitch, 1951), (Cunningham and Hindman, 1954) and plutonium (Cunningham, 1954) are also available. The properties of uranium (De Witt, 1960) and (Steindler, 1963) have also been reviewed. 306 κ. w. BAGNALL

2. The Trivalent Actinides A. General chemistry The stable for actinium, americium and the heavier actinide elements is +3, both in solution and in solid compounds. There is no evidence for this oxidation state for thorium or protactinium in aqueous solution, or for the latter in solid compounds, and one would expect such species to reduce water. Although there have been a number of reported preparations of solid bipositive or tripositive thorium halides, by way of reduction of the tetrahalide with thorium or alu­ minium at high temperatures, the results are often conflicting and it is only recently that the lower valency thorium iodides have become well established. In aqueous solution uranium(III) oxidizes readily, but neptunium(III) and plutonium (III) are a good deal more stable in this respect and their complexing behaviour is easier to investigate. Many of the data on the halide complexing of the actinides(III) in aqueous solution are derived from work on methods of separating the actinides from one another or from flssion products. Such complexing in aqueous solution is generally rather weak, as with the lanthanides(III), associa­ tion with halide ions being mainly through electrostatic interactions. It seems that halide ions can only displace the hydration water from the actinide(III) ions in very concentrated halide solutions; such stability constant data as are known indicate that the stabilities of the acti- nide(III) halo complexes are comparable to those of the lanthanides, although somewhat more stable than the latter where /-electrons are involved in the bonding. Spectrophotometric evidence for the UCla"'" ion has been obtained (Shiloh and Marcus, 1962) and similar studies show that neptunium(III), plutonium(III) and americium(III) (Shiloh and Marcus, 1962,1964,1966) form the ions MCP+and MCI2+. Americium(III) species of these types have also been identified by ion exchange (Grenthe, 1962; Peppard et aL, 1962), while extraction of americium(III) from solutions of high concentration indicates the formation of anionic complexes such as AmCl4- or AmClg^- (Marcus et aL, 1964), as also indicated by cation exchange studies (Diamond et aL, 1954; Choppin and Chetham-Strode, 1960; Choppin and Dinius, 1962). There is some evidence that the complex CmCP+ is more stable than PuCP+ or AmCP+ (Ward and Welch, 1956) and there is qualitative evidence showing that the higher actinides tend to form chloro complexes even more readily (Choppin and Chetham-Strode, 1960; Choppin and Dinius, 1962; Isaac et aL, 1960). ion complexes more weakly than chloride ion (Shiloh and Marcus, 1962, 1964) and there is no evidence for iodo complexes. Anionic complexes of a number of actinide trihalides THE HALOGEN CHEMISTRY OF THE ACTINIDES 307 have been identified in fused media and temperature-composition diagrams for plutonium trifluoride and trichloride in alkali and alkaline earth or chloride phases have been summarized (Leary, 1962), The electronic configurations of the lighter actinides are still by no means certain, since the 6d- and 5/-electron energies are very similar even at uranium; thus spectral and magnetic studies of the ion can be interpreted on the basis of a S/^Bii^-configuration (Dawson, 1951; Jezowska-Trzebiatowska, 1963) whereas the magnetic behaviour of UCI3 in LaClg approaches that of Nd(EtS04)3.9H2O and is consistent with a 5/3-configuration (Handler and Hutchison, 1956). 5/^-configurations for the higher actinides are, however, well established, for example by magnetic studies (Dawson et al., 1951).

B. Trifluorides The known actinide trifiuorides have the LaFg type structure (Table II), in which the central atom lies on a twofold axis and has 9 atoms at nearly equal distances (Zalkin et al., 1966). They are insoluble in water and hydrated salts are precipitated from aqueous solutions of the tervalent actinides on addition of hydrofluoric or a soluble fluoride. The anhydrous compounds are obtained by drying a slurry of the precipitated fluoride in concentrated (CmFg at 200°; Feay, 1954) or heating the precipitated fluoride in fluoride at 400° (Asprey et al., 1965a), and by heating the

TABLE II. Crystallographic data for the actinide trifluorides^

Colour Symmetry and Lattice parameters (A) Calculated space group «0 Co (gcm-3)

AcFs White Hexagonal, PQJmmc - 4-17 7-53 7-88 Dtj, (LaFg type) UF3 Violet-red Hexagonal, P6.Jmmc - or black DU (LaFg type) 4-146 7-348 8-95

NpF3 Purple or Hexagonal, P6Jmmc - black Dtn (LaFg type) 4-108 7-273 9-12

PuFg Purple Hexagonal, PQ^jmrnc - Dtn (LaFg type) 4-087 7-240 9-32 AmFg^ Pink Hexagonal, PQ^/mmc - Dtn (LaFg type) 4-067 7-225 9-53 CmFgto White Hexagonal, P6^/mmc - Dtn (LaFg type) 4-041 7-179 9-70

a Corrected data collected in Table IV of the review by Katz and Sheft (1960) unless stated otherwise. to Asprey et al. (1965a). 308 κ. w. BAGNALL sesquioxides in (AcFg at 700°, Fried et al., 1950, and AmFg at 650°, Fried, 1951; Westrum and Eyring, 1951). The more readily oxidized trifluorides (UFg, NpFg, PuFg) are usually prepared under reducing conditions; thus UFg is conveniently made by the reduction of the tetrafluoride in the absence of water with hydrogen at 1000° (Spencer-Palmer, 1944), with in a vacuum at 900°, aluminium subliming from the reaction vessel (Runnalls, 1953): UF4 + Al UF3 + AlF t and with finely divided uranium in an atmosphere at 1050° (Warf, 1949): 3 UF4 -f U ^ 4 UF3 or with at 560° (reduction to uranium occurs at 600°; Schwarz and Vaughan, 1953). An oxyfluoride, and at high tempera­ tures, an , is formed if water vapour is present. begins to disproportionate above 1000°; it precipitates from aqueous silver perchlorate and is oxidized to uranyl fluoride by boiling water. Its magnetic properties have also been recorded (Nguyen-Nghi et al, 1964). The neptunium and plutonium compounds are prepared by the action of an equimolar mixture of hydrogen and hydrogen fluoride on the dioxides, NpFg at 500° (Fried and Davidson, 1948) and PuFg at 600° (Florin, 1949). The latter is also obtained by heating almost any plutonium compound in hydrogen-hydrogen fluoride; thus pluto- nium(III) oxalate reacts at 550-660° (Reavis et αί.,1959). It is also formed when hydrated (PuF4.2-5ll20) is heated in a vacuum, apparently by way of hydrolysis to the dioxide which then reacts with the tetrafluoride (Dawson et al., 1954b):

3PUF4 + PuOg ^ 4 PuFg + O2 and by heating plutonium (III) or (IV) oxalate in dichlorodifluoro- (Freon 12) at 400-450° (Burger and Roake, 1952, 1961). Phase diagrams describing the fluorocomplexes formed in fused salt media have been summarized in some detail (Leary, 1962). A few tetra- fluoro complexes have been prepared by solid state reactions; thus the plutonium (III) and americium (III) salts, NaMF4, have been made by heating with the actinide trifluoride, or sodium carbonate or fluoride with the actinide sesquioxide or dioxide in a stream of hydrogen fluoride either alone (MgOg) or mixed with hydrogen (MO2) at 450-650°. They are hexagonal, isostructural with NaLaF^ (Keller and Schmutz, 1964). THE HALOGEN CHEMISTRY OF THE ACTINIDES 309

By analogy with , one would expect americium to form stable dihalides, but the only evidence for these to date is a report of the reduction of americium(III) in a fluoride matrix by its own radiation or by at 600°, the presence of Am'^+ being shown by its electron spin resonance spectrum (ground state ^Srj/^ as against '^FQ for Am^+). Reoxidation occurs on heating at about 500° (Edelstein et al, 1966).

C. Trichlorides The properties of the known actinide trichlorides are somewhat similar to those of the compounds; some crystallographic data are given in Table III. Actinium trichloride, a white solid, is pre­ pared by heating the with ammonium chloride at 250° in a vacuum (Farr et al., 1953) or by heating the hydroxide or oxalate in tetrachloride at higher temperatures (Fried et al., 1950); it sublimes in a vacuum at 960°. The uranium compound is best prepared

TABLE III. Crystallographic data for the actinide trichlorides*

Colour Symmetry and Lattice parameters (A) Calculated space group Density Go Co (gcm~^)

AcClg White Hexagonal, C6Jm --Cln 7-62 4-55 4-81 UC13 Rede Hexagonal, G6Jm--Gin 7-442 4-320 5-51 NpClg Green^ Hexagonal, C6Jm--GQU 7-405 4-273 5-58 PuCla Emerald Green Hexagonal, CG^Im --Ce% 7-380 4-238 5-70 AmCl3i> Pink Hexagonal, CGJm--Gl, 7-390 4-234 5-78 CmClg^ White Hexagonal, GQ^jm--Gin 7-368 4-228 5-81 a Corrected data collected in Table IV of the review by Katz and Sheft (1960) unless stated otherwise. ^ Asprey et al. (1965a). c Also described as olive green (Johnson et al., 1958). ^ Frequently described as white, probably due to a fine state of subdivision of the compound. by the action of on the at 250° (Johnson et al., 1958); reduction of the tetrachloride with hydrogen at 550-650° under pressure or with such as at 450-480° (Young, 1958) or with hydrogen at 300-350° have also been used. It dispro­ portionates at 840° (Shchukarev et al., 1956b) and is a strong reducing agent, being oxidized by water. at 250° oxidizes it to the tetra­ chloride, while or react to form uranium (IV) mixed halides; uranyl chloride and are formed when it is heated in air. It is insoluble in , , . 310 κ. w. BAGNALL and nonpolar and reacts with gaseous at 450-500° to form the amidochlorides UNHgCla and U(NH2)2C1. At higher temperatures (^^800°) these compounds decompose to form the imidochloride and the nitride UN1.73_1.75 (Berthold and Knecht, 1965a). The imidochloride has the same structure as the oxychloride (Berthold, 1966). Neptunium trichloride is prepared by chlorination of the dioxide with a mixture of hydrogen and carbon tetrachloride at 350-400° or by reduction of the tetrachloride with hydrogen at 450° (Fried and Davidson, 1948) or ammonia at 350-400° (Sheft and Fried, 1953) and it is also formed in the preparation of neptunium tetrachloride by chlor­ ination of the dioxide with hexachloropropene, probably as a result of reduction of the tetrachloride by carbonaceous material (Bagnall and Laidler, 1966). The plutonium compound is made on the milligram scale by reaction of the dioxide with carbonyl chloride at 500-850° (Ras- mussen and Hopkins, 1961; Boreham et al,, 1960); reaction is appreci­ able even at 350-400° (Tolley, 1953). It is also formed by the action of carbon tetrachloride on the dioxide at 450-500° (Fomin et al,, 1958a) but for gram scale preparations the action of carbonyl chloride on plutonium(III) carbonate at 500-550° or of hydrogen chloride on plutonium(III) or plutonium(IV) oxalate at 140-500° is more effective (Boreham et al,, 1960). Reaction of a mixture of hydrogen and hydrogen chloride with oxalates (Garner, 1959) or of hexachloropropene with plutonium(III) oxalate at 180-190° (Christensen and Mullens, 1952; Harder et al,, 1958) and of hydrogen chloride with plutonium hydride (Abraham et al., 1949; Reavis et al,, 1959) or direct preparation from the elements (Abraham et al,, 1949) are also satisfactory. The plutonium compound can also be made by heating the dioxide mixed with carbon, sulphur or in chlorine, by the action of sulphur dichloride and chlorine on the dioxide at 800°, by the action of phosphorus pentachloride on the dioxide or by liquid phase chlorination of plutonium peroxide with sulphur monochloride and chlorine at 280° (Davidson and Katz, 1958). Thermal decomposition of dipyridinium hexachloroplutonate(IV) in a vacuum at 390-600° or in argon at 220-470° does not yield a pure product (Harder et al,, 1958), and the action of hydrogen chloride on plutonium dioxide at 200-1000° yields a mixture of trichloride and oxychloride (Davidson and Katz, 1960). The green anhydrous compound is hygroscopic, forming the blue hexa- hydrate, which is isomorphous with the compound (David­ son and Katz, 1960). Thermal analysis of the PUCI3-KCI indicates the existence of KaPuClg (m.p. 685°) and, possibly, KaPuClg (Benz et al., THE HALOGEN CHEMISTRY OF THE ACTINIDES 311

1959); KaUClg and K2UCI5 have been identified in the UCI3-KCI system (Kraus, 1943) but there is no evidence of compound formation in the PuCla-LiCl and PuCla-NaCl systems (Bjorklund et al, 1959). Similarly, there is evidence for Rb3(Cs3)PuCl6, Rb(Cs)Pu2Cl7 and RbgPuCls in the PuCl3-RbCl and PUCI3-CSCI fused salt systems (Benz and Douglass, 1961b) and for the formation of complexes of the type M3PUCI9 (M = Sr, Ba) under similar conditions, but there is no evidence of compound formation with magnesium or calcium (Johnson, K.W.R. et al, 1961). The chloro complex, CS3PUCI6.2H2O, has been isolated from 6N solution of plutonium (III) in the presence of an excess of ; it melts in air at 100° with oxidation of the plutonium to the dioxide (Stevens, 1965). Americium- (III) behaves in a more complex manner than plutonium(III), the hydrated complex CsAmCl4.4H20 being obtained from llM hydro­ chloric acid; in the presence of , however, the hexa­ chloro complex, CsgNaAmClg, is obtained (Bagnall et al, 1967c). The anhydrous triphenylphosphonium salt (Ph3PH)3AmCl6, crystallizes from alcoholic hydrochloric acid, and the visible spectrum of its solution shows that the AmClg^" group is octahedral (J. L. Ryan, personal communication). Americium trichloride is prepared by the reaction of the dioxide with carbon tetrachloride at 800° to 900° (Fried, 1951 ; Hall and Markin,1957) or with hydrogen chloride (Broido and Cunningham, 1950). It sublimes in a vacuum at 850°. Both americium and curium trichlorides, however, are most easily made by evaporating to dryness a hydrochloric acid solution of the trivalent element containing ammonium chloride and subliming the last from the residue (Asprey et al, 1965a). The califor­ nium compound has been made on the submicrogram scale by the action of hydrogen chloride on the sesquioxide at 450° (Cunningham, 1961).

D. Tribromides Most of the actinide tribromides (Table IV) have been made by heating the oxide at moderate temperatures with , formed in situ from the elements; thus AcBr3 is formed at 750° (Fried et al, 1950), NpBrg from the dioxide at 350-400° in the presence of excess aluminium to reduce the tetrabromide which is formed (Fried and Davidson, 1948) and AmBr3 from the dioxide at 500° (Fried, 1951). Americium and curium tribromides are, however, most easily prepared by heating the trichlorides with ammonium bromide at 400-450° in hydrogen (Asprey et al, 1965a). Thorium tribromide is said to be formed by reduction of the tetra­ bromide by hydrogen at 360° in the absence of moisture (Shchukarev 312 κ. w. BAGNALL

TABLE IV. Crystallographic data for the actinide tribromides*

Colour Symmetry and space group Lattice parameters (A) Calculated density «0 60 CO (gcm-3)

AcBrg White Hexagonal C6Jm — Ci» 8-06 4-68 5-85 UBrg Red Hexagonal C6Jm —Glj^ 7-942 — 4:'U0 6-53 a-NpBrg Green Hexagonal CQJm—C^j^ 7-917 — 4-382 6-62 jg-NpBrg Green Orthorhombic Cmcm — DU 4-11 12-65 9-15 6-62 PuBrg Green Orthorhombic Cmcm — DU 4-09 12-62 9-13 6-69 AmBrgto White Orthorhombic Cmcm —DU 4-064 12-66 9-144 6-79 CmBrgto White Orthorhombic Cmcm — D\l 4-048 12-66 9-124 6-87

* Corrected data collected in Table IV of the review by Katz and Sheft (1960) unless stated otherwise. t> Asprey et al. (1965a). et al., 1956a) but this claim, hke others for thorium trichloride and tri­ bromide, is very doubtful. Uranium tribromide is made by the action of on the hydride at about 300° (Spedding et al., 1958) and, less satisfactorily, by hydrogen reduction of the tetrabromide at 600-700° and by reaction of stoicheiometric quantities of the elements at about 570° (Eastman et al., 1958a). It disproportionates above 900° and is more hygroscopic than the trichloride; it dissolves in water with the evolution of hydrogen and with dry ammonia forms the ammine UBrg.eNHg. Uranium tribromide is insoluble in non-polar solvents and dissolves in, or reacts with, polar solvents and cannot be recovered from its solutions unchanged (Spedding et al., 1958). Uranium is soluble in the molten tribromide but lower halides have not been isolated (Eastman et al., 1958b; Corbett et al., 1963). It reacts with glass or quartz at high temperatures, forming the tetrabromide, dioxide and disilicide, the last presumably being formed by reaction of silica with uranium metal liberated in the disproportionation. Plutonium tribro­ mide is best made from the elements at 300° (Davidson et al., 1949) or by the action of hydrogen bromide on the hydride at 600° (Reavis et al., 1959) or on plutonium(IV) oxalate hexahydrate at 500° (Fomin et al., 1958b). The reaction of plutonium dioxide with hydrogen bromide, or with a mixture of and bromine, is never quantitative even at temperatures above 800°. It is also reported to be formed by reaction of plutonium dioxide with bromine and sulphur (Davidson and Katz, 1958) and by evaporating a solution of plutonium- (IV) hydroxide in 5M hydrobromic acid to dryness in a stream of hydrogen bromide and subsequent heating in hydrogen bromide at 300° or with ammonium bromide at 350° and 10~^ mm (Davidson and Hyde, THE HALOGEN CHEMISTRY OF THE ACTINIDES 313

1958). Very little is known about these compounds, or about the cor­ responding iodides, apart from preparative details, crystallographic data (Tables IV and V) and magnetic data for UBrg and UI3 (Dawson, 1951).

E. Tri-iodides The actinide tri-iodides are usually made by reaction of aluminium iodide or ammonium iodide with the oxide; Aclg, although not definitely identified, appears to be formed at 500-700° (Pried et al., 1950), Nplg from the dioxide and aluminium iodide at 350-400° (Fried and Davidson, 1948) and Amlg similarly at 500° (Fried, 1951) or by heating the tri­ chloride with ammonium iodide at 400° in hydrogen, a method which is equally applicable to the curium compound. Americium tri-iodide is not reduced by hydrogen at high temperatures (Asprey et al., 1965a). Crystallographic data are summarized in Table V. Uranium tri-iodide is

TABLE V. Crystallographic data for the actinide tri-iodides*

Colour Symmetry and space group Lattice parameters (A) Calculated density «0 Κ Co (gcm-3)

UI3 Black Orthorhombic, Cmcm- 4-32 14-01 10-01 6-76 Npla Brown Orthorhombic, Cmcm - 4-30 14-03 9-95 6-82 Pul3^ Bright Orthorhombic, Cmcm-'DU 4-33 13-95 9-96 6-92 green Amls^.c Yellow Hexagonal, Β'^-ΟΙ^ 7-42 — 20-55 6-04 Cml3c White Hexagonal, 7?--σ|^ 7-44 — 20-4 6-37

a Corrected data collected in Table IV of the review by Katz and Sheft (1960) unless otherwise stated. i> Asprey et al. (1964). c Asprey et al. (1965a).

formed by reaction of the hydride with methyl iodide at 275-300° (Ayres, 1944) or with iodine vapour (Corbett et al., 1963), by reduction of the tetraiodide in hydrogen (Katz and Rabinowitch, 1951, p. 538) and from stoicheiometric quantities of the elements at 700-750° (Popov and Senin, 1957) or at 525° at low pressure (Gregory, 1944), although reaction of iodine vapour with the massive metal is extremely slow (Corbett et al., 1963). Uranium tri-iodide melts at 766-5° (Popov and Senin, 1957) and attacks glass at 800° (Ayres, 1944). The plutonium compound is formed by heating the metal in at 450° (Hagemann et al., 1949) or with mercuric iodide at 500° in a sealed tube (Asprey et al., 1964). The earlier, and conflicting, reports of the preparation of thorium tri-iodide and di-iodide have now been resolved by recent work which shows that a black tri-iodide, which may contain the Th^+ ion, is 314 κ. w. BAGNALL

formed by the reduction of the tetraiodide with thorium metal in tantalum or vessels. It is not, however, isomorphous with the uranium compound. Two forms of the di-iodide have also been identified, likewise made by reduction of the tetraiodide with thorium metal ; the dull black α form is obtained at 600°, but the reaction is never complete, and the golden β form is obtained at 800°. The χ to β transformation occurs sluggishly at 600-700°; both are of hexagonal symmetry. All three compounds disproportionate at higher temperatures and decom­ pose water vigorously with the evolution of hydrogen (Scaife and Wylie, 1964).

F. Mixed halides Many uranium (III) mixed halides have been recorded, prepared by thermal decomposition or hydrogen reduction of the uranium(IV) mixed halides, and by fusing the stoicheiometric quantities of the tri­ halides; a detailed account of these is given by Gregory (1958).

G. Oxyhalides Many of the actinide trihalides are converted to oxyhalides by vapour phase hydrolysis, the products usually being identified by X-ray crystallography (Table VI); most of them are known to be insoluble in

TABLE VI. Crystallographic data for the actinide (III) oxyhalides

Colour Symmetry and space Lattice parameters (A) Calculated group or structure type density

«0 bo Co (gcm-3)

AcOFa White Cubic, CaFa 5-94 8-28 PuOFi> Metallic Tetragonal, PbFCl 4-05 — 5-72 9-70 (P4/nmm-Z)ift) AcOCla White Tetragonal, PbFCl 4-25 — 7-08 7-23 {Pélnmm-Dlj,) UOClc Red Tetragonal, PbFCl 4-00 — 6-85 8-78

PuOCld Green or Tetragonal, PbFCl 4-01 — 6-79 8-8 blue-green {P4:lnmm-Dl^) AmOCle White Tetragonal, PbFCl 4-00 — 6-78 8-96 (P4/nmm-DÎ,) AcOBra White Tetragonal, PbFCl 4-28 — 7-41 7-9 {P4:lnmm-Dlj,) PuOBrd Deep Green Tetragonal, PbFCl 4-02 — 7-57 9-1 {P4:lnmm — Dlj,) PuOId Bright Green Tetragonal, PbFCl 4-04 — 9-17 8-5 {P4:lnmm-Dlj,) a Fried et al. (1950). d Zachariasen (1949a). ^ Zachariasen (1951). e Templeton and Dauben (1953). c Shchukarev and Efimov (1957). THE HALOGEN CHEMISTRY OF THE ACTINIDES 315

water, but soluble in dilute acids. The actinium compounds are made by- hydrolysis with ammonia and water vapour, AcOF and AcOCl being obtained at 900-1000°, AcOBr and AcOI at 500°. AcOF, in contrast to LaOF, cannot be made by heating the trifluoride in air (Fried et at., 1950). The only recorded uranium(III) oxyhalide is the red chloride; this remains in the residue, mixed with uranium dioxide, left when uranium trichloride is sublimed; it is separated from the oxide by dé­ cantation in water, to which it is quite inert, in contrast to the uranium trihalides (Shchukarev and Efimov, 1957). Neptunium (III) oxyhalides are not known, presumably because they have not been sought, but all four plutonium(III) compounds have been described. The oxyfluoride was observed when the trifluoride was melted in argon, presumably a result of the presence of traces of water (Robinson, 1944); the chloride has been made by heating the hydrated trichloride in a sealed tube at 400° (Abraham et al., 1949) and by hydro­ lysis of the trichloride with a mixture of hydrogen, water vapour and hydrogen chloride at 400-520° (Davidson and Katz, 1960). The bromide is obtained by hydrolysis of the tribromide at 400° (Da\âdson et al., 1949) or by heating the dioxide in moist hydrogen bromide at 750° (Sheft and Davidson, 1949), and the iodide by heating dried pluto- nium(IV) hydroxide with hydrogen and hydrogen iodide at 750° (Hage­ mann et al. 1949). Americium (III) oxychloride is obtained by vapour phase hydrolysis of the trichloride or by heating the sesquioxide in a mixture of hydrogen chloride and water vapour at 500° (Koch and Cunningham, 1954), a reaction which, at 450°, has been used to prepare the compound (Cunningham, 1961).

3. The Tetravalent Actinides A. General chemistry Tetrafluorides are known for all the actinides from thorium to curium inclusive and, by analogy with the lanthanides, should also form a tetrafluoride; compounds with halogens of higher , however, become increasingly less stable on passing up the series from thorium. Thus the simple tetrachlorides and tetra­ bromides of plutonium and of the higher actinides are unknown, although complexes derived from both plutonium tetrachloride and tetrabromide can be prepared. Plutonium trichloride (Benz, 1962) and tribromide (Fomin et al., 1958b) become appreciably more volatile in the presence of the appropriate halogen, which suggests that these tetrahalides may exist in the vapour phase under such conditions. 316 κ. w. BAGNALL

Tetraiodides of neptunium and of the higher actinides do not exist and even uranium tetraiodide is relatively unstable to heat. The tetrafluorides are all insoluble in water and are not appreciably hygroscopic, whereas the other tetrahalides are all very hygroscopic, readily forming hydrates in moist air. The quadrivalent actinides can be regarded as nearly pure Chatt-Ahrland A-type elements, the fluoro complexes being the most stable, and the iodo complexes the least stable of the halo complexes; in general, oxygen donor ligands appear to form complexes with the tetrahalides more readily than nitrogen donor ligands and simple phosphorus and sulphur donor ligands do not form stable complexes. The ions of protactinium and the higher actinides all have the 5/^-configuration, established from absorption of the tetrahalides (J0rgensen, 1959; Pa^+—Fried and Hindman, 1954; Axe et al., 1960; —Jezowska-Trzebiatowska and Bukietynska, 1961), magnetic susceptibility data (U^+—Dawson, 1951; Jezowska-Trzebia­ towska, 1963) and paramagnetic resonance absorption (Pa^+—Axe et al., 1961). The general methods used for the preparation of thorium (Katzin, 1954) and uranium (Katz and Rabinowitch, 1951; Gregory, 1958) tetrahalides have been exhaustively reviewed.

B. Tetrafluorides has been made by the reaction of the metal with hydrogen fluoride, for example in a sealed tube at 225° (Muetterties and Castle, 1961) or by reaction of fluorine with the tetrachloride or tetrabromide at room temperature (Moissan and Martinsen, 1905) and by the action of hydrogen fluoride on thorium hydride at 250-350° (Lipkind and Newton, 1952), which is the equivalent to the use of the finely divided metal because of thermal decomposition of the hydride, or on the tetrabromide at 350-400° (Chauvenet, 1911). It is more con­ veniently made by reaction of hydrogen fluoride with the low-fired dioxide at 550° (Newton et al., 1952) or with anhydrous thorium acetate (Gentile and Snyder, 1957) and by dehydration of the tetrafluoride hydrates (ThF4.2-5H20, ThF4.0-5H2O) at 250-300°, usually in a vacuum (D'Eye and Booth, 1955, 1957; Gagarinskii and Mashirev, 1959a). Reaction of (Freon 12) with thorium dioxide at 330-400°, or with the hydrated tetrafluoride or ammonium fluoro complexes at 350-500°, provides a useful alternative route to the anhydrous tetrafluoride (Cacciari et al., 1956, 1957). Anhydrous protactinium tetrafluoride, a reddish brown solid, is obtained by hydrofluorination of the dioxide or hydrated pentoxide (Sellers et al., 1954) and by the action of an equimolar mixture of THE HALOGEN CHEMISTRY OF THE ACTLNIDES 317 hydrogen and hydrogen fluoride on the pentoxide at 500° (Stein, 1964). It is slowly hydrolysed in moist air and is isomorphous with the uranium compound. is made in much the same way as the thorium compound, by heating the metal with liquid hydrogen fluoride in a sealed tube at 250° (Muetterties and Castle, 1961), by the action of hydrogen fluoride on the dioxide (e.g. Dawson et al,, 1954a), a process used on the industrial scale (Kuhlman and Swinehart, 1958) and by the reaction of the dioxide with sulphur tetrafluoride at 500° (Johnson, C. E., et al, 1961) or of the trioxide with Freon 12 at 400° (Booth et al, 1946; Cacciari et al, 1956, 1957). It is also obtained by heating uranium dioxide with an excess of or bifluoride (Braddock and Copenhafer, 1943) a reaction in which the pentafluorouranate(IV), NII4UF5, is flrst formed (Van Impe, 1954; Neumann et al, 1962). This decomposes to the tetrafluoride above 320° (Galkin et al, 1961). A similar reaction with uranium(IV) acetate at 450° also yields uranium tetrafluoride (Sahoo and Patnaik, 1959). The structure of uranium tetrafluoride consists of 8 fluorine atoms arranged around the uranium atom in a slightly distorted antiprism configuration (Larson et al, 1964), an 8-coordinate arrangement which probably applies to the remaining actinide tetrafluorides, all of which have the same crystal symmetry (Table VII). The preparation of the tetrafluorides of the higher actinides requires progressively stronger oxidizing conditions. Neptunium tetrafluoride is

TABLE VII. Crystallographic data for the actinide tetrafluorides*

Colour Sjrmmetry and Lattice parameters (A) Calculated space group density (gcm-3) Κ Co

ThF^ White Monoclinic, C210—0^2 η 131 1101 8-6 5-71 α2=126±1° Green Monoclinic, C2lc—C^ji 12-73 10-75 8-43 6-70 aa=126°20' NpF, Green Monoclinic, C2lc—G^ji 12-70 10-64 8-41 6-8 α2-126°10' PUF4 Brown Monoclinic, C2lc--G92h 12-62 10-57 8-28 7-0 α2=126°10' AmF^ Tan Monoclinic, (72/c—(7f ^ 12-49 10-47 8-20 7-34 α2=126°10' CmFi Greenish- Monoclinic, C2lc—Clf^ 12-45 10-45 8-16 7-49 tan α2=126°±30' a Corrected data collected in Table IV of the review by Katz and Sheft (1960) unless otherwise stated. to Larson et al. (1964). 318 κ. w. BAGNALL

obtained by the action of a mixture of hydrogen fluoride and oxygen on the trifluoride at 500° (Fried and Davidson, 1948), conditions under which uranium would oxidize to uranyl fluoride. The plutonium com­ pound is made by the action of fluorine on the trifluoride at 300°, and of a mixture of hydrogen fluoride and oxygen on the trifluoride at 550° or on the dioxide at 550-600° (Florin and Heath, 1944). The hydrated tetrafluoride, obtained from aqueous solution, can be dehydrated in a mixture of hydrogen fluoride and oxygen at 350° (Meyer and Zvolner, 1944). Plutonium tetrafluoride is also formed by reaction of the dioxide with sulphur tetrafluoride at 600° (Johnson, C.E., et aL, 1961) and by heating the dioxide with ; the ammonium penta- fluoroplutonate(IV) formed in this reaction decomposes at about 280° (Maly et aL, 1961; Tolley, 1954). Plutonium tetrafluoride appears to disproportionate above 1200°, with the appearance of a more volatile species (Mandleberg and Davies, 1961), presumably the hexafluoride formed by reaction of the tetrafluoride with traces of oxygen; the only species which sublimes below 1000° is the tetrafluoride (Berger and Gaumann, 1961). Americium tetrafluoride is obtained by the action of fluorine on the trifluoride or dioxide at 500° (Asprey, 1954) and the curium compound by the action of fluorine on the trifluoride at 400°, but only with the longer-lived , curium-244, radiation damage inhibiting this reaction with the short-lived curium-242 (Asprey et aL, 1957). The existence of curium tetrafluoride shows clearly that the stability of the half-filled 5/-shell is much lower than in the lanthanide case. The visible spectra of these solid tetrafluorides have been recorded (Asprey and Keenan, 1958). Hydrated tetrafluorides, MF4.2-5H20, of thorium (D'Eye and Booth, 1955, 1957), uranium (Katz and Rabinowitch, 1951; Dawson et aL, 1954a) and plutonium (Dawson et aL, 1954b; Deichmann and Tananaev, 1961) are precipitated from aqueous solutions of the quadrivalent ele­ ments by hydrofluoric acid; the white compound precipitated from protactinium(IV) solution (Haissinsky and Bouissières, 1951) is also probably of this type. A neptunium compound of this form should exist, but does not appear to have been recorded. Hydrated americium and curium tetrafluorides cannot be obtained from aqueous solutions since both americium(IV) and curium(IV) are unstable in water in the absence of high concentrations of fluoride ion. The hydrates MF4.2-5H20 lose water on heating, the thorium com­ pound yielding the hemihydrate, ThF4.0-5H2O (D'Eye and Booth, 1955, 1957), and the uranium compound yielding UF4.0-4H2O (Gagarinskii and Mashirev, 1959b; Gal'chenko et aL, 1960). Both can be completely THE HALOGEN CHEMISTRY OF THE ACTINIDES 319

dehydrated by heating, usually in a vacuum. The precipitation of UP4.2-5H20, which exists in both cubic and monoclinic modifications, from sulphate solution by fluoride ion appears to take place by way of an intermediate UFg^^ species (Tananaev and Savchenko, 1962b) and a fluoro-oxalate, UF2(C204).1-5H20, has been obtained by heating UF4.2-5H20 with saturated oxalic acid at 100° and the anhydrous compound (UF)2(C204)3 by heating UF4.2-5H20 with oxalic acid dihydrate at 200° (Tananaev and Savchenko, 1962a). Similar behaviour has been reported for protactinium(IV), the compound PaF2(S04).2H20 being isolated from aqueous solution (Stein, 1965) and for plutonium(IV), particularly in sulphate solution (Deichmann and Tananaev, 1961). The of UF4.2-5H20 in hydrofluoric-perchloric acid mixtures can likewise be explained by the presence of UF2^+ species (Savage and Browne, 1960). There are many published procedures for the preparation of UF4.2-5H20 (see Katz and Rabinowitch, 1951); a particularly con­ venient one is by electrolytic reduction of uranyl fluoride solution at a cathode (Nikolaev and Luk'yanychev, 1961). A hydrate of composition UF4.4/3H2O is also known (Gagarinskii et al., 1965).

C. Tetrachlorides (i) Preparation and Properties Reviews of the preparation and properties of thorium (Flahaut, 1963) and uranium (Oxley, 1962) tetrachlorides have recently appeared; these discuss the following, and other, preparative procedures in greater detail. The tetrachlorides are commonly made by chlorination of the dioxide with carbon tetrachloride vapour, the thorium compound at about 800° (Matignon and Delépine, 1901, 1908), protactinium tetrachloride at 500° (Sellers et al., 1954), uranium tetrachloride at 450° (Katz and Rabinowitch, 1951) or 500° (Harrison, 1958) and the neptunium com­ pound at 530° (Fried and Davidson, 1948). Mixtures of thorium or uranium oxides and carbon react quite readily with chlorine at elevated temperatures, a procedure long used for the preparation of thorium (Berzelius, 1829) and uranium (Péligot, 1842b) tetrachlorides; the reaction proceeds rather better in the presence of ferric chloride in molten KCl-NaCl at 800° (Gibson et al, 1960). The chlorination of uranium oxides by carbon tetrachloride or hexachloropropene proceeds by way of UOCI3 (Budaev and Vol'skii, 1958). Mixtures of carbon tetra­ chloride and chloroform have also been used to chlorinate uranium dioxide at 400-500° (Rosenfeld, 1960). Thorium tetrachloride is conveniently made from the elements at 320 κ. w. BAGNALL

800° (Fowles and Pollard, 1953), by reaction of the carbide with chlorine (Dean and Chandler, 1957, who have also reviewed the preparative procedures available for this compound) or of hydrogen chloride with the hydride at 250-350° (Lipkind and Newton, 1952) or with the metal (Kruss and Nilson, 1887) and by dehydration of the hydrate with thionyl chloride (Bradley et al., 1954; Freeman and Smith, 1958), or by heating with pyridine hydrochloride (Didchenko, 1959). It can be purified by heating with ammonium chloride, followed by sublimation through thorium metal turnings (Skaggs and Peterson, 1958). Pure thorium tetrachloride is also said to be obtained by the reaction of carbon tetrachloride with thorium tetraiodide at 100-200° (Watt and Malhotra, 1960). Other methods of preparing thorium tetrachloride include thermal decomposition of ammonium pentachlorothorate at 500° and chlorination of thorium dioxide with carbonyl chloride at 650-700° (Chauvenet, 1911). A more convenient procedure for the preparation of protactinium tetrachloride is by hydrogen reduction of the pentachloride at 800° (Elson et al., 1950; Sellers et al., 1954); because of the volatility of both the starting material and the product the reaction is best carried out in a sealed tube at 400-500° or, more safely, with aluminium (Brown and Jones, 1966c). Uranium tetrachloride is best made by reaction of UO3 or UgOg with hexachloropropene (Hermann and Suttle, 1957), but reaction of hexa­ chloropropene with NpOa.HgO or NpgOg yields a mixture of neptunium tetrachloride and trichloride (Bagnall and Laidler, 1966). Uranium tetrachloride is also obtained by the action of chlorine on the trichloride at 250° or of a mixture of chlorine and (1:10) on the hydride. There is thermal and X-ray evidence for a crystal transformation at 545° (Johnson et al., 1958). Other methods include reaction of the tetrafiuoride with aluminium or trichloride at 250-500° in a sealed tube (Calkins and Larsen, 1945; Fried, 1945), reaction of sulphur mono­ chloride with under reflux (Uhlemann and Fischbach, 1963), and reaction of the elements at about 650° (Reynolds and Wilkinson, 1956). Thorium and uranium tetrachlorides have an 8-coordinate structure with the metal-chlorine bonding intermediate between covalent and ionic, four of the Th—Cl distances being 2-46 A and four 3-11 A; in UCI4 the U—Cl distances are 2-41 and 3-09 A (Mooney, 1949). Prot­ actinium and neptunium tetrachlorides are isostructural with uranium tetrachloride. Crystallographic data are summarized in Table VIII. A wide variety of hydrated and partially hydrolysed species derived from thorium and uranium tetrachloride have been recorded in the early THE HALOGEN CHEMISTRY OF THE ACTINIDES 321

TABLE VIII. Crystallographic data for the actinide tetrachlorides, tetrabromides and tetraiodides*

Colour Symmetry and space group Lattice parameters (A) Calculated density «0 60 Co (gcm-3)

ThCl^ White Tetragonal, làjamd- 8-473 — 7-468 4-60 PaCl4 Greenish- Tetragonal, I^tjamd--Dl\ 8-377 — 7-482 4-72 Yellow UCI4 Green Tetragonal, 14:jamd- 8-296 — 7-487 4-87 NPCI4 Red-brown Tetragonal, /4/amcZ- 8-29 — 7-46 4-92 ThBr^ White Tetragonal 8-963 — 7-946 5-69 UBr, Brown Monoclinic, 21c —I— 10-92 8-69 7-05 5-55 j3=93°9' NpBr^ Reddish- Monoclinic, 21c —1 — brown Thl^to White Monoclinic P2i/„ 13-216 8-068 7-766 6-00 )δ = 98-68° a Corrected data collected in Table IV of the review by Katz and Sheft (1960) unless otherwise stated. i> Zalkin et al. (1964). literature; summaries of the preparation and properties of these com­ pounds are given by Flahaut (1963), Katz and Rabinowitch (1951), Katzin (1954) and Oxley (1962). The uranium tetrachloride hydrates (4-5 to 9 HgO) have recently been the subject of further investigation (Pommier, 1966). Studies of the conductivity of uranium tetrachloride in aqueous indicate the presence of complex species such as UCI3+ and UClg^"^ (Roach and Amis, 1962) and similar species have been shown to exist in aqueous hydrochloric acid solutions of thorium(IV) (Zebroski et al,, 1951; Waggoner and Stoughton, 1952) and plutonium(IV) (Grenthe and Noren, 1960). The visible spectra of uranium tetrachloride in nonaqueous solvents have also been recorded and discussed (Ewing, 1961; Jezowska-Trzebiatowska et al,, 1958).

(ii) Complexes Triscyclopentadienyl chlorides are known for both thorium and uranium, the former obtained by reaction of cyclopenta- dienide with thorium tetrachloride in ether (Ter Haar and Dubeck, 1964), the latter by reaction of sodium cyclopentadienide with uranium tetrachloride in (Reynolds and Wilkinson, 1956). Both compounds are very sensitive to moisture. The uranium compound is monoclinic (P2i/^), with the three cyclopentadiene rings and the chlo­ rine atom arranged approximately tetrahedrally around the uranium atom (Chi-Hsian Wong et al,, 1965). 322 κ. w. BAGNALL

Most of the known complexes of the tetrachlorides are those formed by oxygen donor ligands; uranium tetrachloride forms an adduct with phosphorus oxychloride, UCI4.4 POCI3, from the components and this decomposes to UCI4.POCI3 when heated in a vacuum (Panzer and Suttle, 1960b). Adducts of the form UCI4.2L, where L is tri-n-butyl phosphate (TBP) or di-isoamylmethyl phosphonate, are said to be extracted by these ligands from aqueous hydrochloric acid solutions of uranium(IV) (Shevchenko et al., 1961); however, studies of the absorp­ tion spectra of mixtures of uranium tetrachloride and TBP in carbon tetrachloride indicate that UCI4.3 TBP is formed, the failure to form the 8-coordinate 1:4 complex probably being due to steric hindrance (Lipovskii and Yakovleva, 1964). The existence of a 1:1 complex with TBP has also been established by absorption spectroscopy (Jezowska- Trzebiatowska et al., 1958). Phosphine oxide complexes, UCI4.2R3PO, are obtained by oxidation of a mixture of uranium tetrachloride and the appropriate phosphine in ethanol by chlorine, or directly from the components in tetrahydrofuran (Gans and Smith, 1963) or methyl cyanide (Day and Venanzi, 1966a). The configuration of UCl4.2Ph3PO has been shown by infrared spectro­ scopy to be trans octahedral (Day and Venanzi, 1966a). Trimethyl­ phosphine oxide also forms a 1:3 complex with uranium tetrachloride and a 1:1 adduct is formed by the last with methylene and ethylene bisdiphenylphosphine oxide, as would be expected (Gans and Smith, 1964a). Adducts with triphenylphosphine or with triphenylphosphine sulphide cannot be obtained (Gans and Smith, 1964a), contrary to an earlier report (Allison and Mann, 1949) of the formation of the trialkyl- phosphine complexes UCI4.2R3P. The supposed uranium tetrachloride complex with triphenylphosphine and cyclohexanol (Majumdar et al., 1964) is a phosphine oxide complex (Fitzsimmons et al., 1966). Adducts with 1,2-dimethylthioethane (UCI4.2L), tetra-P-methylenediphosphine (UCI4.L) and 1,2-dimethoxyethane (UCI4.2L), prepared from the com­ ponents, have also been reported (Mannerskantz et al., 1963). The complexes formed by thorium and uranium tetrachlorides with tri-iV^iV-dimethylphosphoramide, MCI4.2L, are remarkably stable to heat, subliming unchanged in a non-static vacuum at 220-235° (Bag­ nall et al., 1966a). The P=0 shifts in the infrared spectrum are 159 and 167 cm~^ respectively for the thorium and uranium compounds, indica­ tive of very strong bonding by way of the phosphorus oxygen atom. Adducts of thorium and uranium tetrachlorides with nitrosyl chloride have also been recorded, ThCl4.2NOCl (Perrot and Devin, 1958) and UC14.1-8-2-3NOC1 (Addison and Hodge, 1961); the latter is formed THE HALOGEN CHEMISTRY OF THE ACTINIDES 323 in the reaction of uranium metal with nitrosyl chloride, the former from the component halides. These may be nitrosonium salts. Adducts with alcohols, UCI4.4ROH (R - Me, Et, n-Pr and i-Pr) are obtained by evaporation of solutions of uranium tetrachloride in these alcohols, whereas solvolysis occurs with the butyl alcohols (Bradley et al., 1962); thorium tetrachloride forms adducts of this type even with n-butyl alcohol, reacting only with tertiary alcohols (Bradley et ah, 1954). Thorium tetrachloride also forms complexes with aldehydes, ThCl4.2L, obtained when acetaldehyde (Rosenheim et ah, 1903) or cinnamaldehyde in ether (Rosenheim and Levy, 1904) are heated with thorium tetra­ chloride; benzaldehyde reacts with the tetrachloride under these con­ ditions, hydrogen chloride being evolved. The complexes formed by the tetrachlorides with amides fall into two groups; thorium tetrachloride forms crystalline compounds of the type ThCl4.4L with A^A^- (DMP) (Moeller and Smith, 1958), and iV^i\^-dimethylacetamide (DMA) (Bagnall et ah, 1964b), both prepared from the components; the latter is a non-electrolyte in nitro- methane or acetone. Infrared spectra of the DMA complex indicate that the ligand is coordinated by way of the carbonyl oxygen atom. The DMA complex decomposes stepwise in vacuum to the complexes ThCl4.3DMA, ThCl4.2DMA and ThCl4.DMA. The uranium, neptunium and plutonium tetrachloride-DMA com­ plexes all have the composition 2MCI4.5DMA; they are easily made by treating the dicaesium hexachlorometallate(IV), CsgMCle, with DMA in acetone solution and crystallizing the product from the filtrate (Bagnall et ah, 1961). The three complexes are isostructural and the ligand is coordinated by way of the carbonyl oxygen atom; they are non-electro­ lytes in nitromethane. However, thermal decomposition studies indicate that one of ligand is bridging and it seems probable that the structure is of the form shown in Fig. 1 (p. 324), with the metal atoms exhibiting 8-coordination (Bagnall et ah, 1965b). Simple 6-coordinate complexes of uranium, neptunium and plutonium tetrachlorides, MCI4.6L, are formed with acetamide and are made in the same way as the DMA complexes; the infrared spectra show a carbonyl frequency shift of 120 cm-i, indicative of strong bonding (Bagnall et ah, 1961). iV'-Methylacetamide (NMA) yields the simple complex UCI4.4NMA under similar circumstances; it is a non-electrolyte (Bagnall et ah, 1964b). The magnetic and spectral properties of these uranium tetra­ chloride complexes have also been investigated (Bagnall et ah, 1964a). 324 κ. w. BAGNALL

Cl .CL DMA X/^DMA Cl—U: -DMA-i-U—Cl Cl dma' Cl Cl

FiG. 1.

The analogous complex of uranium tetrachloride with DMP has also been recorded; its composition is variously reported to be UCI4.3DMP (Lamisse et al, 1964) or 2UCI4.5DMP (Cans and Smith, 1964b), the latter being the more likely. The complex was prepared by dissolving uranium tetrachloride in DMF and either evaporating the solution in a vacuum or precipitating it from solution with . Thioacetamide does not form a complex with uranium tetrachloride (Bagnall et al, 1961). Complexes of thorium and uranium tetrachlorides with NNN'N'- tetramethyl dicarboxylic acid amides have recently been prepared (Bagnall et al, 1966b). These have the compositions 2MCI4.3L (L == glutaramide and 3,3-dimethylglutaramide), 2ThCl4.3L and UCI4.L (L = a, a-dimethylmalonamide), ThCl4.2L and 2UCI4.3L (L = malon- amide); they are probably polymeric. 2ThCl4.3HMGA (HMGA = 3,3- dimethylglutaramide) is essentially dimeric in methyl cyanide. Complexes with dimethylsulphoxide (DMSO), ThCl4.5DMSO and UCI4.3DMSO, are readily formed; both are non-electrolytes and are monomeric in boiling methyl cyanide. It is unfortunate that the only in which these compounds will dissolve for such measurements has some donor properties, so that the unusual 9- and 7-coordination in the compounds cannot be established unambiguously (Bagnall et al, 1966a). The S=0 band shifts in the infrared spectrum are 108 and 103 cm"^ respectively for the thorium and uranium compounds. Both decompose on heating in a vacuum, losing ligand at below 100° (ThCl4. 5DMS0) or 125° (UCI4.3DMSO) and ultimately yielding ThOClg (above 450°) or oxides of uranium (above 475°). The analogous 1:3 complex of uranium tetrachloride with diphenylsulphoxide has also been reported (Selbin et al, 1966). Adducts of thorium and uranium tetrachloride with tetrahydrofuran (Herzog et al, 1963), of thorium tetrachloride with dioxan (Feltz, 1966) and of uranium tetrachloride with acetone and dioxan (Gans, 1964), all of the form UCI4.3L, are obtained from the components, but the THE HALOGEN CHEMISTRY OF THE ACTINIDES 325

coordination number of thorium and uranium in these compounds is uncertain. Complexes of the tetrachlorides with nitrogen donors are less well- known; the ammine ThCl4.6NH3 is obtained by passing dry ammonia gas over thorium tetrachloride, even at 100° (Matthews, 1898a). This dissociates without appreciable ammonolysis above 200° although ammonolysis occurs slowly when thorium tetrachloride is heated in ammonia gas above 200° (Fowles and Pollard, 1953); an octammine has also been reported to be precipitated from ethereal solutions of the tetrachloride by ammonia gas, and aliphatic amine adducts, ThCl4.4L, are likewise precipitated by amines (Matthews, 1898a); adducts with aromatic or tertiary amines, and with heterocyclic bases, such as ThCl4.4Ph2NH, ThCl4.3L (toluidine) (Matthews, 1898a), ThCl4.2 [(CH3)2(i)-H2NC6H4)N] and ThCl4.L (a-picoline) are obtained in the same way (Prasad and Kumar, 1961, 1962). Uranium tetrachloride complexes with ammonia (e.g. UCI4.2NH3), primary amines (UCl4.L,UCl4.2L) and with hydrazine (UCI4.6L) have been reported (Kalnins and Gibson, 1958). Adducts with up to 10 of ammonia can form with both gaseous and liquid ammonia, but isothermal decomposition of these in nitrogen at 20° yields UCI4. 4NH3, stable to 45° (Berthold and Knecht, 1965b), although a later report (Selbin et al., 1966) gives 80° as the temperature at which the tetrammine is formed from the octammine in a vacuum. Complexes with ethylenediamine (UCl4.4en) and piperidine (UCI4.4L) have also been recorded (Selbin et al., 1966). Uranium tetrachloride reacts with ammonia gas above 350° with reduction and subsequent ammonolysis, the end product being a nitride (Berthold and Knecht, 1965a). The complex with o-phenanthroline, UCI4.2L, is obtained by reaction with the tetrachloride in dimethylformamide, but in ethanol this ligand, and 2,2'-bipyridyl, react to give a mixture of the hexachlorouranate(IV), (LH)2UCl6, and an ethoxy complex, UCl3(OEt).2L (Gans and Smith, 1964b); thorium tetrachloride behaves in a similar manner (Fitzsim­ mons et al., 1965). Complexes with methyl cyanide, UCI4.2L (Gans, 1964), UCI4.4L (BagnaU et al, 1966d), ThCl4.4L (BagnaU et al, 1966d; Feltz, 1966) and with benzyl cyanide, ThCl4.2L (Perrot and Devin, 1958) or ThCl4.4L (Feltz, 1966) are also known. Pyridine complexes, 2UCI4.3L (Barr and Horton, 1952) and UCI4.2L (Selbin et al, 1966) have been reported; the latter formulation seems the more likely. The Th—CI and U—CI vibra­ tions appear at 256 and 262 cm"^ in the infrared spectra of the tetra- chloride-methyl cyanide complexes (Bagnall et al, 1966d) as compared with 245 and 260 cm-^ for the tetrachlorides (Brown, 1966). 326 κ. w. BAGNALL

D. Tetrabromides (i) Preparation and Properties The actinide tetrabromides have scarcely been investigated; the uranium and are best made from the elements, the thorium compound at about 700° (Moissan and Étard, 1897) and the uranium compound at 650° (Spedding et al., 1958). The thorium com­ pound, which is now known to be dimorphic (Scaife, 1965), can also be prepared by reaction of the hydride with hydrogen bromide at 250-350° (Lipkind and Newton, 1952) and it has commonly been made by heating a mixture of the dioxide and carbon in bromine vapour at about 900° (e.g. Young, 1934); an alternative method of brominating the dioxide involves distilling sulphur monochloride onto it at 135° in a slow current of hydrogen bromide, a reaction which is said to yield ThOBrg if carried out at 125° (Bourion, 1907). Although protactinium tetrabromide can be made by hydrogen reduction of the pentabromide, reduction with aluminium at 400-450° is preferable. It has the same structure as the high temperature form of thorium tetrabromide and is not isostructural with uranium tetrabromide (Brown and Jones, 1966c). Uranium tetra­ bromide has also been made by heating the tribromide in bromine at 300° (Spedding et al., 1958), by the action of on uranium trioxide at 165° (Prigent, 1960) or uranium dioxide at 175° (Douglass and Staritzky, 1957), and by heating uranium dioxide in a mixture of bromine and carbon disulphide vapour at 600° (Prigent, 1958; Rohmer and Prigent, 1949) or, mixed with carbon, in a mixture of bromine and (Zimmermann, 1882), It sublimes at 600° in a stream of bromine vapour diluted with nitrogen (Spedding et al., 1958). Neptunium tetrabromide has been made by heating the dioxide with an excess of aluminium bromide at 350°; it sublimes at 500°, apparently with some decomposition to the tribromide (Fried and Davidson, 1948). Hydrates such as ThBr4.7H20 (Chauvenet, 1911) and UBr4.8H20 can be crystallized from solutions of the actinide(IV) in hydrobromic acid (Rammelsburg, 1842), but these cannot be dehydrated without decomposition. Electron diffraction of the vapour shows that the UBr4 molecule is a distorted tetrahedron οΐΟ^ν symmetry (Rambidi et al., 1961) and crystal­ lographic data are available for the known tetrabromides.

(ii) Complexes A few complexes with oxygen or nitrogen donors have been recorded; thorium and uranium tetrabromide complexes with iViV-dimethylacet- amide, MBr4.4DMA (Bagnall et al., 1966d), in which the C-O band of THE HALOGEN CHEMISTBY OF THE ACTINIDES 327 the hgand shifts by 34 cm-^ in each case, with dimethylsulphoxide, MBr4.6DMSO, in which the S =0 band shifts in the infrared spectrum are 102 and 112 cm-^ respectively, and with tri-(iViV-dimethyl)- phosphoramide (HMPA), UBr4.2HMPA and ThBr4.3HMPA, in which the P==0 band shifts are 184 and 130 cm-^ respectively (Bagnall et al,, 1966a) are obtained from the ligand and tetrabromide in nonaqueous media. The protactinium tetrabromide complex, made in the same way, has the composition PaBr4.2HMPA, in contrast to the thorium complex (Brown and Jones, 1966c). The DMSO complexes decompose on heating in a vacuum, yielding ThOBrg at 400° and UOgBrg.DMSO at 140-170°; this last decomposes to a mixture of UgOg and UOaBrg at higher temperatures. ThBr4.3HMPA, which is hygroscopic and behaves as a 1:1 electrolyte in nitromethane, is monomeric and a non-electrolyte in benzene; it decomposes to ThBr4.2HMPA (zl,(P=0) 174 cm-i) at 150° in a vacuum, and this sublimes at 250° in a non-static vacuum, like UBr4.2HMPA, which sublimes at 200° under similar conditions. Both of the MBr4.2HMPA complexes are almost non-conducting in nitro­ methane, but only the uranium compound is non-hygroscopic. Magnetic susceptibility data have been recorded for the uranium tetrabromide complexes with DMA (Bagnall et al,, 1966d), DMSO and HMPA (Bagnall et al,, 1966b). The triphenylphosphine oxide complex, UBr4. 2Ph3PO, prepared from the components in methyl cyanide solution, has the trans octahedral configuration, like the tetrachloride complex (Day and Venanzi, 1966a). Ammines of thorium tetrabromide with 8-20 molecules of adducted ammonia have been reported (Young, 1935) to exist at 0° and adducts with methyl cyanide (ThBr4.4L), pyridine (ThBr4.3L), (ThBr4. 4L), ethanol (ThBr4.4L), ethyl acetate (ThBr4.2L), ethyl benzoate (ThBr4.3L), benzaldehyde (ThBr4.4L), methylphenyl (2ThBr4. 7L) (Young, 1934) and ethylamine (ThBr4.4L) (Matthews, 1898b) have been recorded. Ethyl benzoate reacts with thorium tetrabromide at the to yield thorium benzoate whereas benzaldehyde and methylphenyl ketone under similar conditions yield derivatives of thorium oxydibromide (Young, 1934). Methyl cyanide adducts of protactinium (Brown and Jones, 1966c) and uranium (Bagnall et al,, 1966d) tetrabromides, MBr4.4L, are also known. The Th—Br and U—Br vibrations appear at 188 and 193 cm-^ in the infrared spectra of the tetrabromide-methyl cyanide complexes (Bagnall et al,, 1966d).

E. Tetraiodides The tetraiodides are best prepared from the elements, the thorium compound at 300-400° (Moissan and Étard, 1897; Anderson and D'Eye, 328 κ. w. BAGNALL

1949; Scaife and Wylie, 1964) and the uranium one at about 570° (Katz and Rabinowitch, 1951, p. 535), although reaction at lower temperatures is probably better because of its thermal instability (e.g. Bagnall et al., 1965a). They are also claimed to be obtained by heating the dioxides with aluminium iodide, thoria at 230° and the uranium compound at 400° (Chaigneau, 1957), although the known instability of uranium tetraiodide suggests that the product would contain much UI3 in the latter case. Thorium tetraiodide has also been made by heating the hydride with hydrogen iodide at 250-350° (Lipkind and Newton, 1952), and by heating the metal in a mixture of hydrogen and iodine (Fischer et al., 1939). Protactinium tetraiodide, a dark green solid, has been prepared by reduction of the pentaiodide with hydrogen or aluminium in a vacuum (Brown and Jones, 1967a). Thorium tetraiodide has an unusual layer structure in the solid state; each thorium atom is surrounded by eight iodine atoms at the corners of a deformed square antiprism and the polyhedra share edges and tri­ angular faces to form layers which are only weakly bonded to each other (Zalkin et al, 1964). Like the other halides, adducts are known with DMF, DMA and methyl cyanide. The DMF complex, UI4.4DMF, obtained from a solu­ tion of uranium(IV) in hydriodic acid (Lamisse et al, 1964), is said to be stable in air, which is surprising; the DMA complexes, Thl4.6DMA and UI4.4DMA, prepared under anhydrous conditions from the components, either alone or in methyl cyanide solution, are very susceptible to moisture (Bagnall et al, 1965b). A complex with methyl cyanide, Thl4.4L, sparingly soluble in excess of the ligand, is also known (Bag­ nall et al, 1965a); the analogous uranium tetraiodide complex is unstable. The existence of definite hydrates and ammines of the tetraiodides is rather doubtful, but thorium tetraiodide is known to react with potassamide in liquid ammonia to give an amidoiodide, Th(NH2)2l2- 3NH3 (Watt and Malhotra, 1959).

F. Mixed halides Many mixed halides of uranium(IV) have been described (Warf and Baenziger, 1958). The general preparative methods are by treatment of a uranium trihalide with a halogen of higher atomic number and by heating together the stoicheiometric quantities of two tetrahalides. All the compounds which contain iodine are thermally unstable. Thermo­ dynamic data for the three possible chlorofluorides have been reported (Maslov, 1964). Complexes of UI2CI2 and UI3CI with DMA, UI2CI2.5DMA and UI3CI. THE HALOGEN CHEMISTRY OF THE ACTINIDES 329

5DMA, have been reported; the first is obtained by treating the complex 2UCI4.5DMA with a large excess of in DMA or in methyl cyanide, and this reacts with sodium iodide in nitromethane to give the UI3CI complex. Conductivity studies suggest that these compounds are dimers (Fig. 2(a, b) ) in which the uranium is 8-coordinate, although 9-coordination is not impossible (Bagnall et at., 1965b).

DMA DMA DMA.^ I4 Cl DMA DMA

DMA DMA DMA^^Vj^/ I4 DMA^^ DMA

(b)

FIG. 2(a). [UlaCla-SDMAJg; (b) [UI3CI.5DMAL.

G. Halo complexes A very large number of actinide(IV) fiuoro complexes have been identified, prepared by precipitation from aqueous solution or by heat­ ing together the component salts. The simplest of these are of the form A^M^^Fg; when obtained from aqueous solution they are sometimes precipitated as the monohydrates, but the hydration water is evidently held only very weakly since the anhydrous salts have often been reported when a particular preparation has been repeated with slightly more stringent drying conditions. Some of these salts are listed in Table IX. Some salts are also obtained by heating the actinide dioxides with ammonium fluoride or bifluoride at moderate temperatures; in the re­ action with uranium dioxide (Van Impe, 1954) the complex NH4UOF3 appears to be formed initially, and the compounds (NH4)2UF6 and NH4U2F9 are formed at about 390° (Neumann et al., 1962). Plutonium dioxide reacts at 125° (Maly et al., 1961; Tolley, 1954). The intermediate hydrates which can be isolated are readily dehydrated at about 150° 330 κ. w. BAGNALL

TABLE IX. Fluoro complexes of the type A^M^^Fs

A M Colour H2O References

Na Th White 1 Tananaev and Lu Chzhao-Da (1959a) K Th White —a Zachariasen (1948a). Na,K U Green — Zachariasen (1948a) Na u Green 1 Tananaev et al. (1962) NH4 u Green — Rodriguez et al. (1958) NH4,Na u Green — Schulz etal. (1958) Ν2Η5+,ΝΗ2θΗ + Th;U White; green — Sahoo and Patnaik (1961) NH4 Np Bright green — LaChapelle et αΖ. (1949) Na,K,Rb Pu Green — Anderson (1949a); Alenchikova et al. (1958b) Li,Na, Pu Green — Seaborg (1960) K,Rb,Cs Na Pu Green — Deichmann and Tananaev (1961) a KThFg.HaO and RbThFg.SHgO have also been reported; the same describes the preparation of anhydrous KThFg by heating thorium tetrafluoride with an excess of , the latter being removed by washing with water (Rosenheim et al., 1903).

(U(IV) ) or 200° (Pu(IV) ). The only pentafluoroamericmm(IV) salt known is KAmPg, made by the action of fluorine on potassium ameri­ cium (V) carbonate (Asprey, 1954). In addition to the simple pentafluorocomplex salts, a variety of salts of composition 7M^F.6M^^F4 have been reported and, from considera­ tion of the cation radius ratios M+/M^+, a number of fluoro complexes have been predicted for systems which have not yet been investigated (Thoma, 1962). A single crystal study (Brunton, 1966) of the complex originally reported as 7LiF.6UF4 (Harris et al., 1959) has shown it to be LiUFg, and since this is isostructural with ''7LiF.6ThF4" (Harris et al., 1959), the compound must be LiThFg. The 7:6 stoicheiometry does exist, however, where the ratio lies between 0-99 and 1-68 and all such sodium, potassium, ammonium, and salts are of rhom­ bohedral symmetry. This suggests that KThF^, KUF5, KPuFg, NaUFg, NaPuFg and RbUFg are really the 7:6 compounds. It has been pre­ dicted (Thoma, 1962) that stable 7:6 and 1:1 complexes will both exist where M+/M^+ lies between 1-59 and 1-68; examples are RbUFg and 7RbF.6UF4 (Thoma et al., 1958) and the analogous ammonium salts (Benz et al, 1963). Salts of the type A^M^^Fg are also obtained from aqueous solution (Table X) and with an excess of alkali fluoride the quadrivalent actinides yield fluorides of the type AaMi^Fg (Th—Rosenheim et al, 1903; Tananaev and Lu Chzhao-Da, 1959b; Pu (pink Na and NH4 salts)— Alenchikova et al, 1958b). (NH4)2UF6,(NH4)4UF8 and species such as THE HALOGEN CHEMISTRY OF THE ACTINIDES 331

TABLE X. Fluoro complexes of the type AiM/F\

A M Colour H2O References

Na,K Th White» — Zachariasen (1948a) K,NH4 Th White — Tananaev and Lu Chzhao-Da (1959b) Κ u Green — Zachariasen (1948a) Κ Np Green — LaChappelle et al. (1949) Κ Pu Pink — Seaborg (1960) Cs Pu Light red- 3H2O Anderson (1949a); Alenchikova brown et al. (1958b) a KThgFg.eHaO has also been reported (Rosenheim et al., 1903).

7NH4P.6UF4 have also been obtained from aqueous solution (Penne- man et al., 1964a). The protactinium compound, 7RbP.6PaF4, has been made by heating RbPa^P^ in hydrogen at 450° (Asprey et al., 1965b). The americium(IV) hexafluoro complex, RbgAmPg, an orange-pink solid, has been made by treating americium(IV) hydroxide with IM hydrofluoric acid saturated with or by adding 12M rubidium fluoride to a solution of rubidium americium(V) carbonate in IM and allowing the mixture to stand overnight (Kruse and Asprey, 1962), the reduction presumably being due to the products of the a-radiolysis of the solvent. Stable aqueous solutions of americium(IV) are obtained by treating americium(IV) hydroxide with saturated aqueous ammonium fluoride; the red solid phase in equilibrium with the solution is the octafluorocomplex salt, (NH4)4AmF8. Sparingly soluble fluorocomplex salts are also obtained with potassium, rubidium and caesium fluorides in place of ammonium fluoride, but their com­ positions have not been reported (Asprey and Penneman, 1962). A few alkaline earth hexafluorometallates(IV) are also known; CaUFe.HaO precipitates from aqueous solution and dehydrates readily without hydrolysis at 250-300° in argon (Tolley, 1959). The structures of the alkaline earth and hexafluorothorates(IV) and uranates(IV), prepared by heating the component salts together, have been recorded (Zachariasen, 1949a) and some aspects of the SrThFg and BaUFg systems have been investigated (D'Eye and Ferguson, 1959). The solid phases in equilibrium with aqueous hydrofluoric acid of varying concentration have, in the case of the thorium tetrafluoride system, been identified as ThF4.HF.H2O (35·2-70·7% HF) and ThF4.4HF (75-90-3% HF), whereas the soHd phase in contact with dilute hydrofluoric acid is ThF4.0-7-l-5H2O (Buslaev and Gustyakova, 1965). The hexafluorometallate ion is probably present in solutions of the tetrafluorides in 15M ammonium (Pa(IV)—Haissinsky et al., 1961; 332 κ. w. BAGNALL

Am(IV)—^Asprey and Penneman, 1961) or caesium (Cm(IV)—Keenan 1961) fluoride. All of the anhydrous species obtained from aqueous solution, and a large number of other fluoro complexes, have been identifled in fused salt systems, notably CsF-ThF4 (Thoma and Carlton, 1961), LiF and NaF-ThF4 (Thoma et al, 1959a), KF-ThF4 (Asker et al, 1952) and NaF and KF-ThF4 (Kaplan, 1955); the uranium tetrafluoride systems (NaF-LiF-UF4 (Thoma et al, 1959b), LiF and NaF-UF4 (Barton et al, 1958), KF and RbF-UF4 (Thoma et al, 1958) have also been thoroughly investigated. The system LiF-ThF4-UF4 contains a mixture of fluoro complexes which form a continuous series of solid solutions, as do ThF4 and UF4 (Weaver et al, 1959). Crystallographic studies of the products obtained by heating the actinide tetrafluorides with fluorides in varying proportions have shed a great deal of light on the nature of the species which can be formed; the principal systems investigated include NH4F-UF4 and PUF4 (Benz et al, 1963) and NH4F-PaF4 (Asprey and Penneman, 1965), in which octafluorometallates(IV) have been obtained, NaF and KF-ThF4 and UF4 (Zachariasen, 1948a), LiF-ThF4 (Harris et al, 1959) and NaF-ThF4 and UF4 (Thoma et al, 1963). The halo complexes formed by halogens of higher atomic number are increasingly less stable, indicating the essentially A-type character of the actinides. The species formed are also much simpler than in the case of the fluoro complexes, almost invariably being of the type AgM^^Xg. Thus the chloro complexes isolated from aqueous solution are usually of this form, but the procedure is only suitable for the preparation of salts of the larger unipositive cations because of the lower solubility of the halocomplex salts formed by them; some examples are given in Table XI. The pale green sodium (Moissan, 1896), potassium and salts, A^gUCle, and calcium, and salts, A^UCle, have been made by passing uranium tetrachloride vapour over the alkali or alkaline earth chloride at red heat (Aloy, 1899, 1901b). Nonaqueous solvents also provide a convenient route to the hexahalo complexes; tetraethylammonium hexachlorothorate(IV), which exists in two crystal modifications (Brown, 1966), and uranate(IV) have been made by mixing thionyl chloride solutions of the appropriate tetrachloride and tetraethylammonium chloride, evaporating the solvent and precipitat­ ing the complex with acetic anhydride (Adams, D. M. et al, 1963) and these salts, and the corresponding tetramethylammonium compounds, are also easily made from methyl cyanide solution (Brown, 1966; Feltz, 1966), from which they crystallize when the solution is cooled in ice. THE HALOGEN CHEMISTRY OF THE ACTINIDES 333

TABLE XI. Chloro complexes of the type AjM^^ Clg prepared from aqueous solution

A M Colour References

NH4,Li Th White Chauvenet (1909, 1911) NTa, Rb,Cs pyH,quinHa Th White Rosenheim et al. (1903); Rosenheim and Schilling (1900) Cs Th White Ferraro (1957) Cs Pa Green Brown and Jones (1967b) Cs,NMe4,NEt4 U Green Dieke and Duncani> (1949); Ferraro (1957) ΝΜθ4, NEt4 U Green Staritzky and Singer (1952) PyH* U Green Rosenheim and Kelmyd (1932) RgPHC U Green Gans and Smith (1963, 1964a) Cs Np Yellow Bagnall et al. (1961) NEt4 Np Yellow Ryan (1961) Cs,NEt4, Pu Yellow Anderson (1949b) pyHjquinH» NMe4,NEt4 Pu Yellow Staritzky and Singer (1952) Net4 Pu Yellow Ryan (1961) Cs Pu Yellow Miner et al. (1963)

* py, pyridine; quin, quinoline. Prepared from alcoholic hydrochloric acid solution. ^ These compoimds are not as susceptible to oxidation as the authors state. *5 From ethanolic solution. ^ Dihydrate.

The method has also been used for the preparation of the analogous protactinium(IV) hexachloro complexes, for which thionyl chloride can­ not be used since it oxidizes protactinium(IV) (Brown and Jones, 1966c, 1967b). The Th—Cl, U—Cl (Brown, 1966) and Np—Cl (Brown, 1966; Bagnall and Laidler, 1966) vibrations in the infrared spectra of the hexachlorometallates(IV) appear at about 253 cm-^, 253-259 cm-^ and 267 cm-i respectively. The hydrated potassium enneachloro complex, KThaClg (Clève, 1874) and hydrated pentachlorothorates(IV), A^ThClg (A = Li, Na,K,NH4) and hexachlorothorates(IV), AigThClg (A = Rb,Cs,NH4) have been ob­ tained from aqueous solution (Chauvenet, 1909). Hydrated ammonium hexachlorothorate(IV) yields NH4ThCl5 on heating, and the hydrated rubidium and caesium salts can be dehydrated by heating in hydrogen chloride at 150°, whereas the lithium, sodium and potassium salts form complexes of the type AiTh(0H)Cl4 at 200° and AThOClg at about 400° (Chauvenet, 1909). The anhydrous hexachlorothorates(IV), AigThClg (A = Li, Na, K, Rb,Cs) and octachlorothorates(IV), Ai4ThCl8 (A = Rb, Cs) are reported to be formed by fusing together the stoicheiometric quantities of alkali halides and thorium tetrachloride; lithium, sodium and potassium apparently do not form octachlorothorates, a difference from rubidium and caesium which was detected by measurement of the 334 κ. w. BAGNALL heats of solution of the products of these reactions (Chauvenet, 1911). These reactions are clearly worth further investigation, for the systems have not been studied since first reported, but the anionic species ThClg-, ThClg^" and ThCl7^~ are said to be formed in fused mixtures of thorium tetrachloride with sodium, potassium, caesium and (III) chlorides (lonov et al., 1960), but formation of the heptachloro- thorate(IV) was not confirmed by later work on the - thorium tetrachloride system (Desyatnik et al., 1966). The penta- chlorouranate(IV) ion is said to be present in uranium tetrachloride fused with a mixture of potassium and cuprous chlorides at 180° (Taube, 1962). Although plutonium tetrachloride is unknown, hexachloropluto- nates(IV) are readily obtained from aqueous solution (Table XI, p. 333); the rubidium salt is formed when a mixture of and plutonium dioxide is heated in carbon tetrachloride at 750° (Fomin et al., 1958a), and the sodium, potassium, rubidium and caesium salts are formed when a mixture of the alkali chloride and plutonium tri­ chloride is heated in chlorine, at about 50° above the of the alkali chloride (Benz and Douglass, 1961a), indicating that the tetra­ chloride is formed under these conditions. Oxidation to plutonium(IV) does not occur with mixtures of plutonium trichloride with lithium, calcium or barium chloride, and the stabilities of the alkali metal salts (and the amounts formed) increase with increasing atomic number of the alkali metal. Magnetic susceptibility data have been recorded for CsgUCle and (NMe4)2UCl6 (temperature independent paramagnetic) and for (NMe4)2PuCl6 (temperature dependent) (Candela et al., 1959) and crystallographic data are available for Cs2ThCl6,Cs2UCl6 (Siegel, 1956), (NMe4)2Th (and Np)Cl6, (NEt4)2Th (and Np)Cl6 (Brown, 1966), CsgPuCle (Zachariasen, 1948b) and in many of the modern references given in Table XI (p. 333). The absorption spectra of the UXe^- (X = Cl,Br,I), NpXg^- and PuXg^- ions (X = Cl,Br) have been investigated, the optical being 1-5 for U(IV), 1-75 for Np(IV) and 2-05 for Pu(IV) (Ryan and Jorgensen, 1963). Pyridinium hexabromothorate(IV), (pyH)2ThBr6, is reported to be obtained anhydrous from alcoholic hydrobromic acid solutions of the tetrabromide and pyridinium bromide (Rosenheim and Schilling, 1900; Rosenheim et al., 1903), but there is less risk of hydrolysis if non­ aqueous solvents are used, as in the preparation of tetraethyl- and tetramethylammonium hexabromothorates(IV) and hexabromoura- nates(IV) from methyl cyanide solutions of the tetrabromides and the tetra-alkylammonium bromide, the halo complexes crystallizing when THE HALOGEN CHEMISTRY OF THE ACTINIDES 335 the mixture is cooled in ice (Brown, 1966). The hexabromoprotac- tinates(IV) have been made in the same way (Brown and Jones, 1967b). The dark green sodium and potassium hexabromouranates(IV) have been made by heating the alkali bromide in uranium tetrabromide vapour, a procedure which appears to be less successful with the alkaline earth bromides (Aloy, 1901b) and the triphenylphosphonium salt crystallizes from aqueous acetone-hydrobromic acid (Jorgensen, 1963). Tetraethylammonium hexabromouranate(IV) has been made from ethanolic hydrobromic acid, from which the salt is precipitated with acetone (Ryan and Jorgensen, 1963), a procedure successfully used by these authors for the preparation of the corresponding bright yellow neptunium and deep red plutonium compounds; tetramethylammonium hexabromouranate(IV) has also been isolated from 6N hydrobromic acid (Satten et al., 1965), a study being made of the energy levels of the ion in an octahedral field. The Th—Br and U—Br vibrations appear at 177-179 cm"^ and 178-181 cm-^ respectively in the infrared spectra of the tetraalkylammonium hexabromometallates(IV) ; crystallo­ graphic data are also available for some of these salts (Brown, 1966). Jorgensen (1963) has investigated the spectra of mixed chloride- bromide complex anions in nitromethane solution, obtaining the con­ secutive formation constants for the species UBrClg^" and UBr2Cl4^~, the values of which demonstrate once again the typically A character of uranium(IV). The optical absorption spectra of octahedrally coordi­ nated in triphenylphosphonium hexachloro- and hexabromo- uranate(IV) have also been reported (Pappalardo and Jorgensen, 1964). Triphenylbutylphosphonium tetrachlorodibromouranate(IV), (Ph3BuP)2UCl4Br2, prepared from the phosphonium bromide and uranium tetrachloride in methyl cyanide solution, has the trans octa­ hedral configuration (Day and Venanzi, 1966a). Although the spectrum of the hexaiodouranate(IV) ion in methyl cyanide has been recorded (Ryan and Jorgensen, 1963), solid hexaiodo- metallates(IV) have only recently been obtained. The yellow thorium and red uranium salts, AgM^^Ig (A = BU4N+, Ph4As+) are made by reaction of the tetraiodides with the appropriate cation iodide in methyl cyanide solution, the tetraphenylarsonium salts being the more stable (Bagnall et al., 1965a). The blue protactinium(IV) salt, (Ph3MeAs)2Pal6, has been prepared in a similar manner (Brown and Jones, 1967b).

H. Oxyhalides The only recorded oxyfluoride is the thorium compound, ThOF2, made by heating together the stoicheiometric quantities of the dioxide and tetrafluoride at 900° in an inert atmosphere (D'Eye, 1958; Darnall, 336 κ. w. BAGNALL

1960), the reaction being reversed at higher temperatures; the uranium compound cannot be obtained in this way (Spedding and Wilhelm, 1944). The thorium compound has also been made by heating the tetra­ fluoride hydrate to red heat (Chauvenet, 1911) and the structure of a specimen obtained by hydrolysis (when the tetrafluoride was heated in air) has been recorded (Zachariasen, 1949a). Thorium oxychloride is made from the dioxide and tetrachloride at 840° (Smirnov and Ivanovskii, 1956; Yen Kung-Fan et al, 1963) or by heating thorium tetrachloride octahydrate above 250° in hydrogen chloride (Chauvenet, 1911). An adduct with methyl cyanide, ThOCl2.2L, is reported to be formed by hydrolysis of the thorium tetrachloride adduct with the stoicheiometric quantity of water (Feltz, 1966). The yellow-green uranium compound has been made by dissolving the dioxide in an excess of the molten tetrachloride at 600°, the excess of the last being removed in a vacuum at 450° (Kraus, 1942a, 1944). This procedure seems to give a purer product than that obtained by heating uranium dioxide in the vapour of the tetrachloride at 475° (Davidson and Streeter, 1946). The uranium compound is insoluble in a wide range of organic solvents, but is soluble in water, acids, and, with reaction, in molten pyridinium chloride; its absorption spectrum has been recorded (Ewing, 1961). The structure is probably an oxygen bridged , no band assignable to the U=0 vibration being observed in the infrared spectrum (850-1000 cm-^) and oxygen bridge vibrations appearing at 735 and 720 cm-^ (Selbin and Schober, 1966). The yellow neptunium oxychloride has been made by vapour phase hydrolysis of the tetrachloride at 500° (Fried and Davidson, 1948). The oxychlorides of protactinium(IV) (Brown and Jones, 1967a), thorium(IV), uranium(IV) and neptunium(IV) (Bagnall et al, 1967a) are, however, more easily made by heating the tetrachlorides with the stoicheiometric amount of (III) oxide. The bridging oxygen vibration in these products appears at about 600 cm-^. Thorium oxybromide, ThOBrg, has been made by heating the dioxide with sulphur monochloride and hydrogen bromide at 125° (Bourion, 1907), by boiling an aqueous solution of thorium tetrabromide and heating the residue to 160° (Moissan and Martinsen, 1905) and by heat­ ing hydrated thorium tetrabromide (Chauvenet, 1911). The uranium compound, a greenish-yellow to yellow solid, has been prepared by the action of bromine on the oxide-sulphide, UO2.2US2, at 600° (Spedding et al, 1958). It is also formed by the decomposition of UOBrg at room temperature (Shchukarev et al, 1958a) or, more rapidly, at 300°. Its infrared spectrum shows bands at 500 and 546 cm~^ which have been assigned to U—Ο vibrations (Prigent, 1960). It is also formed, but not in THE HALOGEN CHEMISTRY OF THE ACTINIDES 337

a pure state, by heating uranium dioxide with the tetrabromide (Gregory, 1958). As with the oxychlorides, the best way of preparing the oxybromides of protactinium(IV) (Brown and Jones, 1967a), thorium(IV) and uranium(IV) (Bagnall et al., 1967a) is by heating the tetrabromides with antimony (III) oxide at 150°. They all dispropor­ tionate above 500° in a vacuum and, like the uranium compound, the M—Ο vibrations appear at about 500 cm-^, indicating that the com­ pounds are oxygen bridged . Both uranium(IV) oxychloride and the oxybromide can be reduced to the corresponding tervalent oxyhalides (Gregory, 1958). Neptunium(IV) oxybromide is stated (Zachariasen, 1949b) to be iso- structural with the uranium compound, but no preparative details have been recorded; X-ray powder data, which have not been inter­ preted, are available for uranium(IV) oxychloride and oxybromide (Zachariasen, 1949b). Thorium oxyiodide is obtained by heating together the dioxide and tetraiodide at 600°; its structure is probably an infinite chain of thorium atoms linked by oxygen bridges; Th—0 bands have not been observed in the infrared spectrum between 4000 cm~^ and 650 cm~^ (Scaife et al., 1965). Protactinium(IV) oxyiodide, a pink solid, is formed to some extent when the tetraiodide reacts with silica above 500° (Brown and Jones, 1967a).

4. The Pentavalent Actinides A. General chemistry Simple pentahalides and oxyhalides have been isolated only for pro- tactinium(V) and uranium(V) and, in the case of neptunium(V), the hydrated oxytrifluoride is known. However, fluoro complexes of all the elements from protactinium(V) to plutonium(V) have been prepared and a few oxyhalo complexes of these elements, and of americium(V), have been recorded. The actinide pentahalides, like their (Z-transition element analogues, are very sensitive to moisture and the uranium(V) compounds disproportionate immediately on exposure to moist air, or in water and oxygenated solvents, but are more stable in dry halo- genated hydrocarbons, such as chloroform, bromoform or carbon tetra­ chloride, and in solvents with donor properties, such as thionyl chloride, with which both protactinium and uranium pentachlorides form stable complexes; there is also some evidence for the existence of a neptunium pentachloride complex in thionyl chloride. Some crystallographic data are given in Table XII. 338 κ. w. BAGNALL

TABLE XII. Some crystallographic data for the actinide pentahalides*

Colour Symmetry and Lattice parameters (A) Calculated space group density

«0 0 Co (gcm^ -3)

White Tetragonal, Ti2d 11-53 5-19 6-28 Black Cubic, /43m(?) 8-507 — — 6-83d (or •Pa,F,)o Pa^OF.e White Cubic 8-4065 — — — a-UFg White to Tetragonal, /4/m 6-525 — 4-472 5-81 pale blue β-VF, White to Tetragonal, U2d 11-473 5-209 6-45 pale blue — U^F^ Black Cubic, /43m 8-471 — — 7-06 Black Distorted UF4 — — — — UCI5 Red-brown Monoclinic — — — — PaCl^e Yellow Monoclinic, C2/c 7-97 11-35 8-36 3-74 i3=106-4 °

^ Values from the corrected data collected by Katz and Sheft (1960) unless otherwise stated.

t> Stein (1964). c gtein (1965). d Calculated as PagF» (Author), e Dodge et al. (1967).

B. Pentafluorides Protactinium pentafluoride, a white, crystaUine sohd isomorphous with JS-UFQ, is made by heating the tetrafluoride with fluorine at 700°; it is less volatile than , niobium and tantalum pentafluorides, subliming in a vacuum above 500° (Stein, 1964). The colourless di­ hydrate is obtained by evaporating a solution of protactinium(V) in concentrated hydrofluoric acid to dryness (Grosse, 1934a; Stein, 1964). It decomposes to the oxyfluoride, PagOFg, at 160° (Stein, 1964). The infrared spectra of the protactinium(IV) fluorides and (V) oxyfluorides have been recorded, the Pa—F vibration appearing at 400 cm-^ in PaF^ and Pa^F^^ and at 450 cm-^ in PaaOFg (Stein, 1965); the Pa—Ο vibrations in the last appear at 790, 740 and 690 cm~^. , first obtained by Grosse (1958c) by reaction of uranium tetrafluoride with the hexafluoride at 95-100°, exists in two crystalline forms, both of tetragonal symmetry. The high-temperature α-form is made by the action of fluorine on uranium tetrafluoride at 150° (Agron et al., 1958) or by reaction of the tetrafluoride with at 230-250° (Wolfed al., 1965a), a reaction which yields the β-ΘοτίΆ below 125°. jS-Uranium pentafluoride is obtained by the action of hydrogen fluoride on uranium pentachloride (Agron et aL, 1958), a reaction previously investigated by Ruff and Heinzelmann (1911), and is precipitated on addition of to a solution of nitrosonium THE HALOGEN CHEMISTRY OF THE ACTINIDES 339 hexafluorouranate(V) in anhydrous hydrofluoric acid (Geichman et aL, 1962a). The α-form is best made by reaction of hydrogen bromide with uranium hexafluoride at 65° (Wolf aL, 1965b), a reaction which can also be made to yield the j8-form by suitable temperature control (Hobbs, 1962). Both forms are white to chalky blue in appearance; disproportionation occurs slowly above 150° (Priest, 1958). However, a-UFg melts at 348°; vapour pressure data are available for both the solid and liquid (Wolf et aL, 1965a), and magnetic susceptibility data for jS-UFg have been recorded (Nguyen-Nghi et aL, 1964). Other actinide pentafluorides are unknown, although an abnormal increase in the vapour pressure of plutonium tetrafluoride at 900° in a vacuum has been tentatively ascribed to disproportionation to plu­ tonium tri- and pentafluorides; however, the volatile product, which has never been characterized, was stable in air, so that it is unlikely to have been a pentafluoride (Dawson et aL, 1954b). Apart from PaF5.2H20, mentioned above, complexes of the penta­ fluorides with oxygen and nitrogen donor ligands are unknown.

C. Intermediate fluorides Fluorides of composition intermediate between MF4 and MF5 have been recorded for protactinium, uranium and plutonium; Pa^Fi, (or PagFg), a black solid isostructural with U2F9, has been made by heating pro­ tactinium pentoxide with hydrogen fluoride and hydrogen at 500° (Stein, 1965) and by thermal decomposition of ammonium hepta- fluoroprotactinate(V) (NH4)2PaF7 (Brown, 1965). The black uranium compounds U2F9 and U4F17 have also been recorded, the former ob­ tained by heating uranium tetrafluoride in the vapour of the hexa­ fluoride at 200° or by partial decomposition of uranium pentafluoride at 100-200°, and the latter by heating together the stoicheiometric quantities of uranium tetra- and pentafluorides at 215° or by heating uran­ ium tetrafluoride in the vapour of the hexafluoride for two days ; the rate of diffusion of the hexafluoride into the solid is apparently the controlling step of the reaction (Agron et aL, 1958). These uranium compounds are more stable to hydrolysis than the pentafluoride and both decompose to a mixture of the tetra- and hexafluorides at high temperatures. The preparation and properties of these compounds, and of U5F22, have been described in some detail in a recent report (Nguyen-Nghi, 1961). An analogous plutonium compound, a brick-red solid thought to be PU4F17, remains as a residue in the reaction of fluorine with plutonium tetrafluoride at 500-600°; its X-ray powder pattern is said to resemble that of U4F17 (Mandleberg et aL, 1956). 340 κ. w. BAGNALL

D. Pentachlorides (i) Preparation and Properties Protactinium pentachloride is a pale yellow solid which melts at 301° (Grosse, 1934b); the Pa—Cl vibrations in the infrared spectrum appear at 362 and 322 cm-^, which suggested that the compound was at least dimeric (Bagnall and Brown, 1964). A full structure analysis has now shown that protactinium is 7-coordinate in the compound, the structure consisting of infinite chains of pentagonal bipyramidal PaCl, groups which share pentagon edges (Dodge et al., 1967). It is soluble in dry methyl cyanide, in tetrahydrofuran, in alcohols with reaction and is also slightly soluble in carbon tetrachloride. The compound was first obtained, although not definitely identified as such, by heating the pentoxide in carbonyl chloride at 550° (Grosse, 1934b) and, later, by heating the pentoxide in carbon tetrachloride vapour at 300° (Sellers et al., 1954), when it was shown that hydrogen reduction of the product yielded the tetrachloride. It has also been obtained in about 50% yield by thermal decomposition of the thionyl chloride adduct, 2PaCl5.SOCl2, at 150° in a vacuum (Bagnall and Brown, 1964) and by heating the pentoxide with (A. G. Maddock, personal communi­ cation). Since severe losses occur when protactinium pentoxide is heated in a stream of carbon tetrachloride vapour, either alone or mixed with chlorine, the chlorination is best carried out either by heating a mixture of the pentoxide and carbon with chlorine and carbon tetrachloride at 500-700° in a sealed tube, a reaction which also yields the oxychloride, PagOClg, from which the pentachloride is separated by vacuum sublima­ tion at 200°, or by heating low fired protactinium pentoxide in a sealed tube at 300-500° with thionyl chloride vapour (Brown and Jones, 1966a). Protactinium pentachloride is appreciably less volatile than and tetrachlorides, a property which has been used to advan­ tage in the separation of protactinium from pitchblende residues, by heating them, mixed with graphite, in chlorine at 800° and condensing the protactinium pentachloride in a trap maintained at 150°, a tempera­ ture at which silicon and titanium tetrachlorides remain in the vapour phase (Conte et al., 1964). Uranium pentachloride, usually described as dark red crystals or a brown red powder, is dimeric in carbon tetrachloride (Goren et al., 1946), explaining the observed diamagnetism of the pentachloride in this solvent (RiidorflF and Menzer, 1957). It is very soluble in thionyl chloride and in carbon disulphide. Since the compound is thermally unstable even at 100° and dispro­ portionates readily, its melting point is unknown and it is difficult to THE HALOGEN CHEMISTRY OF THE ACTINIDES 341

obtain in a pure state. Liquid phase chlorination of uranium trioxide or triuranium octaoxide at 250° with a mixture of carbon tetrachloride and chlorine in a sealed tube appears to be fairly satisfactory (Michael and Murphy, 1910), but it is probably best prepared by heating uranium tetrachloride in chlorine at about 550° and quenching the vapour (Webb, 1943), although the experimental conditions must be carefully controlled. It is also formed in most of the chlorination reactions used for the preparation of the tetrachloride, from which it can be separated by recrystallization from liquid chlorine (Grosse, 1958a). It can also be recrystallized from carbon tetrachloride (Gans, 1964). The methods used for its preparation have been discussed in some detail by Katz and Rabinowitch (1951, pp. 489-491). Uranium(V) chloride alkoxides, of the form UC1^(0R)5_^, have been made by reaction of the penta-alkoxide with hydrogen chloride; they are green liquids, soluble in non-polar solvents (Jones et al., 1956). The magnetic behaviour of uranium pentachloride (and of the thionyl chloride adduct, UCI5.SOCI2) has been reported to be consistent with a %άλ (Rtidorff and Menzer, 1957) and with a 5/^ (Handler and Hutchinson, 1956) configuration for the ion, the latter being the more likely since the absorption spectrum of the thionyl chloride adduct in carbon tetrachloride fits the 5/^ configuration very satisfactorily (Karracker, 1964). Other observations on the absorption spectra of uranium penta­ chloride in various solvents are also available (Sterett and Calkins, 1949; Rohmer et al., 1952; Bagnall et al., 1964c).

(ii) Complexes A few complexes of the pentachlorides with oxygen or nitrogen donors are now known; the orange-red uranium and pale-yellow protactinium pentachloride-phosphine oxide complexes, MCI5.R3PO, are conveni­ ently prepared by the reaction of protactinium pentachloride with the phosphine oxide in methyl cyanide or, better, methylene dichloride, and by treating caesium hexachloroprotactinate(V) with the phosphine oxide in methylene dichloride (Brown et al., 1966), a reaction success­ fully used for the preparation of the uranium pentachloride complexes (Bagnall et al., 1965c). As with the niobium and tantalum pentachloride analogues, the position of the P=0 vibrational frequency in these complexes has shifted by over 200 cm*^ from that of the free ligand, but the protactinium and uranium complexes remain unchanged in the presence of an excess of the ligand (Brown et al., 1966), unlike the niobium and tantalum complexes, which react to give the oxychloride complexes MOCI3.2R3PO {Δν (P=0) = 25 cm-i) (Brown et al., 1966; Copley et al., 1965), a difference which might be due to 77-backbonding 342 κ. w. BAGNALL

from the oxygen of the hgand into readily accessible 5/-orbitals in protactinium(V) and uranium(V). uranium pentachloride forms a complex with tri-iV^iV-dimethyl phosphoramide, prepared in the same way as the phosphine oxide complex, but it disproportionates rapidly, as do the complexes with aryl or alkaryl , so that these com­ pounds have not been isolated (Bagnall et aL, 1965c). The adducts with thionyl chloride, yellow 2PaCl5.SOCl2 (Bagnall and Brown, 1964) and red UCI5.SOCI2 (e.g. Bradley et al, 1957), the former obtained by dissolving freshly precipitated protactinium(V) hydroxide in thionyl chloride and evaporating the resulting solution in a vacuum and the latter by refluxing uranium trioxide with thionyl chloride until dissolution is complete, followed by vacuum evaporation, are more correctly formulated as hexachlorometallates(V), SO(PaCl6)2 and SOC^UClg). The orange-red uranium pentachloride-phosphorus penta­ chloride complex, UClg.PClg, obtained by heating uranium trioxide with phosphorus pentachloride (Cronander, 1873), ionizes as PCI4+UCI6" in phosphorus oxytrichloride, a complex of composition 2UCI4.UCI6.6POCI3 being recovered from the anolyte; the last is also obtained by adding small amounts of water to phosphorus oxytrichloride solutions of the phosphorus pentachloride complex (Panzer and Suttle, 1961). A dark red complex with trichloroacryloyl chloride, 5UCI5. CClg^CClCOCl, which melts at 96° (sealed tube) is obtained as an intermediate in the chlorination of uranium trioxide with hexachloro­ propene (Panzer and Suttle, 1960a), but no structural data have been reported.

E. Pentabromides Protactinium pentabromide, a red solid, is best prepared by the action of bromine on a mixture of protactinium pentoxide and carbon at 600-700° in a sealed evacuated tube; the oxybromide, PaOBrg, is obtained as a by-product of the reaction (Brown and Jones, 1966b). Earlier preparative work, in which the product was not definitely identified, includes heating the pentoxide with hydrogen bromide, carbon tetrabromide or a mixture of thionyl bromide and hydrogen bromide, and the pentachloride with hydrogen bromide or thionyl bromide (Sellers et al, 1954), the product of the reaction of the pent­ oxide with aluminium bromide subsequently being identified as the pentabromide (A. G. Maddock, personal communication), although this preparative method is not very satisfactory because of the difficulty of separating the product from aluminium bromide. Uranium pentabromide is obtained by heating uranium trioxide with carbon tetrabromide at 110-130°, careful temperature control being THE HALOGEN CHEMISTRY OF THE ACTINIDES 343 necessary in order to avoid the formation of uranium tetrabromide or the oxybromide, UOBrg (Prigent, 1954a, 1960); it is also said to be formed by heating UO3 with carbonyl bromide. Uranium pentabromide is insoluble in carbon tetrabromide and is soluble in, and decomposed by, water, alcohol and acetone. The only recorded complex of an actinide pentabromide is the methyl cyanide adduct, PaBr5.3CH3CN (Brown and Jones, 1966b).

F. Pentaiodides The black protactinium compound is formed when the pentoxide is heated with aluminium iodide at 500° (A. G. Maddock, personal com­ munication) and is probably formed in the analogous reaction with ammonium iodide (Sellers et al., 1954), although the product of the latter reaction has not been formally identified. Protactinium penta­ iodide is formed in about 70% yield by reaction of the pentoxide with silicon tetraiodide at 600° in a vacuum, but it is best prepared from the elements at 450° or, more conveniently, by reaction of the pentachloride or pentabromide with silicon tetraiodide at 180°. It sublimes in a vacuum at 450° and is soluble in methyl cyanide (Brown et al., 1967). The known thermal instability of uranium tetraiodide renders it extremely unlikely that uranium or higher actinide pentaiodides will be isolable.

G. Mixed halides The only compound recorded is the butoxy derivative, UCl2Br(OBu)2, obtained by treating UCl2(OBu)2 with bromine in tetrahydrofuran; it reacts with sodium cyclopentadienide in tetrahydrofuran to form the butoxytriscyclopentadienide, U(C5H5)3(OBu)2 (Ethyl Corporation, 1963, Report TID-19367).

H. Halo complexes Protactinium(V) is very stable to hydrolysis in aqueous hydrofiuoric acid, in contrast to its behaviour in the other halogen acids, and uran- ium(V) is likewise stable in concentrated (or anhydrous) hydrofiuoric acid, one of the few solvents in which it does not disproportionate. Evaporation of the protactinium solution yields the pentafluoride di- hydrate, as already mentioned, whereas cooling the blue uranium(V) solution in concentrated hydrofluoric acid to —10° yields blue crystals of the hexafluorouranic(V) acid, HUF6,2-5H20 (Asprey and Penneman, 1964a). Fluoro complexes of the types A^MFg, A2MF7 and AgMFg are now known for both protactinium, uranium and neptunium, but only the hexa- and heptafluoro complexes of plutonium have been made so 344 κ. w. BAGNALL far. Ammonium, potassium and rubidium hexailuoroprotactinates(V) (Asprey and Penneman, 1964b) have been obtained by evaporating equimolar quantities of protactinium(V) and the alkali fluoride in hydro­ fluoric acid to dryness. However, the salts made in this way always contain some heptafluoroprotactinate(V) and it is advisable to evaporate to small volume, discarding the first crop of crystals (the heptafluoro complex) and then to add a further quantity of 20M hydrofluoric acid, finally evaporating the solution until crystallization occurs (Keller and Chetham-Strode, 1965). A better method of preparing these salts is by fluorine oxidation of equimolar quantities of protactinium tetrafluoride and the alkali metal fluoride (Asprey et aL, 1965b,c). The protactinium compounds (Brown and Easey, 1966) are isostructural with the uranium(V) analogues, possessing orthorhombic symmetry (Charpin, 1965). Potassium heptafluoroprotactinate(V), K2PaF7, first prepared by Grosse (1934b, 1935) by treating the hydrated pentafluoride with aqueous potassium fluoride, is remarkably stable to hydrolysis and can be recrystallized from water; the caesium salt, however, cannot be obtained from aqueous solution by evaporation because of its suscepti­ bility to hydrolysis and is very soluble in water or aqueous hydrofluoric acid. However, this, and the ammonium, potassium and rubidium salts, are easily obtained by precipitating them from 17M hydrofluoric acid solution with a large volume of acetone (Brown and Easey, 1966), a procedure which is unsuccessful in the case of the smaller lithium cation and which yields only the octafluoroprotactinate(V), NagPaFg, in the case of sodium (Brown and Easey, 1965, 1966), the last being also obtained even when a hydrofluoric acid solution containing 2 moles of sodium fluoride per mole of protactinium(V) is evaporated to dryness. The preferential crystallization of NagPaFg has also been noted by Bukhsh et aL (1966). Potassium, rubidium and caesium octafluoro- protactinates(V), which cannot be prepared from hydrofluoric acid solution, are made by heating together the stoicheiometric quantities of the heptafluoro complex salt and the appropriate alkali fluoride at 450° in dry argon or even in air, and the lithium salt has been made by evaporating to dryness the stoicheiometric quantities of and protactinium(V) in hydrofluoric acid solution and dehydrating the product at 450° in air (Brown and Easey, 1966). The Pa—F vibrations in the infrared spectrum appear at 523, 454 cm-^ in KPaFg, 430, 356 cm-^ in K2PaF7 and at 401 cm~^ in KgPaFg, increasing coordination leading to an increase in the wavelength of the Pa—F stretching vibra­ tion as would be expected (Brown and Easey, 1966); some crystallo­ graphic data have been reported for these compounds by the authors THE HALOGEN CHEMISTRY OF THE ACTINIDES 345

quoted above, as well as a full structure analysis of KgPaFy (Brown and Smith 1965; Brown et al, 1967), and the Raman spectra of RbPaFg and Rb2PaF7 have been recorded (Keller and Chetham-Strode, 1965). The corresponding uranium(V) fluoro complexes have also been investigated in some detail; greenish-white nitrosonium hexafluoroura- nate(V), NOUFg, which is of pseudo-cubic symmetry, has been pre­ pared by reaction of nitric oxide with uranium hexafluoride, an analogous reaction occurring with hexafluoride, but not with tung­ sten hexafluoride, which remains unchanged; no reaction occurs with nitrous oxide (Ogle et al, 1959; Geichman et al, 1962c). The nitro­ sonium salt is also made by the reaction of uranium hexafluoride with nitrosyl chloride (Geichman et al, 1963), a reaction which to the analogous product with , and by reaction of the pentafluoride with (Geichman et al, 1962c). The nitrosonium salt is decomposed by acetone, and trichloro- ethylene and is insoluble in carbon tetrachloride, Freon-113, chloro­ benzene and nitrogen dioxide (Ogle et al, 1959). Nitrosonium hexa- fluorouranate(V) reacts with fluorine, or in anhydrous hydrofluoric acid, uranium hexafluoride being evolved; the solid is reduced to uranium tetrafluoride by hydrogen at 300-350° or by carbon monoxide at 300° (Geichman et al, 1962a). The nitronium compound is likewise obtained by the action of nitrogen dioxide on uranium hexafluoride (Geichman et al, 1962b). The kinetics of hydrolysis of the nitrosonium salt over the range 68-231° have also been studied (Massoth et al, 1960). The white ammonium salt, NH4UF6, was originally made by reaction of an excess of uranium hexafluoride with ammonia (Rampy, 1959b), although it has been reported that the product of this reaction at 25° is a mixture of uranium pentafluoride and ammonium pentafluoroura- nate(IV) (Galkin et al, 1960). However, Rampy (1959b) found that the product of the reaction was soluble in 48% hydrofluoric acid, forming a blue solution from which pale green KUFg was precipitated on addition of potassium fluoride; he also obtained some indications of the formation of K2UF7. The ammonium salt is best prepared by heating uranium pentafluoride with ammonium fluoride in a sealed tube at 80-85° (Penneman et al, 1962), or by prolonged heating of the hexafluoride with ammonium fluoride at 120°. It decomposes, with the evolution of fluorine, at 150° in a vacuum or in argon (Nguyen-Nghi et al, 1965a,b). Geichman et al (1962a) then obtained lithium, sodium, silver and calcium hexafluorouranates(V) by heating the nitrosonium salt with the appropriate nitrates until no further evolution of dinitrogen tetraoxide occurred. The white calcium compound was also made by heating a 346 κ. w. BAGNALL

mixture of uranium tetrafluoride and in fluorine at 210° and the sodium, potassium and silver salts were obtained from 48% (Na,K) or anhydrous (K,Ag) hydrofluoric acid. The alkaH metal salts are best prepared from solutions of the pentafluoride in concen­ trated aqueous (10-27M) hydrofluoric acid and the appropriate alkali fluoride (Asprey and Penneman, 1964a), or by treating a mixture of the pentafluoride and alkali fluoride with anhydrous hydrofluoric acid (Sturgeon et al., 1965), a procedure successfully used for the preparation of the blue sodium salt, which is dimorphic, and the pale yellow-green ammonium, potassium, rubidium and caesium salts, for which X-ray crystallographic data are available. Analysis of the optical absorption spectrum of CsUFg shows that the UFg- ion has a shghtly distorted octahedral configuration (Reisfeld and Crosby, 1965). The magenta caesium hexa- and rubidium heptafiuoroneptunates(V) (Asprey et al., 1966), the analogous green fluoroplutonates(V) (Penne­ man et al., 1965), and rubidium octafluoroneptunate(V) (Bagnall et al., 1967b) have been made by heating the appropriate quadrivalent actinide fluoride compounds in fluorine at 250-300° (Np) or 300-400° (Pu). Caesium hexafluoroneptunate(V) can also be prepared by the action of fluorine on a 1:1 mixture of and neptunium tetrafluoride in anhydrous hydrofluoric acid (Asprey and Penneman, 1967). The lithium, sodium, potassium, rubidium and caesium hexafluo- rouranates(V) can also be made by heating together the stoicheiometric quantities of uranium pentafluoride and the alkali fluoride at 300°; when a 2:1 mixture of alkali fluoride and uranium pentafluoride is treated in this way, all, except lithium, which forms only LiUFg, yield a mixture of the hexa- and octafluoro complexes. Apart from the sodium salts, these, when heated at 350°, react to give the heptafluo- rouranates(V), identified as new phases by X-ray powder photography; they are not isostructural with the heptafluoroprotactinates(V). The octafluorouranates(V) of all except lithium are prepared in a similar manner, using the appropriate quantity of alkali fluoride. The corre­ sponding ammonium salts are made in the same way, but at a lower temperature; these salts and the alkali metal compounds are almost white (Penneman et al., 1964b). Sodium octafluorouranate(V) has also been made by heating sodium heptafluorouranate(IV) in fluorine at 390° and its magnetic behaviour has been recorded, together with X-ray crystallographic data (Riidorfif and Leutner, 1960). Both silver hexafluorouranate(V) and the octa­ fluorocomplex have been made from j8-uranium pentafluoride and silver fluoride at 350-400°; crystallographic data for these compounds, and THE HALOGEN CHEMISTRY OF THE ACTINIDES 347 their infrared spectra, have been recorded (Bougon and Plurien, 1965). Lithium and silver hexafluorouranates(V) are said to decompose with the evolution of fluorine, at 400° and 230° respectively (Nguyen-Nghi et al, 1965b). The pale-yellow caesium, tetramethylammonium and tetraphenyl­ arsonium hexachloroprotactinates(V) (Bagnall and Brown, 1964) and the corresponding deep-yellow to orange hexachlorouranates(V), and the dimethylammonium salt of the latter (Bagnall et al, 1964c) have been prepared from solutions of the components in thionyl chloride (alkylammonium and arylarsonium salts) or in a mixture of and thionyl chloride (caesium salts). Bright yellow tetra­ methylammonium octachloroprotactinate(V) and the pale yellow octa- chlorouranate(V) have also been isolated from thionyl chloride solution. The infrared spectra of these compounds have been recorded; the Pa—CI vibration appears at 308 cm-^ in NMe4PaCl6 and at 290 cm"^ in (NMe4)3PaCl8, consistent with the increased coordination number of the metal ion, and at 310 cm-^ in both hexa- and octachlorouranates(V), probably because of decomposition of the latter in the Nujol mull. The magnetic properties of the uranium(V) chloro complexes have also been recorded. Conductio-metric titration of uranium pentachloride (UCI5. SOCI2) against pyridine in thionyl chloride has given some evidence for the existence of the heptachlorouranate(V) ion, but no salts of this ion have been isolated (Bagnall et al, 1964c). X-ray diffraction data for some of the hexachloro compounds are available (Bagnall and Brown, 1964). Although analogous neptunium(V) compounds have not been isolated, tetraphenylarsonium oxypentachloroneptunate(V), (Ph4As)2NpOCl5, dissolves in thionyl chloride to give a dark-red solution which probably contains the hexachloroneptunate(V) anion; the absorption spectrum of the solution has been recorded, but the neptunium species decom­ poses rapidly; on addition of carbon disulphide a mixture of the hexa- chloroneptunate(IV) and an unidentified neptunium(V) chloro complex precipitates from the solution (Bagnall and Laidler, 1966). The orange tetraethylammonium hexabromoprotactinate( V), NEt4PaBr6 (Brown, 1965) and the brown triphenylmethylarsonium hexaiodoprotactinate(V) have been prepared from a methyl cyanide solution of the components (Brown et al, 1967).

I. Oxyhalides Protactinium oxyfiuoride, PagOFg, a white, hygroscopic solid iso- structural with U2F9 (body-centred cubic), is slightly volatile in vacuum above 500° ; it is made by thermal decomposition of the pentafluoride 348 κ. w. BAGNALL dihydrate at 160° and by reaction of the pentoxide with fluorine at 550° or with an equimolar mixture of hydrogen fluoride and oxygen at 500°. It decomposes above 800°, yielding the pentafluoride among other, unidentified, products (Stein, 1964). The uranium analogue, UgOFg, a white solid, is obtained by heating uranium tetrafluoride at 850° in an intermittent oxygen flow; it is unstable in air and is very hygroscopic. It decomposes in a vacuum at 300° (Kirslis et al., 1950):

2U2OF8 ^UFe + UO2F2 + 2UF4

The corresponding protactinium oxychloride, PagOClg, is obtained as a by-product of the reaction of a mixture of chlorine and carbon tetra­ chloride with protactinium pentoxide mixed with carbon and a second crystal modification of this compound is obtained by heating the penta­ chloride with the stoicheiometric amount of oxygen in a sealed tube at 350-400°. Thermal decomposition of PagOClg at 270° in a vacuum, or treatment of the pentachloride with the appropriate amounts of oxygen at 350-400°, yields the oxychloride Pa203Cl4 and there is some evidence for the formation of PaOCla. Thermal decomposition of Pa203Cl4 at 520° in a vacuum yields the dioxochloride PaOgCl. All of these com­ pounds are oxygen bridged polymers (Brown and Jones, 1966a). Compounds of the general form MOX3 are also known; UOF3 is thought to be formed as an intermediate in the reaction between uranium dioxide and hexafluoride at 500°, the final products of which are uranium tetrafiuoride and uranyl fiuoride (Rampy, 1959a). The green hydrated neptunium analogue has been prepared by the action of hydrogen fluoride on neptunium pentoxide at 40° (Bagnall et al., 1966c). Reddish-brown UOCI3 is usually prepared by heating an equimolar mix­ ture of uranium tetrachloride and uranyl chloride at 370° (Shchukarev et al., 1958b; M. D. Adams et al., 1963); it is formed as an intermediate in the reaction of uranium dioxide, triuranium octaoxide or uranium(IV) oxychloride with carbon tetrachloride, and in the reaction of uranium dioxide with hexachloropropene; a brown compound of composition U2O3CI3 is also formed in these reactions. Uranium oxytrichloride is insoluble in benzene or carbon tetrachloride, but is soluble, with de­ composition, in methanol, ethanol and in water (Budaev and Vol'skii, 1958). Its heat of formation has been reported as —283-4 (Shchukarev et al., 1958b) and —281-4 kcal mole-^ (Kao-P'in K'uo, 1959), in reason­ able agreement. A dark-brown ethanol adduct, UOCl3.EtOH, is obtained by the action of ethanol on the thionyl chloride complex, UCI5.SOCI2 (Bradley et al., 1957). The oxochloro complex, CSUOCI4, has been made by reaction of the hexachloro complex, CsUClg, with antimony(III) oxide (Bagnall et al., 1967a). THE HALOGEN CHEMISTRY OF THE ACTINIDES 349

Yellowish-green protactinium oxytribromide, PaOBrg, is formed as a by-product in the preparation of the pentabromide by the action of bromine on a mixture of the pentoxide and carbon at 600-700°; it is best prepared by heating the pentabromide in oxygen at 350°. A less satisfactory preparative procedure is to heat a mixture of the pentoxide and pentabromide at 400°. The oxytribromide can be separated from the pentabromide by vacuum sublimation of the latter at 300-350° or by dissolving out the pentabromide in methyl cyanide, in which the oxytribromide is insoluble. It is not isostructural with NbOBrg and the Pa-0 vibrations appear in the infrared spectrum at 513, 364 and 298 cm-i, indicating that the compound is probably an oxygen bridged polymer. It disproportionates in a vacuum at 500°, yielding the penta­ bromide and the white dioxybromide, PaOgBr (Brown and J ones, 1966b). Uranium oxytribromide is a brownish solid made by reaction of uranium trioxide with carbon tetrabromide at 110° (Prigent, 1953, 1954b, 1960). It oxidizes to UgOg in air at 140° and is insoluble in carbon tetrabromide, but soluble in chloroform and bromoform, and, with de­ composition, in water, acetone or ethanol. Its absorption spectra in chloroform and in bromoform have been recorded (Kaufman and Rohmer, 1961). The heat of formation at 298° is —233-8 kcal mole-^ (Shchukarev et al., 1958a). A few oxyhalocomplex salts of the type A2MOX5 have been recorded; the pale yellow-green dip3rridinium uranium salt, (pyH)2UOCl5, is said to be precipitated from ethanolic solutions of UCI5.SOCI2 on the addition of pyridine and hydrogen chloride (Bradley et al., 1957), although it is possible that the product may be an equimolar mixture of the hexachlorouranate(IV), (pyH)2UCl6, and tetrachlorodioxouranate(VI), (pyH)2U02Cl4, resulting from disproportionation. The yellow caesium and tetraphenylarsonium neptunium(V) salts are easily made by treat­ ing freshly precipitated neptunium(V) hydroxide with concentrated hydrochloric acid saturated with the appropriate cation chloride; the Np=0 and Np—CI vibrations appear at 921 and at 275, 252 cm-^ respectively in the infrared spectrum and magnetic susceptibility data for the compound have been recorded (Bagnall and Laidler, 1966). There is also some evidence for the formation of the analogous pro- tactinium(V) bromo complex, since protactinium oxytribromide, which is insoluble in methyl cyanide, slowly dissolves in that solvent in the presence of tetra-alkylammonium bromides (D. Brown, personal com­ munication). A few dioxohalides of the type MOgX have been recorded; although the simple dioxofluorides are unknown, apart from the neptunium com­ pound, made by controlled hydrogen reduction of neptunyl fluoride 350 κ. w. BAGNALL

(Bagnall et al., 1966c), a number of alkali metal or ammonium deriva­ tives, A^M02F2 (M = Np, Pu, Am) have been isolated. The grey-green rubidium neptunium(V) and lavender rubidium plutonium(V) salts are precipitated when a cooled solution of the actinide(V) in dilute acid is added to saturated aqueous rubidium fluoride at 0°, and the ammonium plutonium(V) compound is precipitated when solid ammonium fluoride is added to a solution of plutonium(V) at pH 6 (Keenan, 1965). The white, cream or tan americium(V) salts are likewise precipitated when a saturated solution of the alkali fluoride is added to a solution of americium(V) in dilute nitric (Asprey et al., 1954) or hydrochloric acid (Keenan, 1965). All of these salts are of rhombohedral symmetry (Asprey et al., 1954; Keenan, 1965). The protactinium dioxochloride, Pa02Cl, has been mentioned above and there is spectroscopic evidence for the uranium(V) chloride, UO2CI, in fused -potassium chloride solutions of uranyl chloride, formed from the latter by decomposition above 600° in an inert atmo­ sphere or at 450° in a vacuum. Concentrated solutions are greenish- yellow and dilute solutions are yellow (Adams M. D. et al., 1963). It is possibly formed by electrolytic reduction of uranyl chloride in molten salts (Wilks, 1962). The analogous plutonium(V) chloride is possibly formed by the oxidation of plutonium(III) or (IV) with a 2:1 mixture of chlorine and oxygen in fused lithium chloride-caesium chloride or potassium chloride eutectics (Swanson, 1964). Although the corresponding neptunium chloride has not been re­ ported, the caesium chloro complex, CS3NPO2CI4, a bluish-green salt, is precipitated when acetone is added to a near neutral solution of nep- tunium(V) containing caesium chloride; the neptunium(V) solution is obtained by dissolving neptunium (V) hydroxide in the minimum of dilute hydrochloric acid; the Np=0 and Np—Cl vibrations appear at 810, 794 and at 264, 245 cm-^ respectively in the infrared spectrum and magnetic susceptibility data for the compound have been recorded (Bagnall and Laidler, 1966). The analogous americium(V) complex, a pale green solid, is obtained by treating americium(V) hydroxide with a solution of caesium chloride in concentrated hydrochloric acid. It is isostructural with the neptunium compound and the Am=0 vibra­ tion appears at 800 cm-^ in the infrared spectrum (Bagnall et al., 1967b). Protactinium(V) dioxobromide, Pa02Br, obtained in the dispro­ portionation of the oxytribromide, as mentioned earlier, is stable to 700° in a vacuum and is insoluble in methyl cyanide. The Pa—0 vibrations in the infrared spectrum appear at 642, 576 and 376 cm-^, indicating that the compound is probably an oxygen-bridged polymer (Brown and Jones, 1966b). The dark-brown uranium analogue is obtained by heating THE HALOGEN CHEMISTRY OF THE ACTINIDES 351 uranium trioxide with hydrogen bromide at 250°; it decomposes in nitrogen at 500° to the dioxide and bromine; its infrared spectrum appears to be similar to that of uranyl bromide, bands being observed at 940, 890 and 850 cm~^. At —20° the above reaction yields uranyl bromide monohydrate, and at room temperature, a brown-black solid is obtained, possibly U02Br.2HBr (Levett, 1965). Protactinium oxotri-iodide, PaOIg, is a dark brown solid made by reaction of the pentaiodide with antimony(III) oxide in a vacuum at 150°. It disproportionates above 450°, yielding the yellow-brown dioxo- iodide, Pa02l, and the pentaiodide. The dioxoiodide has also been obtained by reaction of the pentaiodide with the appropriate quantity of antimony(III) oxide at 150°. Both of these oxoiodides are formed as by-products of the reaction of the pentoxide with silicon tetraiodide. Their infrared spectra indicate that they are both oxygen bridged polymers like the analogous oxochlorides and oxobromides (Brown et al, 1967).

5. The Hexavalent Actinides A. General chemistry Only four actinide hexahalides are known, comprising uranium, neptunium and plutonium hexafluorides and , but oxyhalides of the form MO2X2 are comparatively well known for all three elements; although the americyl ion, Am02^"'', exists both in solution and in solid compounds, the attempted preparation of the hexa­ fluoride by reaction of the oxide ^^^AmgOg with fluorine in the presence of platinum, using PtFg as the carrier gas, was unsuccessful (Tsujimura et al, 1963), in conformity with the observed marked decrease in the thermodynamic stabilities of the hexafluorides with increasing atomic number (Table XIII). However, the use of the longer-lived (7600 yr) Am might prove more successful since the failure with ^^lAm may

TABLE XIII. Some physical properties of the hexahalides

M.P. B.P. or Triple ) Point Heat of ΔίΖ" fusion fusion (°C) sublimation Temp. Press. formation (cal mole-^ ) (cal mole-i temperature (kcal mole~^, deg-i) CO CO (mm Hg) gas)

56-82 64-05 1139-6 -523 4570 13-61 53 55-18 55-10 758-0 -463b 4198 12-79 PuF^e 50-75 62-16 51-59 533-0 -392b 4456 13-72 UCle

have been due to decomposition resulting from the more intense

α-radiation from the shorter lived (458 yr) ^^lAm. The vibrational spectra of the hexafluorides indicate that they have

regular octahedral symmetry (Oh) (UF^—Bigeleisen et aL, 1948; Claasen et aL, 1956; Gaunt, 1954; 1956; NpFg and PuFg—Malm et aL, 1955; Weinstock and Claasen, 1959); uranium hexachloride is also of near octahedral symmetry (Zachariasen, 1948d). The crystal structures of some of them are also known (Table XIV).

TABLE XIV. Crystallographic data for the hexahalides

Colour Symmetry Space group Lattice paramete rs, A Density «0 bo Co (gcm-^)

UFe^ White Orthorhombic Dll-Pnma 9-900 8-962 5-207 5-060 NpFgb Bright Orthorhombic Dll-Pnma 9-91 8-97 5-21 5-00 orange PuFgd Reddish Orthorhombic Dll-Pnma 9-95 9-02 5-26 — brown UCle^ Greenish Hexagonal Dla-CSm 10-97 — 6-04 3-56 black

» Hoard and Stroupe (1958). ^ Seaborg and Brown (1961). c Zachariasen (1948d). d Florin et al. (1956).

B. Hexafluorides

The hexafluorides are low melting, volatile (Table XV) solids, com­ monly prepared by the reaction of a lower fluoride with fluorine above -220° (UFe), 500° (NpFg) or 750° (PuFg). Neptunium and plutonium hexafluorides are less stable to heat than uranium hexafluoride and their preparation in this way requires a higher temperature, so that it is neces­ sary to allow liquid fluorine to drip onto the heated lower fluoride, giving a high local concentration of fluorine (NpFg—^Malm et aL, 1958; PuFg— Weinstock and Malm, 1956; Florin eiaZ., 1956). Alternatively, the neptu­ nium compound can be made by heating the lower fluoride on a

TABLE XV. Vapour pressure data for the hexafluorides

Range i(°C) State logio p{mm)

UFe^ 0-64 Solid 6-38363 + 0-0075377 t ~ 942·76/(ί + 183-416) 64-116 Liquid 6-99464 - 1126-288/(i + 221-963) 116-230 Liquid 7-69069 - 1683-165/(i + 302-148) KpFe^ 0-55-10 Solid 18-48130 - 2892-0/i - 2-6990 log ί 55-10-76-82 Liquid 0-01023 - 1191-1/ί + 2-5825 log ί PuFei> 0-51-59 Solid 0-39024 - 2095-0/ί + 3-4990 log t 51-59-77-17 Liquid 12-14545 - 1807-5/ί - 1-5340 log ί

a Oliver et aL (1953). to Weinstock et al. (1959). THE HALOGEN CHEMISTRY OF THE ACTINIDES 353 filament in fluorine (Seaborg and Brown, 1961). In either procedure the volatile hexafluoride is carried by convection to a condenser or cold surface on which it is trapped. A more convenient preparative procedure is to react neptunium or plutonium metal or a lower fluoride with at room temperature (Malm et al., 1959). How­ ever, when anhydrous liquid hydrofluoric acid is added to caesium hexafluoroneptunate(V), disproportionation occurs (Asprey and Penne­ man, 1967), indicating an easier low temperature route to neptunium hexafluoride; it is possible that the plutonium(V) fluoro complexes may behave in the same way. The formation of plutonium hexafluoride by fluorination of the tetrafluoride at 200-375° in a flow system has been investigated as a method of recovering plutonium from neutron irradiated uranium reactor fuels (Adams et al., 1957; Robb et al., 1957; Steindler et al, 1958, 1959); the procedure could be used for the laboratory scale preparation of the compound. Uranium hexafluoride can also be prepared by the action of on the oxides of uranium (Emeléus et al, 1948; Emeléus and Woolf, 1950) or uranium metal (Vogel and Vogel, 1951), by the action of chlorine trifluoride on uranyl fluoride (Ellis and Forrest, 1960), and by the action of fluorine on uranium metal or any uranium compound under suitable conditions. It is usually purified by distillation (Ellis et al, 1958; Mears et al, 1958), but the formation and decomposition of the alkali metal fluoro complexes can also be used as a purification method provided fluorine is present (Gathers et al, 1958). Uranium hexafluoride is formed when uranium tetrafluoride is heated in dry oxygen at 800° (Fried and Davidson, 1958), 2UF,+ 02->UFe-fU02F2

a useful preparative reaction for which free fluorine is not required. Some pentafluoride is also formed, either by the reaction

3 UF4 + O2 -> 2 UF5 + UO2F2

or, above 750°, by the reaction UF4 +UFe->2UF5 (Ferris, 1957, 1959). The uranyl fluoride produced in the reaction can be converted to uranium tetrafluoride, for the production of more hexa­ fluoride, by reducing it in hydrogen to the dioxide which is then reacted with hydrogen fluoride. Some UgOg is also formed in the oxidation of uranium tetrafluoride, by way of decomposition of the resulting uranyl fluoride, which occurs above 700° (Ferris and Gabbard, 1958), the rate of decomposition in an atmosphere of dry helium being first order with 354 κ. w. BAGNALL

respect to uranyl fluoride (Ferris and Baird, 1960). Plutonium hexa­ fluoride is also formed by oxidation of the tetrafluoride and by the reaction of plutonium dioxide with a mixture of hydrogen fluoride and oxygen, 2 PuOg + 12 HF -^ O2 2 PuFg + 6H2O

a reaction which probably involves the intermediate formation of the tetrafluoride, or with fluorine

Pu02 + 3F2->PuFe + 02 (Mandleberg et al., 1956). At 600° plutonium trifluoride yields only the tetrafluoride and dioxide, a reaction which is reversed in a vacuum:

4 PuFg + 02^ 3PUF4 + PUO2

(Fried and Davidson, 1949; Dawson et al., 1954b). Fluorination of uranium trioxide or uranyl fluoride with sulphur tetrafluoride above 300° (Oppegard et al., 1960) is also satisfactory; uranyl fluoride is formed as an intermediate in the reaction with the trioxide: U03 + 3SF4-^UFe + 3SOF2

Uranium hexafluoride is reduced by sulphur tetrafluoride above 500°

UFe + SF4->UF4 + SFe whereas plutonium hexafluoride is reduced at 30° (Johnson, C. E. et al., 1961), so enabling a separation of the two elements (Steindler, 1962). Uranium hexafluoride has been the most studied of these compounds because its volatility is of importance for the separation of the rarer fissile isotope, ^^^U, from the non-fissile ^ssxj by methods. The volatilities of the liquid hexafluorides are in the expected order UFg > NpFg > PuFg, but the neptunium compound is anoma­ lous in having the highest vapour pressure of the three in the solid state over the ranges so far investigated, an observation which apparently cannot be related to its paramagnetism. Uranium hexafluoride (Henkel and Klemm, 1935) and plutonium hexafluoride (Gruen et al., 1956) exhibit weak, and almost temperature independent, paramagnetism. Unpaired electrons are absent in UFg, but two non-bonding 5/-electrons are present in the plutonium compound, and these occupy the lowest (ffi) level with paired spins, the /-term being split in an octahedral fleld into three levels, f^^fs and f^. It should be noted that the ground state in PuFg is non-degenerate whether the spins are paired or not (Grifiith and Orgel, 1957). The molar susceptibility of NpFg is only 443 χ 10"^ c.g.s. units at 300°K and 887 χ 10"^ at 64°K, markedly lower than the THE HALOGEN CHEMISTRY OF THE ACTINIDES 355 calculated spin only values of 1240 χ 10-^ and 5810 χ 10*^ c.g.s.u. or those calculated for the unquenched orbital angular momentum cases (Weinstock and Malm, 1957); the lowest susceptibility arises when the splittings produced by both the spin-orbit interaction and electric field are approximately equal (Gruen and Hutchison, 1954). Paramagnetic resonance absorption data for neptunium hexafluoride can be interpre­ ted on the basis of a 5/^-configuration (Hutchison and Weinstock, 1960). Since the volatile hexafluoride could be utilized for the recovery of uranium from irradiated nuclear fuels (Bernhardt et al., 1959; Hyman et al, 1956), the UFg-ClFa and UFg-BrFg phase diagrams (Fischer and Vogel, 1954; Ellis and Johnson, 1958) and the kinetics of the reaction of UF4 with fluorine (Labaton and Johnson, 1959) and chlorine trifluoride (Labaton, 1959; Nikolaev and Shishkov, 1962; Nguyen-Nghi, 1963), as well as of the reactions of sulphur tetrafluoride with uranium trioxide and uranyl fluoride (Johnson and Fischer, 1961) and of fluorine with uranium trioxide, triuranium octaoxide (Iwasaki, 1964) and uranium dioxide (Yahata and Iwasaki, 1964) have been investigated in some detail.

(i) Handling and Stability The three hexafluorides do not react with quartz or Pyrex in the absence of water or hydrogen fluoride, both of which cause virtually unlimited hydrolysis of the hexafluorides as a result of reaction of hydrogen fluoride with silica and its regeneration:

SiOg + 4 HF -> SiF^ + 2 H^O MF, + 2 H2O -> MO2F2 + 4 HF This is prevented, in the case of the uranium compound, by handling it in the presence of anhydrous sodium or potassium fluoride which takes up hydrogen fluoride and water, for example as KHF2.2H2O (Grosse, 1958b); the use of silica-free aluminium phosphate glasses is also recommended (Grosse, 1958d). Similar measures would presumably be effective with the other hexafluorides. All the hexafluorides are decomposed to lower fluorides by α-radia­ tion, the effect being scarcely noticeable with uranium hexafluoride made from natural uranium, but detectable with uranium enriched in the somewhat shorter-lived ^^^U or ^^^V, which have a higher specific α-activity (Shiflett et al., 1958). The decomposition is not serious with the neptunium compound, but the rate of α-radiation decomposition in solid plutonium hexafluoride is as high as 2% per day (Steindler, 1963) and the compound is best stored in the vapour state in order to minimize the decomposition, a large proportion of the α-particles then being 356 κ. w. BAGNALL absorbed by the walls of the container; the lower the gas pressure and the smaller the diameter of the container, the greater the probability that the α-particles will hit the walls rather than molecules of the hexafluoride. Uranium hexafluoride is relatively stable to y-radiation under reactor conditions (Hull, 1947), whereas the plutonium com­ pound is quite readily decomposed (Steindler et al., 1964). Similarly, both neptunium and plutonium hexafluorides are susceptible to photo- decomposition, whereas the uranium compound is stable. Thermal decomposition of plutonium hexafluoride to the tetrafluoride is com­ plete in 1 h at 280°C (Weinstock and Malm, 1956) but is quite slow at 200°C; the kinetics of the decomposition have also been investigated (Fischer et al, 1961; Trevorrow et al, 1961).

(ii) Chemistry Although a great deal is known about the formation and the physical properties of the hexafluorides, few data are available on their com­ plexing and other chemical properties. Uranium hexafluoride forms fluoro complexes with the alkali metal fluorides (Ruff and Heinzel- mann, 1911; Martin et al, 1951); the sodium compound was initially thought to have the composition NagUFg (e.g. Adams et al, 1958) but i^F exchange studies (Sheft et al, 1961) suggested that the compound was NagUFg; this was subsequently confirmed by Katz (1964). No fluorine exchange was observed between UFg and lithium, potassium, silver or fluorides labelled with ^^F (Sheft et al, 1961). The kinetics of the formation of the sodium complex salt have been investi­ gated (Massoth and Hensel, 1958, 1959; Peka, 1965). This work has been extended recently, and it has been shown that reaction of uranium hexafluoride with sodium fluoride dispersed in n-perfluoroheptane at 100° yields the white heptafluoro complex, NaUF,, which decomposes at 100° in a vacuum to the yellow octafluoro complex, NagUFg, which is of body-centred tetragonal symmetry. The corresponding potassium salts have been obtained in the same way (Malm et al, 1966). Other recent work on these and similar systems indicates that both MgUFg and M3UF9 may be formed (M = Na, K, Cs), but that there is no reaction between uranium hexafluoride and lithium, strontium or (Peka, 1966). A complex of composition 2 NaF.UFg.HF has been recorded, formed from UFg and NaHFg (Katz, 1963). Other heptafluoro complexes are well known; the pale yellow ammo­ nium salt, NH4UF7 (cubic, a^ = 13-11 Â) is obtained by condensing uranium hexafluoride in a suspension of ammonium fluoride in C2H2CI4 (Volavsek, 1961) or by adding solid ammonium fluoride to a solution of uranium hexafluoride in chlorine trifluoride; the caesium salt is THE HALOGEN CHEMISTRY OF THE ACTINIDES 357

precipitated when a solution of caesium fluoride in chlorine trifluoride is added to uranium hexafluoride (Nikolaev and Sukhoverkhov, 1961). The ammonium salt decomposes to a mixture of a- and j3-uranium pentafluoride at 170° in a vacuum, and to the tetrafluoride at 450° (Volavsek, 1963). The corresponding hydrazinium salt, N2H5UF7, separates as yellow crystals when a solution of uranium hexafluoride and hydrazinium fluoride in anhydrous hydrofluoric acid is allowed to stand for 24 h (Frlec et aL, 1964). The greenish-yellow nitrosonium and greenish-white nitronium salts, NOUF7 and NO2UF7, are formed directly from uranium hexafluoride and the corresponding fluoride (Geichman et al., 1963); the UF7- ion may have a pentagonal bipyra­ midal structure. Addition compounds with a few metal tri- and tetra­ fluorides have been reported, for example PbF4.UF6, formed by reaction of lead difluoride with an excess of UFg at 450° (Michallet, 1961; Michallet et al., 1961). A curious red complex with titanium tetra­ chloride, UFe.2TiCl4, which may have monoclinic symmetry (with = 6-39, = 9-87, = 8-05 Â, ^ = 79°10' and density 2-45 g cm-^) is reported to be formed when the components are mixed, even at liquid air temperatures (Michallet et al., 1959) in contrast to the halogen replacement reaction which occurs between and titanium tetrachloride (Cohen et al., 1965). More recent work (O'Donnell et al., 1966) indicates that the products of the reaction of uranium hexafluoride with titanium tetrachloride are uranium hexachloride, uranium and titanium tetrafluorides, and chlorine. Uranium hexafluoride is stable to oxygen, nitrogen, carbon dioxide, chlorine, bromine and ; although stable for some time in carbon tetrachloride, chloroform and s-tetrachloroethane at room temperature, reaction occurs on heating. Thus, above 150° carbon tetrachloride reacts to give uranium tetrafluoride, chlorine and chloro- fluoromethanes (Nairn et al., 1958). It readily fluorinates silicon, , phosphorus and most organic compounds, the last yielding fluorocarbons. Although uranium hexafluoride is reduced to the tetra­ fluoride when heated in hydrogen, the reaction has a high energy of activation and is relatively slow even at 600° (Dawson et al., 1950). It is more readily reduced by hydrogen chloride (250°) or hydrogen bromide (80°), with the formation of hydrogen fluoride and free halo­ gen; reaction with anhydrous liquid ammonia at —70° yields NII4UF5 almost quantitatively (Johns et al., 1958), whereas reaction with ammonia gas yields either ammonium hexafluorouranate(V) or uranium pentafluoride, as mentioned earlier. Hydrogen sulphide and carbon disulphide reduce uranium hexafluoride at 25°, the former yield­ ing sulphur tetrafluoride and hydrogen fluoride, the latter sulphur 358 κ. w. BAGNALL

tetrafluoride and perfluoroalkyl sulphides or, at high temperatures, sulphur hexafluoride and (Trevorrow et aL, 1963). Nitrosyl chloride reduces uranium hexafluoride (and molybdenum hexafluoride, but not the tungsten compound) (Geichman et aL, 1963), as does nitric oxide, yielding the quinquevalent nitrosonium fluoride, NOUFg Î nitrogen dioxide yields the corresponding nitronium salt, NO2UF6, in contrast to molybdenum and tungsten hexafluorides, which do not react (Geichman et aL, 1961, 1962b, c; Ogle et aL, 1959). Uranium hexafluoride, when present in excess, is reduced to lower fluorides by , molybdenum pentafluoride, tungsten tetra­ fluoride and by many metal and non-metal chlorides. However, when the hexafluoride is treated with an excess of aluminium chloride or , uranium hexachloride is formed (O'Donnell et aL, 1966).

C. Uranium hexachloride This is a relatively unstable blackish-green solid which begins to decompose at 120-150° and melts at about 178°; it sublimes at 100° and 10-4 torr (Johnson and Butler, 1944). It is usually made by dis­ proportionation of uranium pentachloride at 80-180° in a high vacuum (10-3_iQ-6 torr), the undecomposed reaction product subliming out of the reaction zone (Jenkins, 1951; Carter, 1956; British patent 818 321, U.K.A.E.A., 1959): 2UCl5->UCl4 + UC16

and, in poorer yield, by reaction of a lower chloride with chlorine above 350° or by the action of carbon tetrachloride and chlorine on uranium trioxide at 65-170° under pressure (Reiber, 1950; Van Dyke and Ewers, 1955). UCI5, UOCI3 and UOCI4 appear to be formed as intermediates in the last reaction (Lowrie and Larson, 1946). The most promising route to the hexachloride appears to be the reaction of the hexafluoride with an excess of aluminium chloride or boron trichloride; it has also been shown that the hexachloride reacts with the hexafluoride to form the tetrafluoride, chlorine being liberated (O'Donnell et aL, 1966). Uranium hexachloride is soluble in carbon tetrachloride and chloro­ form, slightly soluble in perfluoroheptane and insoluble in benzene; it is immediately hydrolysed to uranyl chloride by water and reacts with hydrogen fluoride at room temperature to form UFg:

2UC16 + 10 HF -> 2 UF5 + 10 HCl + Clg

The only recorded complex is a yellow species obtained from uranium hexachloride and α,α'-dipyridyl in carbon tetrachloride (Gans, 1964). THE HALOGEN CHEMISTRY OF THE ACTINIDES 359

D. Oxyhalides (i) General Chemistry The uranyl halides are quite well known, but the corresponding neptunyl, plutonyl and americyl compounds have scarcely been in­ vestigated. Since both PuOg^"^ and AmO^^ are reduced to the tervalent state by iodide in aqueous acid, and NpOg^^ is reduced to the quadri­ valent state under similar conditions, it is doubtful whether the iodides will be obtainable; indeed, it is probable that uranyl iodide has never been obtained pure and in an unsolvated state. Since AmOg^^ is also reduced to the quinquevalent state by chloride or bromide ion in aqueous solution only the fluoride is preparable from such media and the other halides will have to be investigated, if they can be obtained at all, in nonaqueous solvents. Uranyl, neptunyl and plutonyl ions give rise to halocomplex species such as M^^O^f^^- and M^^OgFs" in the appropriate halogen acid and salts of these ions, and of the analogous americyl chloro complex, have been obtained. Some crystallographic data for the simple halides are given in Table XVI.

TABLE XVI. Crystallographic data for the oxyhalides, MO2X2

Symmetry and space group Lattice parameters (A) Calculated Refer­ density ence Κ Co (gcm-8)

U02F2 Rhombohedral, R'^m — D^^ 5-764 (α = 42°43') 6-37 1 NpO^F^ Rhombohedral, R~^m — Dl^ 5-795 (α = 42° 16') 6-41 2 PUO2F2 Rhombohedral, R'^m — D^^ 5-797 (α = 42°) 6-50 3 UO2CI2 Orthorhombic 8-73 8-41 5-73 5-43 4

1, Zachariasen (1948c). 2, Zachariasen (1949a). 3, Alenchikova et al. (1958a). 4, Baenziger and Rundle (1944).

(ii) Fluorides Anhydrous uranyl fluoride, a pale yellow, hygroscopic solid, is made by heating uranium tetrafluoride in oxygen, as mentioned earlier, by the action of hydrogen fluoride on uranium trioxide above 300°C in the presence of oxygen UO3 + 2HF ^ UO2F2 + H2O (Johnson and Clewett, 1946; Kuhlman, 1948), by the action of hydro­ gen fluoride on uranyl acetate at 250° (Brooks et al., 1956) or by the action of fluorine on uranium oxides at 350°. Uranyl fluoride decomposes, without melting, to UgOg at high temperatures as mentioned earlier; it is reduced to UO2 by hydrogen above 450° and some UF4 is formed at higher temperatures, presumably by reaction of the hydrogen fluoride which is formed with the dioxide. 360 κ. w. BAGNALL

Sulphur reduces it to a mixture of the dioxide and tetrafluoride at 500-600° (Rampy, 1961); only the latter is formed when a mixture of uranyl fluoride and sulphur is heated in hydrogen fluoride at 300-400°. The anhydrous neptunyl compound, a pink solid, is formed, mixed with sodium fluoride, by the action of hydrogen fluoride on sodium neptunyl acetate at 300-325° (Fried, 1954); the pure compound is made by heating neptunium trioxide hydrate in hydrogen fluoride at 300° or by vacuum drying the precipitate obtained by adding hydrofluoric acid to a solution of neptunium(VI) (Bagnall et al., 1966c). It is isomorphous with uranyl fluoride (Table XVI). White, gelatinous hydrated plutonyl fluoride is precipitated when methanol and concentrated hydrofluoric acid are added to an aqueous solution containing plutonium(VI) (Anderson, 1949c); the anhydrous salt is obtained when this hydrate is washed with anhydrous hydrofluoric acid and dried over phosphorus pentoxide (Alenchikova et al., 1958a). The formation of plutonyl fluoride has been observed in the hydrolysis of plutonium hexafluoride, identiflcation being by X-ray powder photography, the product being isomorphous with uranyl fluoride (Florin et al., 1956; Mandleberg et al., 1956), but crystallographic data were not recorded in these instances. Americyl fluoride, a brown solid, has been made by evaporation to small volume of an aqueous solution of americium(VI) in hydrofluoric acid, followed by addition of liquid hydrogen fluoride to the frozen solution (T. K. Keenan, personal communication). Aqueous solutions of uranyl fluoride are prepared by dissolving uranium trioxide in hydrofluoric acid and although crystallization is difficult, trihydrates (Brooks et al., 1956) and compounds of the type UO2F2.2HF.4H2O can be obtained from solution (Buslaev et al., 1963). Somewhat similar species are formed in the PuOaFg-HF-HaO system (Alenchikova et al., 1961). Adducts of uranyl fluoride with ammonia, UO2F2.4NH3 obtained with liquid ammonia and U02F2.2(or 3)NH3 with the gas, have been recorded (Unruh, 1909). Uranyl fluoride is very soluble in water and there is spectrophoto­ metric evidence for the formation of UO2F42- ions in solution (Blake et al., 1951), but this has not been confirmed by solvent extraction studies (Day and Powers, 1954). Yellow complex salts such as K3UO2F5, in which the UOgFg^- ion is a pentagonal bipyramid (Zachariasen, 1954), NaU02F3, K3(U02)2F7, K5(U02)2F„ (NH4)3U02F5 and Ba3(U02F5)2 are readily obtained from aqueous solutions. Thus (NH4)3U02F5 is the solid phase in equilibrium with ammonium fiuoride and uranyl fluoride in water (Ferris, 1960). The hydrated hydrazinium salt, U02F2.(N2H5F)3.1-5H20, yields the quadrivalent complex. THE HALOGEN CHEMISTRY OF THE ACTINIDES 361

N2H5UF5.H2O, at 200° in a vacuum and uranium tetrafluoride at 400° (Sahoo and Satapathy, 1964). KUO3F, an orange-red solid which appears to form adducts with acetic or oxalic acid, is made by heating uranium trioxide with an excess of potassium fluoride at 850° and extracting the unchanged potassium fluoride with water (Mitra, 1963) and there is also evidence for the existence of NaUOgF (Ippolitova and Kovba, 1961). Plutonyl fluoro complexes are also known; the pink compounds, all of which are probably hydrated, are precipitated from fluoride solutions of plutonium (VI). The quinolinium salt appears to be of the form C9II7NHPUO2F3.H2O, but the potassium, rubidium, caesium, tetra­ methylammonium and pyridinium salts have not been characterized (Anderson, 1949c).

(iii) Chlorides Anhydrous uranyl chloride is a bright yellow crystalline solid (Table XVI, p. 259) which becomes orange at high temperatures (Ochs and Strassmann, 1952); it melts at 578° and is converted to oxides, such as UgOg, on ignition in air. It is conveniently made by heating uranium tetrachloride in oxygen at 300-350° (Johnson et al,, 1958), less satis­ factorily by heating uranium dioxide in chlorine, a reaction originally due to Péligot (1842b), which, although it does not go to completion, gives higher yields at 800° under pressure (Prigent, 1958). Although it is diflicult to dehydrate the hydrates obtained by evaporating solutions of uranium trioxide in hydrochloric acid to dryness, it can be done by heating them in hydrogen chloride at 300° and then in chlorine and hydrogen chloride at 400° (e.g. Ochs and Strassmann, 1952; Bradley et al,, 1959), but not by refluxing the hydrate with thionyl chloride, which leaves the monohydrate (Hefley et al,, 1963). The heat of forma­ tion of the anhydrous compound is —301-9 kcal mole"^ (Shchukarev et al,, 1958b). The reaction of uranium trioxide with hydrogen chloride yields the monohydrate but is extremely slow in the absence of moisture. The monohydrate, which is monoclinic (Staritzky and Truitt, 1950), can be dehydrated in hydrogen chloride at 300° (Kraus, 1942b). Both mono- and trihydrates can be isolated from aqueous solution and a yellow unstable complex, UO2CI2.HCI.2II2O, which fumes in air, separates from solutions of uranium(VI) in hydrochloric acid on cooling to — 10° (Aloy, 1901a). Neptunyl chloride has not been recorded, but the hydrated plutonyl compound, PUO2CI2.6H2O, has been obtained by vacuum evaporation of aqueous hydrochloric acid solutions of plu­ tonium (VI) at room temperature. The solid is described as pinkish (Studier, 1954), or greenish yellow (Alenchikova et al,, 1959). 362 κ. w. BAGNALL

Uranyl chloride ionizes to UO2CI+ and UO2CI3- respectively in dilute and concentrated chloride solutions in 30-60% ethanol (Hefley and Amis, 1960). There is evidence for the formation of hydrolytic species, such as U02(0H)C1, in dilute solutions of uranyl chloride (Pozharskri et al., 1963) and electrophoresis, electrolysis and infrared studies have led to the suggestion that one of the species present in such solutions might be represented as [(U03H)2] [UCl6(OH)2] (Duval, 1962). The former could be regarded as the parent acid of the oxochloro complex salts AIUO3CI described later. Solid uranyl, neptunyl and plutonyl tetrachloro complexes have been isolated from aqueous solution. The yellow to yellow-green salts M2UO2CI4 (M =^ NH4, K,Rb, dihydrates and Cs, ΝΜθ4, NEt4 and NMcgH, anhydrous) crystallize from aqueous solutions of the appropriate chlorides and hydrochloric acid (NH4, Κ— Berzelius, 1824; Péligot, 1842a; all—Rimbach, 1904). The golden yellow anhydrous sodium and potassium salts are obtained by passing uranyl chloride vapour over the heated alkali metal chloride (Aloy, 1901a) and the potassium salt has also been made by fusing stoicheio­ metric amounts of the components at 280° or by the action of hydrogen chloride on potassium uranate, K2UO4, at 250°. It melts at about 290° and forms the dihydrate in moist air (Lucas, 1964). The yellow-green anhydrous quaternary ammonium, pyridinium and quinolinium (Loebel, 1907), 1,10-phenanthrolinium (Markov and Tsapkin, 1961) and 2,2'-dipyridylium (Markov and Tsapkin, 1959) salts have been prepared from aqueous solution; the pyridinium salt has also been made by passing hydrogen chloride into an ethanolic solution of uranyl chloride hydrate and pyridine (Bradley et al., 1959). The 2,2'-dipyridylium and o-phenanthrolinium salts are also formed by air oxidation of uranium tetrachloride in DMP in the presence of 2,2'-dipyridyl or o-phenanthro­ line (Gans and Smith, 1964b). Similarly, the diphosphonium salts, (R3PH)2U02Cl4, have been made by air or hydrogen peroxide oxidation of the hexachlorouranates(IV) in bofling ethanol (Gans and Smith, 1964a). Mixed halide salts, such as K2U02Cl2Br2, have been recorded, made by fusing uranyl chloride with at 300°, by the action of hydrogen chloride on the salt K2U03Br2 at 290°C, or the ammonium salt at 150°, and by dehydration of the dihydrate which crystallizes from a solution of uranium trioxide in 20% hydrochloric acid containing potassium bromide (Prigent and Lucas, 1960; Lucas, 1964). The anhydrous compound melts at about 290° and forms the dihydrate in moist air. The dark yellow caesium (Bagnall and Laidler, 1966) and tetraethylammonium tetrachloroneptunate(VI) and the yellow tetraethyl-, tetrapropyl- and triethylammonium plutonate(VI) crystallize from concentrated hydrochloric acid solutions of the THE HALOGEN CHEMISTRY OF THE ACTINIDES 363 actinide(VI) and the appropriate cation chloride (Ryan, 1963). The tetramethyl- and tetraethylammonium uranium and plutonium salts have also been made by evaporating 4M hydrochloric acid solutions of the actinide(VI) with the tetra-alkylammonium chloride; all are of tetragonal symmetry excepting the tetraethylammonium uranium com­ pound, which is of monoclinic symmetry (Staritzky and Singer, 1952). Although americium(VI) is rapidly reduced by chloride ion, the dark red tetrachlorocomplex salt, Cs2Am02Cl4, is obtained by treating the americium(V) complex, Cs3Am02Cl4, with concentrated hydrochloric acid, in which neither salt is appreciably soluble. This reaction appears to involve a spontaneous oxidation which has been ascribed to the high lattice stabilization energy of the americium(VI) compound. It exists in two crystal modifications (Bagnall et al., 1967c), one of which is iso- structural with CS2UO2CI4, which is of monoclinic symmetry (Hall et al., 1966). Salts such as K2UO3CI2 have been made by heating the chlorodi- bromouranate(VI) in oxygen at 250° (Prigent and Lucas, 1960, 1961), by heating potassium uranate, K2UO4, in hydrogen chloride at about 200° (Lucas, 1964); the potassium salt is also made by reaction of the stoicheiometric quantities of uranyl chloride monohydrate with potas­ sium hydroxide, and the ammonium salt by treating the monohydrate with gaseous ammonia, reactions which suggest that uranyl chloride monohydrate could be considered as an acid, H2UO3CI2. The potassium and ammonium salts are converted to the corresponding tetrachloro- uranate(VI) by hydrogen chloride at 150° (Prigent and Lucas, 1961). A second series of oxochloro complexes is obtained by heating the with uranium trioxide in a vacuum at up to 600° (Allpress and Wadsley, 1964); these are of monoclinic symmetry and appear to be non-stoicheiometric, the caesium salt being close to CS0.9UO3CI0.9. Derivatives of the diuranyl ion, U205^+, and triuranyl ion, U308^+, such as K2TJ2O5CI4 and K2U3O8CI4, have been made by heating the di- and triuranates, K2U2O7 and K2U3O10J in hydrogen chloride, respec­ tively at 240° and 210-250°. They form respectively tetra- and hexa­ hydrates in moist air (Lucas, 1964). Many adducts of uranyl chloride with oxygen and nitrogen donors, usually of the type UO2CI2.2L, have been reported (Katz and Rabino­ witch 1951, p. 584); all of them are orange to yellow or greenish-yellow crystalline solids. The orange diammine is precipitated from ethereal solutions of uranyl chloride by gaseous ammonia and when solid uranyl chloride is exposed to the gas (Peters, 1909, 1912) but no definite product has been isolated from the reaction of uranyl chloride with anhydrous liquid ammonia (Rosenheim and Jacobsohn, 1906). A 364 κ. w. BAGNALL triammine, as well as complexes with alkyl amines (UO2CI2.2-3L) and hydrazine (UO2CI2.4L) have also been obtained from the components. The ammines are the most thermally stable of these compounds; uranyl chloride, but not the trioxide, is reduced to the dioxide at room temperature by aqueous lOM hydrazine (Kalnins and Gibson, 1959). Adducts with 1,10-phenanthroline (UO2CI2.L and UO2CI2.2L) precipi­ tate from ethanolic solution of the components, whereas the 2,2'-di­ pyridyl complex (UO2CI2.L.2H2O) can be obtained from ethanolic or aqueous solution (Markov and Tsapkin, 1959). The complex with tri- butyl phosphate, UO2CI2.2L, is present in ligand solutions of uranyl chloride (Komarov and Pushlenkov, 1961a) and the absorption spectra of such solutions also indicate the existence of this complex and of the complex UO2CI2.3TBP (Vdovenko et al, 1963). The trialkyl and triaryl phosphine oxide complexes (UO2CI2.2L) are made by oxidation of the corresponding uranium tetrachloride bis-(phosphine oxides) with 100 volume hydrogen peroxide (Gans and Smith, 1964a). The supposed tri- phenylphosphine adduct (Majumdar et al, 1964) is the phosphine oxide complex (Fitzsimmons et al, 1966). Complexes with pyridine-iV-oxides, UO2CI2.3L (L = 4-methylpyri- dine-iV-oxide) and UO2CI2.2L (L = 4-methoxy- and 4-nitropyridine- iV^-oxide) precipitate from acetone solutions of uranyl chloride and the ligand, whereas 4-chloropyridine-iV-oxide precipitates the complex UO2CI2.4L from hot ethanolic solution (Balakrishnan et al, 1966). The ethanol adduct, UO2CI2.2L, is formed in the azeotropic dehydration of hydrated uranyl chloride in benzene-ethanol mixture, from which it is separated by evaporation of the solvent (Bradley et al, 1959) and that with acetic anhydride, UO2CI2.L, is formed by the reaction of uranium trioxide with acetyl chloride (Chrétien and Oechsel, 1938). The acet­ amide complex, UO2CI2.2L.H2O, separates as green crystals when the ligand is added to a methanol solution of uranyl chloride and under similar conditions urea forms the complex UO2CI2.4L (Markov and Tsapkina, 1962), but from aqueous solution urea forms the complex UO2CI2.2L.H2O or, if a large excess of urea is used, UO2CI2.3L.H2O (Markov and Tsapkina, 1959). A variety of complexes is formed with iV-alkyl substituted ureas, UO2CI2.2L with 1,3-dimethylurea and tetra- methylurea, UO2CI2.3L with ethylurea and 1,3-diethylurea, and UO2CI2.4L or UO2CI2.5L with 1,3-dimethylurea (Deptula, 1965). The i\ri\r-dimethylformamide complex, UO2CI2.3L is made by dissolving uranyl chloride in the ligand and evaporating the solution in a vacuum (Lamisse et al, 1964), whereas iViV-dimethyl acetamide forms the com­ plex UO2CI2.2L in acetone solution (Bagnall et al, 1966d). NNN'N'- tetramethylmalonamide (TMMA) and -glutaramide (TMGA) precipitate THE HALOGEN CHEMISTRY OF THE ACTINIDES 365

the complexes UOgCla. 1 ·5Ε from acetone solution and the corresponding derivatives of α,α-dimethylmalonamide (HMMA) and 3,3-dimethyl- glutaramide (HMGA) precipitate the complexes UOaClg.L. UO2CI2. 1 ·5ΤΜ0Α is appreciably dissociated to UOgClg.TMGA and free ligand in boiling methyl cyanide and UO2CI2.HMGA is monomeric in that solvent, with the uranium presumably 6-coordinate, possibly in an octahedral environment. The other dicarboxylic acid amide complexes are insoluble in polar solvents and appear to be polymeric (Bagnall et aLy 1966b). Infrared spectra of the amide and urea complexes indicate that coordination is by way of the carbonyl oxygen.

(iv) Bromides The only actinide(VI) bromide known is the uranyl compound. The anhydrous salt is a blood-red hygroscopic solid which becomes yellow as it hydrates; it is unstable, decomposing slowly even at room tempera­ ture with the evolution of bromine. It is made by heating the tetra­ bromide in oxygen at 150-160°, the product being about 96% pure (Powell, 1944; Powell and Nottorf, 1944; Spedding et al, 1958), by heating UOBrg in oxygen at 150° (98% pure) or by heating uranium dioxide with bromine in a sealed tube (95% pure) (Prigent, 1954a, 1960). Some uranyl bromide is formed when a mixture of uranium dioxide and carbon is heated in bromine vapour; extraction of the pro­ duct with ether yields UOgBrg.EtgO (Unruh, 1909). Aqueous solutions are obtained by dissolving uranium trioxide or uranyl acetate in aque­ ous hydrobromic acid (Sendtner, 1879) or when an aqueous suspension of uranium dioxide is heated with bromine on a water bath (Richards and Merigold, 1902). Evaporation of the solution yields the yellow- orange, deliquescent trihydrate, formerly thought to be a heptahydrate (Sendtner, 1879), also formed when the anhydrous compound is allowed to hydrate (Shchukarev et al, 1959). The trihydrate loses one molecule of water at 60° and decomposes at higher temperatures. It forms solvate hydrates when recrystallized from ether or isopropanol. The yellow basic salt, U02(OH)Br.2H20, crystallizes from acid deficient solutions; this is stable in dilute aqueous solution, but concentrated solutions deposit hydrated uranium trioxide (Peterson, 1961). The yellow to yellow brown hydrated complex halides, M2U02Br4. 2H2O (M = NH4, K), are less stable than the chloride analogues. They are made by dissolving the corresponding uranates in hydrobromic acid and concentrating the resulting solution (Sendtner, 1879). The potassium salt can also be prepared from a 1:1 mixture of the com­ ponents in water; with the stoicheiometric quantities some potassium bromide is included in the crystals of the product (Lucas, 1964); the 366 κ, w. BAGNALL

pyridinium salt separates on cooling when pyridine is added to a solu­ tion of uranium trioxide in boiling alcoholic hydrobromic acid (Loebel, 1907). The anhydrous caesium salt, CsgUOgBr^, has been obtained from hydrobromic acid solution (Ellert et al., 1965) and has been shown to be of monoclinic symmetry (Mikhailov et al., 1965). The dihydrates can be dehydrated without decomposition in nitrogen at 120°; the anhydrous potassium compound melts at about 290° and reforms the dihydrate in moist air. The anhydrous potassium compound yields KaUOgBra when heated in oxygen at 250°; this is also formed when uranyl bromide monohydrate, obtained when the trihydrate is allowed to stand over phosphorus pentoxide in a vacuum, is treated with the stoicheiometric quantity of potassium hydroxide; the corresponding ammonium salt is formed by treating the monohydrate with ammonia gas at room temperature. Both salts react with hydrogen chloride at 150° to give the mixed halo complex, MaUOgClaBra (Prigent and Lucas, 1961). KUOgBr, made in the same way as the chloride, has also been investigated (Allpress and Wadsley, 1964). Adducts with 2, 3 or 4 molecules of ammonia are formed in ethereal or ethanolic solution (Unruh, 1909) and a black adduct with acetic anhydride, U02Br2.2L, is obtained when uranyl acetate is refluxed with acetyl bromide (Paul et al., 1961). Complexes with 2 molecules of tributyl phosphate (Komarov and Pushlenkov, 1961a) or tributyl phosphine oxide (Komarov and Pushlenkov, 1961b) have been prepared in the ligand solution and the complex with 3 molecules of iViV-dimethyl- formamide prepared from the components is of monoclinic symmetry (Kaufman et al., 1963). However, iViV^-dimethylacetamide forms the complex U02Br2.2L (Bagnall et al., 1966d). Phosphine oxide complexes, U02Br2.2R3PO (R = Me,Et,Ph), are obtained from methanol or acetone solution (Gans, 1964). The reported triphenylphosphine adduct (Majum­ dar et al., 1964) is the phosphine oxide complex (Fitzsimmons et al., 1966).

(v) Iodides It is doubtful whether pure anhydrous uranyl iodide has ever been obtained; its adducts are much less stable at room temperature even than those of the bromide. Reaction of ethereal tri­ hydrate with barium iodide (Aloy, 1901b) or of uranyl chloride with sodium iodide in ether (Lamisse and Rohmer, 1963) and evaporation of the filtrate yields an orange-red etherate, stable at 0° but decomposing at room temperature. It is very hygroscopic, very soluble in water and soluble in methanol, ethanol, ether, acetone, pyridine and methyl acetate. Addition of iV^iV-dimethyl formamide to its ethereal solution THE HALOGEN CHEMISTRY OF THE ACTINIDES 367

yields the adduct UO2I2.4L (Lamisse et al,, 1964). A supposed complex with triphenylphosphine, UO2I2.2L (Majumdar et al,, 1964) is the phosphine oxide adduct (Fitzsimmons et al,, 1966; Day and Venanzi, 1966b). Aqueous solutions of uranyl iodide have been made by reduction of uranyl iodate with aqueous sulphur dioxide (Richards and Merigold, 1902) and by double decomposition of uranyl sulphate and barium or calcium iodide (Truttwinn, 1925; Lynds, 1962). Vacuum evaporation of the resulting solution at room temperature gives a dark-brown product which may contain some uranyl iodide hydrate. Ammines have been reported (Unruh, 1909) and the only recorded iodo complex, dark red (Ph3BuP)2U02l4, has recently been prepared from methyl cyanide solution (Day and Venanzi, 1966b).

References

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