The Halogen Chemistry of the Actinides 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-iodides 313 F. Mixed Halides 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 Fluorides 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. Hexafluorides 352 C. Uranium 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 actinium (89) to lawrencium (103) as a second/-transition series, the actinides, originally suggested by Seaborg, is now well established. The earlier members of the group, up to americium (95) exist in a greater variety of valency states (Table I) than do the lanthanides, 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 valence 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 einsteinium (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 ions 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 actinide hexahalide, also resembles its tungsten analogue to some extent. In the pentavalent state the MOion predominates in aqueous media, in contrast to the behaviour of protactinium(V), with which hydrolysis and polymeriza­ tion to oxygen-bridged species occurs readily in the absence of com- plexing anions, a behaviour rather similar to that of niobium and tantalum, and there is no satisfactory evidence for the protactinyl ion, PaOg"^. However, the halide 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 thorium 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 acids 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 halogens; as far as is known, oxygen donors, such as iV^iV^-dialkylamides, substituted phosphine oxides and dimethylsulphoxide, form more stable complexes than simple nitrogen 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, plutonium and americium) and microscale (actinium, curium (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), neptunium (Cunningham and Hindman, 1954) and plutonium (Cunningham, 1954) are also available. The properties of uranium hexafluoride (De Witt, 1960) and plutonium hexafluoride (Steindler, 1963) have also been reviewed. 306 κ. w. BAGNALL 2. The Trivalent Actinides A. General chemistry The stable oxidation state 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 chloride 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). Bromide 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 salt media and temperature-composition diagrams for plutonium trifluoride and trichloride in alkali and alkaline earth fluoride 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 metal atom lies on a twofold axis and has 9 fluorine 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 acid or a soluble fluoride.