complex : thermochemical and structural considerations J. Fuger

To cite this version:

J. Fuger. actinide complex halides : thermochemical and structural considerations. Journal de Physique Colloques, 1979, 40 (C4), pp.C4-207-C4-213. ￿10.1051/jphyscol:1979465￿. ￿jpa- 00218861￿

HAL Id: jpa-00218861 https://hal.archives-ouvertes.fr/jpa-00218861 Submitted on 1 Jan 1979

HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. JOURNAL DE PHYSIQUE Colloque C4, supplément au n° 4, Tome 40, avril 1979, page C4-207

Alkali metal actinide complex halides: thermochemical and structural considerations

J. Fuger

Institute of Radiochemistry, University of Liege, Sart Tilman, B-4000 Liege, Belgium

Résumé. — L'auteur passe en revue l'état actuel de nos connaissances dans le domaine de la thermodynamique des complexes halogènes d' avec les alcalins, en portant une attention toute spéciale aux dérivés chlorés et bromes. Lorsque les données thermodynamiques et structurales sont accessibles, il tente de déduire l'évolution de l'énergie de la liaison actinide-halogène au sein d'une série de composés isomorphes ou analogues. Enfin, la variation énergétique au cours de la formation du complexe halogène à partir des halogénures binaires d'actinides et de métaux alcalins est prise pour en vue de prévoir la stabilité de composés nouveaux, spécialement ceux pour lesquels l'halogénure binaire d'actinide n'a pas été préparé ou est de faible stabilité. Diverses méthodes de préparation sont évoquées.

Abstract. — The present status of our information on the thermodynamics of the actinide halogeno-complexes with alkali metal ions is reviewed, with special emphasis on chloro- and bromo-derivatives. Where enough thermodynamic and structural data are available, attempts are made to deduce the evolution of the energetics of the actinide- bonds along a series of isomorphous or analogous compounds. The energy change upon the formation of the halogeno-complexes from binary actinide halides and alkali is discussed with the aim of predicting the stability of new compounds, especially those for which the corresponding binary actinide halides have not been characterized or are of low stabilities. Possible preparative routes for such compounds are also outlined.

1. Introduction. — For many years the halogeno- instance via aqueous rather than via dry method) or complexes of the actinides, as well as those of the to handle (because they are less hygroscopic) than and of d transition and main the binary halides. elements have received considerable interest from In the present we shall restrict our conside­ the inorganic and the physical . It is probably rations to halogeno-complexes and oxyhalogeno- significant to indicate that at the Internatio­ complexes involving alkali metal ions ('), as thermo­ nal Conference on the of the dynamic data on complexes with organic univalent Actinides in Wroclaw (1976) four were de­ cations are simply not existent and since our overall voted to this topic : these papers, however, were information on complexes with other metal ions are essentially oriented toward spectral and magnetic of relative paucity. studies, leading to energy levels of the actinide cation in a highly symmetrical environment of 2. Fluoro-complexes. — A very large number of anions. Structural of these compounds actinide complexes have been characte­ has also received a lot of attention and has shown rized, in which the actinide cation is displaying a that quite often the coordination about the actinide coordination which can vary from 6 to 9, and prepa­ in a complex halide is not the same as in the binary rative, structural and spectral studies on these halide. On the other hand our information on the compounds have adequately been reviewed [1-3]. thermochemical properties is quite fragmentary : Our knowledge on the enthalpies of formation of This may appear surprising since such data are such species is so far restricted to a number of needed to fully understand the very existence of compounds [4-6] with the general formulae such compounds. The stabilization observed upon M;U02F;, M'(U02)2F5 and M;(U02)2F9 (M' variously formation of such complex halides from the binary Na, K, Rb and Cs). We have also at disposal is itself a source of valuable information with information on the complex compounds of UFf) with respect to the obtention of new compounds for NaF : NaUF„ Na2UF8 and the controversial which the binary either cannot be obtained or is Na3UF9 [7-10] ; however, quantitative thermodyna- of very low stability. Finally, on a practical point of view, these halogeno-complexes often provide the actinides in a wide choice of oxidation states, in the (') The cation, which often behaves like an alkali metal will not be considered here. We have also disregarded form of compounds which are easier to prepare (for the numerous alkali metal hydrated complexes.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979465 C4-208 J. FUGER mic data on fluoride complexes of in a More recently similar data were obtained on the valency state other than six or on complex newly characterized compounds Cs,NpBr6 and of other actinides are essentially lacking except for Cs,PuBr6 by Magette and Fuger [19] and Niffle and studies on the interaction of alkali metal fluorides Fuger [20]. Table I lists the recently assessed with PuF, [I 11. values [21] for the standard enthalpies of formation Under such conditions the establishment of at 298 K of the various actinide (IV) chloro- and thermodynamic interrelationships with fluoro- bromo-complexes, according to reaction (1) complexes seems premature. 3. Chloro- and bromo-complexes. - Here, our information is much broader and, in fact, as early as 1911, Chauvenet [12] obtained results on the enthal- 4 MW4+x(c), AH: (1) pies of formation of various chloro-complexes of (IV) with the general formula M'ThCl,, where M', M and X, are, respectively, the alkali M:ThC16 and M:ThCl,, from the comparison of the metal, the actinide and the halogen in their standard enthalpies of solution in of the binary halides state at 298 K : crystalline (c), (1) and gaseous of these chloro-complexes. In a study involving the (g). These values are consistent with the latest various actinide (IV) compounds of the type auxiliary data recommended by CODATA [22] or Cs,MC16 (M = Th to Pu inclusive) Fuger and compatible with the CODATA selection 1231. Ta- Brown [13, 141 obtained results in good agreement ble I lists also the best values for the enthalpies of with those of Chauvenet for the thorium salt. On the formation, AHcomp,,,,from the binary salts, ac- other hand, Martynova et al. and Vdovenko et al. cording to reaction (2) reported results for the enthalpies of formation of various alkali metal uranium (IV) chloro- complexes 115, 161 and bromo-complexes [17, 181.

Table I. - Thermodynamic data associated with the formation of quadrivalent actinide chloro- and bromo -complexes at 298 K (kJ . mol-I).

Original references (*) on Compound - AH: which data are based - - - LiThC1, 1 619.6 + 4.2 Chauvenet [I23 KuC1, 1481.1 + 3.3 Martynova [15] ;Vdovenko [16] RbUCl, 1 497.9 2 4.2 Vdovenko [16] CsUCl, 1 518.4 +- 4.2 Vdovenko [16] Li,ThCl, 2 038.9 & 6.3 Chauvenet [I21 Li,UC16 1 831.3 + 3.8 Vdovenko [16] Na,ThC1, 2 041.4 + 6.3 Chauvenet [I21 Na2UCl, 1 848.1 + 3.8 Martynova [I51 ;Vdovenko [16] NaKUCl, 1891.623.8 Martynova [15] K,ThCl, 2 110.8 + 6.3 Chauvenet [I21 K2uc16 1931.8 & 3.8 Martynova [15] ;Vdovenko [16] Rb,ThCI, 2 157.3 rt 6.3 Chauvenet [I23 Rb,UCl, 1 956.0 2 3.8 Vdovenko [ 163 Rb4ThC1, 3 063.5 t 8.0 Chauvenet [l2] Rb,UCl, 2 828.0 rt 4.2 Vdovenko 1161 Cs,ThCl, 2 147.6 r 2.1 Chauvenet [12] ;Fuger, Brown [13] Cs,PaC16 2029 + 13 Fuger, Brown [I41 Cs,UCl, 2 011.2 + 4.2 Fuger, Brown [13] ;Vdovenko [16] Cs,NpCl, 1 977.4 t 1.7 Fuger , Brown [13] Cs,PuCl, 1 972.8 + 2.9 Fuger ,Brown [13] Cs4ThC1, 3 053.1 + 8.0 Chauvenet [12] Na,UBr6 1 529.7 t 2.5 Vdovenko [I81 K,UBr6 1 632.6 % 2.5 Vdovenko [l8] Rb,UBr6 1 653.3 2 3.3 Vdovenko 1171 Cs,UBr, 1 710.0 % 3.3 Vdovenko [l7] Cs,NpBr, 1 682.5 + 1.9 Magette, Fuger [19] Cs,PuBr6 1 694.1 + 3.6 Niffle , Fuger [20] CsU,C1, 2 535.1 t 8.0 Vdovenko [16]

(*) When there are more than two authors, only the first one is indicated. ALKALI METAL ACTINIDE COMPLEX HALIDES C4-209

The evaluation of AH,,,,,, is obtained from the the enthalpies of formation of the tetravalent actini- comparison of the enthalpies of solution of the de ions [26]. complex halides and of the binary halides in the As our information on the thermochernistry of the same media. In a number of instances the reported Cs2MX, compounds is now relatively abundant we data arise from measurements by two groups of have attempted to gain some insight on the evolution authors. The agreement between these sets of re- of the enthalpic effect, through the actinide series, sults is good as it can be inferred from the indicated for the formal process described by reaction (3) limits. The values are also in agreement with the recently assessed enthalpies of formation of the actinide tetrahalides [21]. The results of Marty- nova et al., Vdovenko et al. and Fuger et al. clearly show for both the uranium chloro- and bromo- and corresponding to the standard enthalpy of for- complexes a steady change in AH,,,,,,, with the mation of MX:-(g), as well as for the process des- increasing size of the alkali metal cation from cribed by reaction (4) + 5 kJ . mol-I for a salt such as Li2UCl, to ca. - 100kJ . mol-' for Cs2UX, salts. The same trend is observed in the case of the uranyl fluoro- complexes [4-61. The early data of Chauvenet on , however, do not seem to obey AH,(M-X) being the average enthalpy change upon such a simple pattern. On the other hand, where data formation of one M-X bond in MX:-(g) from the are available for a series of actinides, essentially the specified gaseous species. The quantities can be dicaesium compounds, an increase is also observed evaluated through the use of a classical Born-Haber in the stability of the complex salt with regard to the cycle as shown in figure 1. In this cycle AH:(M, g) is binary salts with the decreasing ionic size of the the standard of the actinide actinide cation. In the case of Cs2PuC1, and metal, as recently assessed [26] and listed in ta- Cs2PuBr,, as neither PuCl, nor PuBr, have been ble I11 ; AH:(Cs, g), the standard enthalpy of subli- characterized as compounds, AH,,,,,,, has mation of Cs(c) to Cs(g), 76.07 & 0.02 kJ . mol-' [27] ; been obtained from an estimate of the enthalpy I(Cs, g), the first potential of , formation of these hypothetical tetrahalides based 375.7 kJ . mol-' [28] ;AH:(X, g), the standard enthal- on the extrapolation of the well known enthalpies of py of formation of monoatomic halogen , solution of the lighter actinide tetrahalides and on 321.302 & 0.008 kJ . mol-I for C1 and 111.86 %

Fig. I. - Enthalpy cycle for Cs,MX. C4-210 I. FUGER

0.12 kJ . mol-' for Br [22] ; EA(X), the In the case of the Cs2MC16salts, which have a affinity of the halogen, - 348.8 2 0.4 kJ . mol--'for trigonal structure (D:,-c?~), the value of r, + r, is 10 deduced from the formula proposed by Yatsimirskii C1 and - 324.60 2 0.4 kJ . mol-' for Br 1291 ; - RT 2 [3 I] : is the enthalpic term corresponding to the formation of two moles of gaseous ; U' is the , i.e. the total change in internal energy upon formation of one of Cs2MX6(c)from two moles of Cs'(g) and one mole of MX:-(g), - 3 RT being in which d is the of the compound (g . cm-') the corresponding PV work. and M its molecular weight. The corresponding Although realizing limitations of such relation- hexabromo-compounds of uranium, and ships, but having only in mind to evaluate the trend exhibit the fcc K,PtCI, structure (0;- in A H:(MX:-, g) and A H,(M-X) along a series of Fm3m). In that case we have taken for r the analogous compounds we have chosen to use for the interatomic distance between the actinide and the estimation of the lattice energy the semi-empirical caesium (both fixed by symmetry) as calculated relationship of Kapustinskii as modified by from the lattice parameter. Let us note, however, Yatsimirskii [30] and applicable to salts formed by that application of relationship (6) above for cations having an outer shell of eight electrons these compounds .to values of r which are within 1 % of the selected values. These various structural data are listed in table 11. The results of the thermodynamic calculations are listed in table 111, together with A H:(M, g). These data clearly show, with the increase of the of the actinide, a steady decrease in the enthalpy effects associated with the formation of in which n is the number of ions in the of each MX:-(g) species. On the other hand, through the salt, Zc and Z, are the formal charges of the similar calculations, Vdovenko et al. [16, 181 showed cation (Cs') and of the anion (MX: ) and r, and r, that in the case of the chloro- and bromo-uranates, are the radii (A) of the cation and of the anion the energetics of the M-X bond is virtually indepen- respectively. dent of the of the alkali metal cation.

Table 11. - Structlcral data on Cs,MX6 compounds.

Lattice Compound parameters (A) Ref.

4.671 4.638 4.621 4.596 4.593 Cs-M (A) 4.801 4.799 4.785

Table 111. - Thennodynamic calculatiorts on Cs,MX6 (kJ . mol-'). ALKALI METAL ACTINIDE COMPLEX HALIDES C4-211

It is obvious that the energetics of the MX:-(g) The enthalpy of formation of the only known species deserves more elaborate calculations such as of neptunium, NpI,, has recently been deter- those carried out by Jenkins and Pratt [37] for mined as - 512.8 -t 2. I kJ . mol-' [44], while we can MiMX, compounds of d transition and main group estimate elements with the &PtC1, structure and indeed such calculations are tackled by these authors [38]. Table IV summarizes the few existing thermody- for this hypothetical compound. namic data for the chloro- and bromo-complexes of We thus obtain for the hypothetical reaction (7) the actinides in the V and VI valency state.

Table IV. - Thermodynamic data associated with the formation of penta- and hexavalent actinides chloro - and bromo-complexes at 298 K (kJ . mol-I). AH, = + 19 2 20 kJ . mol-'. We shall accept for this reaction Compound - AH - AH,,,,,., References - - - - AS, = - 16235. K-I. mol-' Cs,NpOCl. 2 450.2 2 4.7 - Bastin and Fuger 1393 Cs,U02CI, 2 204.5 * 2.5 75.7 % 0.5 Tixhon and Fuger [40] by analogy with the corresponding uranium system. Cs,Np02C1. 2 056.6 a 5.0 (86 % 5) Tixhon and Fuger [40] Cs,UO2Br4 2 009.0 2 1.5 60.1 % 0.4 Niffle and Fuger [20] For reaction (8)

No value is given for AHcomplsxin the case of Cs3Np02C1, as Np02C1 is not known and because we deduce from the known experimental data on the U0,CI and PaOzCl are the only such actinide (V) known chloro- and bromo-complexes, known with certainty. Similarly NpO,CI, has not been characterized but its enthalpy of forma- tion can easily be estimated from that of UO,CI,. The results in table IV can also be used as a good and accept AS, 2 - 30 J . K-' . mol-' from the data basis for the estimation of values on the plutonium of Latimer [45] on the entropies of analogous salts and analogues [I]. such as K,PtC16, K,PtBr,, K,IrCI, ... Therefore we obtain for reaction (9), which is the sum of Although many chloro- and bromo-complexes of reactions (7) and (8) the actinides in the + 3 valency are known, the thermodynamic data are scanty. An interesting class of compounds to note displays the general formula Cs,NaMCI, : it can accommodate trivalent cations of almost any size (from La3' to Fe3') [41] while retaining the same high symmetry fcc structure with MC1;- regular octahedra. Thermodynamics of these AS, - 46 J . K-' . mol-' compounds throughout the series has and been thoroughly studied by Morss [42] who also obtained data for the plutonium analogue. From AG, s - 57 2 22 kJ . mol-' these data Morss and Goldman [43] derived values for the enthalpies of hydration of trivalent actinides indicating that Cs,NpI,(c) should be stable toward (U-Bk inclusive) in excellent agreement with those evolution. As Np(1V) is the only species obtained through the use of an electrostatic model. stable in an aqueous medium in presence of the I,/I- couple, this route appears to be the first choice to 4. Prevision of the stability of new halogeno- make if the synthesis is attempted. complexes. - The existence of Cs,PuCl,(c), while Although UO,I, adducts with organic PuCI, has only been characterized in the gas phase have been reported [I], the preparation of this and is unstable in the crystalline form toward compounds and its hydrates has been attempted decomposition into PuC13(c), the existence of several times without success. Brandenburg [46] Cs,BkCl,(c) 131 and of the newly prepared suggested through a correlation method involving Cs,PuBr, 1201, while BkCI, and PuBr, are not likely numerous compounds a value of - 1 000 2 to be characterized, are interesting examples of 4 kJ . mol- ' for its enthalpy of formation. In view of stabilization of an actinide in a given valency state. the accepted value for AH;(UO,, c), - 1 084.9 2 Many such similar situations can be envisioned and 0.8 kJ . mol [22], it is clear that U0,12(c) should be discussed. We selected here to take as examples the unstable toward UO,(c) + I,(c) : therefore for case of the hypothetical Cs,Np16 and Cs,UO,I, Cs,UO,I,(c) to be stable with respects to UO,(c), its compounds, because so far there are no thermody- formation from CsI(c), UO,(c) and I,(c) should be namic data on actinide iodo-complexes. accompanied by an enthalpy effect more negative C4-212 J. FUGER than - 85 kJ . mol-', which appears unlikely in view presently carried out by Morss 1471 : preliminary of the data obtained for Cs,UO,Cl, and Cs2U0,Br4. experiments indicate that a reaction such as Let us note, however, that the synthesis of an analogous iodo-complex with a large organic cation (triphenylbutylphosphonium) has been obtained by crystallization from an organic medium [I]. corresponds to a AH,, of - 28 2 15 kJ . mol-' which Finally, it is certainly appropriate to evoke the is very small. Therefore hopes to obtain more easily possibilities of stabilization of the lower valency divalent actinides by stabilization in a chloride states of the actinides through the formation of complex salt of caesium appear scanty. However, halogeno-complexes of the type CsMX,. None of these systems deserve further studies, particularly these compounds have been prepared, yet, for the with regard to and and also to other actinides but such studies on the lanthanides are univalent ions.


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Dr. FORRESTL. CARTER.- I have several ques- one would agree that UF, is covalent and adding a tions : couple of electrons should not change the bonding 1) In your application of semiempirical calcula- very much. tions of lattice energy have you had occasion to make use of the approximation given by Templeton J. FUGER. - Indeed the actinide hexahalogeno based on . species can be considered as tightly bound species ; 2) In the rare the very interesting series of the MXZ- species is well known in solution. It is << Vernier M phases have been recently discovered. Is however difficult to ascertain the amount of cova- these any evidence of similar among the lent character and the stimulating discussions during actinide halides. this conference have shown that this matter is still 3) Also among the reduced rare halides is pending. the very interesting linear conductor Gd,Cl, in which Pr. J. R. PETERSON. - With respect to the a conducting metal chain is surrounded by an insu- complex halide salts of the type Cs,AnX,, which lating layer of halides. Such a compound isolated halide provides the greatest stabilization of the among the actinides should be very interesting to the An(1V) ? In particular, how can I best stabilize physicists in regard to dimensional waves and f Es(1V) in such a complex halide salt ? delocalization, etc. J. FUGER. -Without any doubt, if I was to select J. FUGER.- 1) Although I am aware of the approximation given by Templeton, I have not a halide for that purpose I would choose fluoride. attempted to make use of it as here my only purpose Alkali metal lanthanide and earlier actinide (IV) was evaluate trends in the MXZ- energetics. fluoride salts have been reported quite a number of 2) I do not think that similar phases have been years ago. The review cited as reference [2] in my paper would be very useful in selecting possible characterized as yet for the pure actinides. A lanthanide-actinide containing cali- synthesis routes. It is also relevant to recall the early fornium (11) was reported by Haire et al. last work of Asprey and coworkers on (IV) fluorides [ASPREY,L. B. and fall at the rare earth research conference in the U. S. KEENAN,T. K., J. Inorg. Nucl. Chem. 16 (1961) 3) I certainly agree fully with you. A lot of such 2601 and on caesium (IV) heptafluoride compounds containing lanthanides in formally non [VARGA,L. P. and ASPREY,L. B., J. Chem. Phys. 48 integral oxidation states (1.5, 2.2, ...) have been (1968) 1391. reported by several authors (Blirnighausen, Eick, Haschke.. .). Pr. BERTAUT.-The formula of Yatsimirskii, is it Indeed future efforts towards studying such empirical or electrostatic? compounds of actinides should be scientifically rewarding. J. FUGER. - The Yatsimirskii formula (Ref. [30]) is a refinement of the Kapustinskii's equation Pr. P. PYYKKO.-Do you have any feeling for the [KAPUSTINSKII,A. F., Zhur. Obshch. Khim. 13 amount of covalent character in the MXZ- group ? If (1943) 4973 which was derived from the Born elec- I am not mistaken, in the alkali hexachloroplumbates trostatic model with empirical correction factors. you can actually see the hindered rotations of the Another relevant reference concerning this equa- PbClz- group by NMR, which indicates the existence tion is KAPUSTINSKII, A. F., Quart. Revs. (Chem. of a tightly bound, well defined group. Also, every- Soc. London) 10 (1956) 283.