CHEMISTRY OF THE TRANSURANIC ELEMENTS By M. W. LISTER M.A. D.PHIL. (CHEMISTRY DIVISION ATOMIC ENERUY RESEARCH ESTABLISHMENT HARWELL) THE elements which have now been discovered beyond uranium [atomic number (Z) = 921 are neptunium (Z = 93) plutonium (Z == 94) americium (Z = 95) and curium (Z = 96). Thus the last three periods of the Periodic Table start as follows Number of places be- yondinert gas . . 0 1 2 3 4 5 6 7 8 9 10 2ndlongperiod. . . Kr Rb Sr Y Zr Nb Mo Tc Ru Rh Pd 3rdlongperiod . . . Xe Cs Ba La Ce Pr Nd 61 Sm Eu Gd Lastperiod . . . . Rn Fr Ra Ao Th Pa U Np Pu Am Cm Written in this way the Periodic Table shows the contrast between the second and third long periods caused by the presence of the rare earths. The reason for this difference as is well known is that in the rare-earth series the fourth quantum shell is filled up from 18 at lanthanum to 32 at lutecium where it is complete whilst in the second long period the third quantum shell cannot expand beyond 18.The availability of the 4f orbits gives the rare earths their characteristic properties. In the last period a similar espansion of the fifth quantum group could take place; and the evidence is very strong that in fact 5f electrons occur in uranium and the succeeding elements. If no 5f electrons were present we should expect the elements neptunium plutonium americium and curium to be analogous to rhenium and the platinum metals. What is perhaps not so clear is the behaviour we should expect for elements containing 5f electrons. In the first place it by no means follows that because the first 4f electron appears in the fourth element beyond the inert gas (in cerium) the first 5f electron will appear in thorium.However we might reasonably expect that when 5f orbits were occupied the properties of the elements would deviate from the sequence found in the first and second long periods; and because an electron in a 5f orbit might to some extent be removed from the region of chemical attack as 4f electrons are that the deviations from first- and second-long-period behaviour would be in the direction of a series of elements of similar chemical properties. These remarks will be amplified later in the comments on the observed chemical behaviour of the transuranic elements. Since the rare-earth series starts in effect at lanthanum the chemistry of the elements from actinium onwards will have to be taken into account though this will be done very briefly for the elements before uranium.The resemblances of the chemistry of neptunium and plutonium to that of uranium are so extensive that a rather more extended account of uranium will have to be given. 20 LISTER CHEMISTRY OF THE TUNSURANIC ELEMENTS 21 Solution Chemistry All the elements from actinium to curium are base metals and with the possible exception of protoactinium form compounds which give simple hydrated ions in aqueous solution. In this it is at once seen that they resemble the rare earths whose most characteristic compounds are their tervalent salts which in solution give hydrated M3+ ions; the platinum metals and also tungsten and rhenium on the other hand give little indication of simple cations in solution.The simple ions of the heavy elements are _I u3 + AcS+ - - T h 4 + - u4+ Np4+ Pu4+ In addition the characteristic oxy-ions are known U022+ NpO,2+ PUO,%+ UO,+ Np02+ PuO,+ and there are compounds such as UOC1 and NpOC1 which possibly give quadrivalent oxy-ions in solution. Some of these ions are somewhat un- stable but where the stability is sufficient a series of salts can be prepared by the ordinary methods and the ions of any element can be converted into each other by suitable oxidising or reducing agents. In considering *the stability of these ions in aqueous solution we must take into con- sideration the following types of reaction which might remove the ion in question (i) oxidation or reduction by water or the associated anion (ii) disproportionation to a higher and lower valency state (iii) hydrolysis and (iv) formation of a covalent complex with the anion.(i) and (ii) can conveniently be considered together since they are both dependent on the oxidation-reduction potentials. All the compounds of actinium are tervalent,l and there can be little doubt that the hydrated Ac3+ ion with a rare-gas structure is present in solution. Thorium gives the well-known series of quadrivalent salts such as thorium(1V) nitrate Th(NO,),. Though tervalent and bivalent thorium compounds such as thorium tri-iodide can be obtained by dry methods these are decomposed by water to give quadrivalent thorium.2 The equilibrium Th3+(aq.) + H20 $ Th4+(aq.) + OH- + &H2 is evidently very much in favour of the right-hand side. In other words the oxidation-reduction potential of the Th(IIIJ1V) couple is very positive.In this thorium resembles hafnium as opposed to cerium. Little is known of protoactinium ions in solution ; the few protoactinium compounds that have been characterised are quinquevalent but it is very improbable that protoactinium forms a simple quinquevalent ion in solution. Recently indications of a lower valency state of protoactinium produced by reduction - - - - - - Am2+ - Np3+ Pu3 + Am3 + Cm3 + - - S. Fried and F. Hagemann AEXD 1891 (References to MDDC or AECD numbers * J. S. Anderson and R. W. M. D’Eye Chemical Society Symposium Oxford here and later are to American atomic energy declassified reports). April 1949; J . 1949. k244. 22 QUARTERLY REVIEWS with zinc amalgam have been obtained. In this state protoactinium gives an insoluble fluoride which is taken as an indication of ter- or quadri- valent protoactini~m.~ The formation of this compound might also explain the cathodic deposition of protoactinium when electrolysed in solutions containing fl~oride.~ Uranium forms two well-known series of salts the uranyl salts of U022+ and the uranous salts of U4+.Moderately strong reducing agents are needed to reduce uranyl to uranous compounds e.g. amalgamated zinc; and conversely most oxidising agents convert uranous into uranyl salts. The evidence that uranyl solutions do in fact contain UOa2+ presumably hydrated comes first from the formula of the salts which always contain a bivalent UO group e.g. U0,S04,3H20 NaUO,( OAc), and secondly from the vibrational fine structure of the spectrum of the solutions,5 which indicates a cation containing more than one atom that is unchanged by considerable variations of pH and thirdly from the electrochemistry of the solutions which is described below.The Raman spectrums can be explained in terms of a bent ion (04-O)++. If the uranyl solution is reduced electrolytically the first stage of the reduction involves a single electron transfer and occurs at a half-wave potential that is little affected by acidity 7 9 8 9 9 lo so that evidently this process is U0,,+ + e- -+ U0,f. This is confirmed by the diffusion constants of these ions calculated from the diffusion current at a dropping mercury electrode by the Ilkovic equa- tion id = 605nD$m*ta where id is the diffusion current per g.-mol. n is the number of charges transferred in the electrode process D is the dif- fusion constant m is the mercury flow rate and t the time of formation of the mercury drops.If n is assumed to be 1 the value of D becomes 0.65 x 10-5 cm.2/sec. whilst the value of D obtained from the conductance of UO2Cl solutions is 0.5 x The UO,+ ion readily dispro- portionates to sexi- and quadri-valent uranium. The rate is proportional to the square of the UO,+ concentration and to the H+ concentration. The rate constant is 130 (moles/l.)-l. sec.-l when the H+ activity is unity so that disproportionation is rapid at UO,+ concentrations above about ~/1000 though the stability increases in less acid solutions. H. G. Heal 7 calculates that at equilibrium in a M-SOhtiOn of UO,,+ and of U4+ N. with respect to H+ the concentration of UO,+ would be only 10-6.The polarogmphic reduction of UO,,+ gives a second wave which is a two-stage reduction uranium(V) to uranium(III) which shows signs of a kink at half its height. Before the kink which presumably corresponds to reduction to uranium(1V) the wave is irregular whilst its second half is cm.2/sec. 9 M. Haissinsky and G. Bouissi6res Compt. rend. 1947 226 573. 4 G. Bouissihres J . Phys. Radium 1941 [viii] 2 72. 6R. E. Connick M. Kasha W. H. McVey and G. E. Sheline MDDC 892. 6H. W. Crandall MDDC 1294. 7 Nature 1946 15'9 225 ; Tram. Faradag Soo. 1949 45 1. 8 I. M. Kolthoff and W. E. Harris J . Amer. Chem. SOC. 1945 67 1484 s D. M. H. Kern and E. F. Orlemann MDDC 1703. l o K. A. Kraus and F. Nelson AECD 2394. LISTER CHEMISTRY OF THE TRANSURANIC ELEMENTS 23 logarithmic Hence the reduction uranium(V) to uranium(1V) is not rever- sible at a dropping mercury electrode whilst the uranium(1V) to uranium(II1) stage is.This is consistent with the view that we have a change of ion type in passing from uranium(V) to uranium(IV) but not from uranium(1V) to uranium(III) i.e. U02+ + 4H+ + e- -+ U4+ + 2H20 u4+ + e- + u3+ This change is confirmed by the formulae of the quadrivalent salts which are not those of an oxy-ion and by the fact that the interchange of uranium between uranium( IV) and uranium(V1) compounds (e.g.,UCI and UO,CI,) occurs at a measurable rate.ll Further the U(VI/IV) couple is not rever- sible at a platinum electrode and so far as it can be observed appears to be strongly dependent on the acid concentration. The U(IV/III) couple is reversible and not acid dependent 7 10 l 2 at a mercury electrode.(A platinum electrode however takes up the potential of a hydrogen elec- trode.) Both the UO,+ and U3+ ions are very easily oxjdised for instance by air at room temperature; UO,+ can be titrated with ferric salts. Neptunium almost certainly gives the same series of ions as uranium namely NpO,,+ NpO,+ Np4+ and Np3+ (all hydrated) but their relative stabilities are altered.13 l4 In particular quinquevalent neptunium is stable in aqueous solution. Sexivalent neptunium requires more vigorous oxida- tion to prepare it than is necessary for sexivalent uranium but less than for sexivalent plutonium. Cold bromate solutions for instance oxidise neptunium(1V) solutions to neptunium(VI) the reaction passing through neptunium(V) and it is believed that it is the first stage that is the rate controlling step ; but the kinetics of the reaction are not simple.Ceric or argentic solutions will also oxidise it to neptunium(VI) as will potassium permanganate. Neptunium(V1) is reduced by stannous chloride rapidly to neptunium(V) and then more slowly to neptunium(1V). The same is true of reduction by hydrazine hydroxylamine or sulphur dioxide. Ferrous iron also reduces neptunium(V1) or neptunium(V) to neptunium(1V) at a rate proportional to the neptunium(V) concentration and to rather more than the first power of the hydrogen-ion concentration suggesting that the slow stage is Np02+ + Fea+ + H,O+ -+ Fe3+ + H20 + Np02H+ followed by a faster conversion into NpQ+. Hydrogen peroxide will reduce neptunium(V1) to neptunium(V) and sodium nitrite also effects reduction only as far as neptunium(V).Chlorine oxidises neptunium(1V) to nep- tunium(V) whilst chloride ion will slowly reduce neptuniurn(V1) to nep- tunium(V). Strong reduction (e.g. electrolytic) is required to produce neptunium(II1) solutions and they are oxidised by air. That sexivalent neptunium solutions do indeed contain NPO,~+ ions is shown by the isolation of NaNpO,(OAc), isomorphous with the corre- llE. Ron& AECD 1909. lS L. B. Magnusson J. C. Hindman and T. J. La Chapelle MDDC 1266 1267 l4 S. Fried and N. R. Davidson MDDC 1332. lnB. J. Fontana MDDC 1453. 1381; J. Amer. Chem. Xoc. 1949 71 687. 24 QUARTERLY REVIEWS sponding uranium compound and by the electrochemistry of the neptunium ions. Neptunium(V1) solutions on reduction with one equivalent of a reducing agent give solutions that are quite stable of a neptunium ion with a characteristic absorption spectrum.This is presumably the NpO,+ ion ; this conclusion is supported by the fact that the Np(VI/V) couple is reversible and is only slightly dependent on the hydrogen-ion concentra- tion (this apparent slight dependence may in any case be due to liquid- junction potentials). The absorption spectrum of the NpO,+ ion is very similar to that of the iso-electronic PuO,~+ ion. The Np(V/IV) couple on the other hand is not reversible at a platinum electrode and it has only been measured indirectly by means of the equilibrium NpO,+ + Faa+ + 4H+ f? Np4f + 2H,O + Fa3+ As with uranium the irreversibility of this couple is attributed to the slowness of removal of the covalently-bound oxygen atoms in the quinque- valent ion.The Np(IV/III) couple is reversible and acid independent so that the ions in these valency states are probably hydrated Np4+ and Np3+. The disproportionation of quinquevalent neptunium analogous to the rapid disproportionation of UO,+ does occur ; but in M-sulphuric acid the rate con- stant k (from d[NpO,+]/dt = k[Np0,+I2) = 5.3 x lo- (moles/l.)-l.min.-l at 25". For the reverse reaction the rate constant is 2-15 (moles/l.)-1 . min.-l. Hence in M-sulphuric acid the equilibrium constant of the reaction 2NpO,+ + 4H+ + NpOaa+ + Np4+ + 2Ha0 is K = [Np0,+]2/([Np0,2+][Np4+]) = 40. It is also found from measure- ments of the " reproportionation '' rate at two temperatures and from a study of the kinetics of oxidation of neptunium(1V) by bromate that the activation energy of the reaction Np(1V) + Np(V1) -+ 2Np(V) is 27 kcals./g.-mol.Plutonium in aqueous solution also gives a similar series of ions Pu022+ PuO,+ Pu*+ and Pus+. The tervalent ion is however much more stable than with uranium or neptunium; Pu02+ is intermediate in stability be- tween U02+ and NpO,+. Plutonium(1V) is probably the most stable valency state of plutonium; to obtain plutonium(V1) it is necessary to use hot bromate l 5 or permanganate l6 solution (though these will oxidise plutonium(1V) slowly in the cold) but ceric nitrate argentic ions hot perchloric acid,17 potassium dichromate,ls nitric acid l9 at moderate con- centrations or in fact any vigorous oxidising agent can be used. Conversely plutonium(V1) is reduced by a variety of reducing agents (e.g.ferrous ions hydroxylamine or sulphur dioxide) initially to plutonium(V) which dispro- portionates and is further reduced. The reduction of plutonium(1V) to plutonium( 111) is much easier than the corresponding process for uranium or neptunium and as will be seen is not much more difficult than the reduction of plutonium(V1). Sulphur dioxide hydrazine iodide ion or 15R. E. Connick and W. H. McVey MDDC 335. l* B. G. Harvey H. G. Heal A. G. Maddock and E. Rowley J. 1947 1010. I'M. Kahn MDDC 391. I s R . E. Connick W. H. McVey J. W. Gofman and G. E. Sheline MDDC 687. 19 R. E. Connick and W. H. McVey MDDC 337 ; K. A. Kraw MDDC 1378. LISTER CHEMISTRY OF THE TRANSURANIC ELEMENTS 25 hydrogen and a platinum catalyst will all give plutonium(II1) solutions. The oxidation of plutonium( 111) to plutonium(1V) needs a moderate oxidis- ing agent and is for instance not effected by air.20 There is no sign of a lower ion than Pu3+.That the plutonium(V1) solutions do in fact contain PUO,~+ ions is shown (i) by the formulae of their salts e.g. NaPuO,(OAc), isomorphous with the uranium and neptunium compounds 21 or the 8-hydroxyquinoline derivative Pu02(CoH6~O),,CoH,N0 ; l6 (ji) by the fact that the absorption spectrum shows vibrational fine structure in the same wave-length region and with nearly the same separation of the lines as does that of U022+ ; and (iii) by the electrochemistry of plutonium solutions. Electrolytic reduction of a plutonium(V1) solution leads in the first place to a solution with a different absorption spectrum from either that of plutonium(V1) or plutonium(IV) and this must be plutonium(V) since only one equivalent is needed to reduce plutonium(V1) to this state.Similarly it can be made under the right conditions by oxidation of plutonium(1V) 22 z3 [this generally leads directly to plutonium(VI)]. The plutonium(VI/V) couple is reversible and the potential is independent of the acid concentration,22 z3 24 which indicates that the quinquevalent ion is Pu0,f. It has been found that this ion disproportionates and no well-defined compound derived from it has been obtained (or at least described) though it is believed to be present in a complex carbonate precipitated from alkaline solution. The kinetics and equilibria involved in the disproportionation of plutonium( V) have been z 3 8 25 It is found that plutonium(V) is fairly stable at pH values from about 2 to 5 ; but a t all acidities it slowly disproportionates to plutonium(1V) and plutonium(V1).The concentrations of the various plutonium ions can be followed by their absorption spectra; the initial process in the disappearance of plutonium(V) appears to be 2PuO,+ + 4Hf + PuO,a+ + Pu4f + 2H20 . ' (1) at least in effect though presumably (as with neptunium) there is an inter- mediate stage involving an oxy-ion of plutonium(1V) (e.g. PuO,H+) which then reacts with more hydrogen ion. A faster equilibrium than the above reaction is then set up PUO," + Pu4* + PuO,2+ + Pu3+ . (2) As soon as appreciable amounts of Pu(II1) are present the reaction PuO,+ + Pu3+ + 4H+ + 2Pu4+ + 2H20 . . (3) occurs [again probably with some intermediate stage involving an oxy-ion of plutonium(IV)] and this reaction is believed to be in practice the most important one in removing plutonium(V).22 259 26 Thus the mechanism of disproportionation of plutonium(V) is more complicated than that of 2oR.E. Connick and W. H. McVey MDDC 338. 21 W. H. Zacharissen MDDC 67. aeR. E. Connick M. Kasha W. H. McVey and G E. Sheline MDDC 749. ei J. C. Hindman AECD 1893. 45M. Kasha MDDC 904. L. H. Gevantman and K. A. Kraus MDDC 1251. *sR. E. Connick MDDC 666. 26 QUARTERLY REVIEWS uranium( V) or neptunium(V) because of the production of plutonium(II1) as well as plutonium(1V) and plutonium(V1); and it is one of the main peculiarities of the plutonium ions in aqueous solution that the 3- 4- and 6-valency states can exist together in comparable concentrations in solu- tions of moderate acidity.Consequently the study of the mechanism of disproportionation of plutonium(V) has to be carried out in solutions con- taining more than negligible amounts of plutonium(III) (IV) and (VI) so that the precise elucidation of the mechanism is difficult. R. E. Connick 25 finds for the equilibrium constant of (2) above in O*5~-hydrochloric acid Kasha 26 finds H = 10-7 in 0-lM-perchloric acid. constant obtained for the disproportionation of Pu(1V) (see below) Combining this with the K = [Pu(III)]~[Pu(VI)]/[PU(IV)]~ = 0.05 we get for the disproportionation of plutonium(V) to plutonium(1V) and (VI) (in 0*5~-hydrochloric acid at 25"). This may be compared with the value of for uranium(V) and 40 for neptunium(V) (both in M-acid). Con- sequently in fairly acid solution very little plutonium(V) will be present at equilibrium but as can be seen from the reaction (1) above [which assumes that plutonium(V) is present as the oxy-ion PuO,+] the stability of plutonium(V) would be much favoured by higher pH.This is in fact found by L. H. Gevantman and K. A. K r a ~ s ~ who also found decreasing stability with rising temperature. The interpretation of these kinetic measurements is complicated by the reduction of plutonium(V1) by the products of the alpha-particle bombardment of the solution which destroys plutonium(V1) at the rate of 1-2% per day. Gevantman and Kraus attribute this reduction to hydrogen peroxide which is known to reduce plutonium(V1) ; plutonium(V) reacts with hydrogen peroxide much more Quadrivalent plutonium in suflticiently acid solution (more than about 0*2N.) is probably largely present as hydrated Pu4+.Thus the Pu(III/IV) but not the Pu(IV/V) or (IV/VI) couple is reversible and reasonably inde- pendent of acid concentration. Plutonium( 111) gives under these conditions the simple hydrated ion Pu3+. This is confirmed by the formulz of the salts of plutonium in these valeneies e.g. Pu(IO,), Pu2(C204),. As was mentioned above the plutonium ions are remarkable in that at moderate acidities the oxidation-reduction potentials of the Pu( III/IV) and (IV/VI) couples are not very different. Consequently all three ions can exist in solution simultaneously in equilibrium 3Pu*+ + 2H,O r? PUO,~+ + 4H+ + 2Pu8+ . ' (4) The equilibrium constant written as K4 = [Pu(III)]~[P~(VI)]/[P~(IV)] was found 26 to have the following values using plutonium concentrations about 0.001~.slowly. K4 252 40.2 0.195 0.0405 Molarity of HCIO 0.052 0-102 0.516 0.994 LISTER CHEMISTRY OF THB TRANSURANIC ELEISIENTS 27 K4 is of course dependent on the hydrogen ion concentration and equation (4) suggests that it should be inversely proportional to its fourth power ; in fact it is inversely proportional to the third power of the perchloric acid concentration but it must be remembered that no account has been taken of activity coeacients. However these results are more consistent with this equation involving only the simple hydrated ions than with those based on hydrolysed ions [e.g. in particular plutonium(1V) as PuOH3+]. It will be seen from these values of K4 that if the acidity falls below about 0 .2 ~ . considerable disproportionation of plutonium( IV) occurs. At low acidities plutonium(V) is also formed possibly by the reverse of equilibrium (2) above or possibly directly thus 2Pu4f + 2H20 + PuO,+ + Pu3+ 3. 4H+ The latter mechanism is supported by the observation that the rate of disproportionation of plutonium(1V) is proportional to [Pu(IV)I2 ; the rate constant is 0.053 (mole/l.)-l. min.-l a t 25" in O*48~-perchloric acid. The oxidation-reduction potentials of the uranium neptunium and plutonium ions have been measured in various solutions. The results in gome cases indicate considerable complex formation with the anion but as is usual the potentials in perchlorate solutions appear to give the most reliable figures for the simple hydrated cations ; the potentials in chloride solutions are not much different.The most thorough investigation of the plutonium redox potentials is that of Kraus,27 who has studied their depen- dence on hydrogen-ion concentration (cf. also ref. 28) ; the following table gives the potentials relative to the normal hydrogen electrode; a smaller negative value for the potential indicates that the oxidised state of the couple is relatively more stable. The uranium potentials are for M-hydro- chloric or -perchloric acid solutions ; somewhat different values have also been reported for uranium(IV/V) I Couple. U. Np (in N-HCI). Pu (in H-€IClO4). III/IV . . . + 0.63 - 0.14 - 0.96 I V / V . * . . - 0.55 - 0.74 - 1.20 V / V I . . . . - 0.06 - 1.14 - 0.93 Pu (in M-HCl). - 0.97 volts - 1.13 - 0.91 *7 K. A. Kraus MDDC 814. as B. J. Fontana NDDC 1603 ; K.A. Krrtus and G. E. Moore MDDC 906 ; and refs. (9) (12) (13) (22) and (24). 2s L. Brewer AECD 1899. **E. F. Westrum AECD 1903. CI 28 __ 1 Ionformed. M4+. . . . . - 23.6 M02+ . . . . + 12.8 MOe2+ . . . . + 9.6 I I QUARTERLY REVIEWS NP. Pu. _- - + 12-9 - 5.9 + 33.1 + 62.9 + 55-44 + 79.7 The lower the value of AH the more stable is the ion relative to ter- valent M3+. Neither americium nor curium forms the series of ions given by uranium neptunium or plutonium. Both elements are predominantly tervalent and there can be little doubt that in solution they give the ions Am3+ and Cm3+. Americium cannot be oxidised by argentic ions in B~-nitric acid nor by potassium permanganate or sodium b r ~ m a t e ~ ~ suggesting that its oxida- tion potential is more negative than - 2 v.(for the III/IV couple). I n alkaline solution it can be oxidised by strong oxidising agents such as sodium hypochlorite ; but the higher oxidation state has not been definitely characterised. Americium(II1) can be reduced by sodium amalgam but not by zinc amalgam.32 The product of the reduction is believed to be Am2+ as it can be co-precipitated with europous and samarous sulphates presumably as AmSO ; americium(II1) is not carried on these precipitates. Curium so far has been obtained only as tervalent compounds and is not affectgd by the reagents which oxidise or reduce americium. These oxidation-reduction potentials show an analogy with the behaviour of the rare-earth ions. If we consider the 3+ rare-earth ions starting with Ce3+ where the first 4f electron appears it becomes progressively more difficult with increasing atomic number (and in fact after praseodymium impossible) to remove a further 4f electron by chemical means.The rising nuclear charge holds the 4f electrons progressively more firmly though it is somewhat screened by the other 4f electrons. Eventually in samarium a point is reached where the electrons are so firmly held that the bivalent ion Sm2+ can be obtained in solution by strong reduction and in europium this reduction is easier. In gadolinium the trends of the redox potentials are sharply interrupted gadolinium is only tervalent and the next element terbium shows signs of quadrivalency. Thereafter the elements are only tervalent until ytterbium is reached which can be bivalent. Thus the trends of the first half of the rare-earth series are repeated.This break a t gadolinium is no doubt because in Gd3+ with seven 4f electrons another electron (to give the unknown Gd2+) would have to be paired with one of the previously-held electrons and it is known from the spectroscopic and magnetic properties of the rare-earth ions that this pairing involves a rela- tively higher energy i.e. a more.unstable ion. In the heavy elements more electrons can be involved in chemical reaction than in the rare earths ; that is to say the outermost electrons are not so firmly held as is generally the case with elements of high atomic number. As with the rare earths the tervalent ions become progressively more stable relative to the quadri- slB. B. Cunningham AECD 1879. 8. G. Thompson R. A. James and L. 0. Morgan AECD 1907.LISTER CHEMISTRY OF THE TRANSURANIO ELEMENTS 29 valent and further on the bivalent relative to the tervalent. This is also true of the M0,2+ ions relative to the M4+ ions the M(1V) becomes more stable relative to M(V1). The changes are more rapid than in the rare- earth series and it is to this that we owe the far less uniform chemistry of the heavy-element series. The 5f electrons seem to provide a less efficient screening of the nuclear charge than do the 4f electrons. Finally in Cm3+ a point is reached where the 5f shell is half full (assuming no 6d electrons) and the Cm2+ ion like the Gd2+ ion is unknown. It should also be mentioned at this point that the relative energy levels of 4f and 5d as compared with 5f and fid electrons are probably somewhat different the 6d-electron levels being relatively of lower energy.As will be seen later the ground state of the atom (un-ionised) of thorium is 6d27s2 ; and there is some not entirely conclusive magnetic evidence for 6d electrons in some uranous compounds. At present no very definite assignment of the un- shared electrons in heavy element compounds can be made in all cases and the position on this question appears to be as follows. It is still possible to maintain that all unshared electrons in compounds of the heavy elements beyond the radon core are in 5f orbits ; but it seems more probable that at the beginning of the series we have 6d orbits which become relatively less stable as we go to higher atomic numbers. By the time uranium is reached the 5f orbits are probably somewhat the more stable and beyond uranium there is no evidence of unshared electrons in 6d levels.Thus the only compounds likely to contain unshared 6d electrons are the lower valency compounds of thorium and protoactinium. Mention was made earlier that the heavy-element ions were probably of a simple hydrated type in acid solution. In fact of course an equilibrium of the type or is set up in solution for any n-valent ion. Investigations have been made of the equilibrium constants of these reactions for the heavy-metal ions by measurements of the pH of solutions of their salts when titrated with alkali,27 and by changes in the absorption spectra and redox potentials in solutions of various pH. The values of the hydrolytic constants so obtained241 2'1 33134135 are given in the table following where K = [H30+J.[MOH(n-l)+]/[Mn+] and pK = - log, K so that pK is equal to the pH at which 50% hydrolysis occurs. Mention3' may be made for comparison of the ceric ion which is largely hydrolysed to CeOH3+ and so is a much weaker base than Th*+ or Pu4+ and of the tervalent rare-earth ions where pK ranges from roughly 8 (in lanthanum) to 6. Plutonium tetrahydroxide is thus a base of very much the same strength as uranium tetrahydroxide. Plutonium( IV) also readily polymerises at pH values above about 1 so that in its solutions if the p H is raised by the Rii%+(aq.) + 2H,O + ICSOH(%-l)+(aq.) + H,Of M%+(aq.) + OH- + MOH@-l)+(aq.) K. A. Kraus and F. Nelson AECD 1864. ap Idem AECD 1888. 3s J. C. Hindman and D. P. Ames MDDC 1213. 30 QUARTERLY REVIEWS Ion. M 3 + . . . . . ~ 1 4 + . . . . .MO,+ . . . MOZ2+ . . . . U. - 1.44 1.15 (10) - 4.7 (36) NP. Pu. 6.95 (in M-c104-) (27) 7.1 (35) 7.2 (34) 1.53 (34) 1.4 (in M-GI-) 9.7 5.7 (Figures in parentheses are references.) addition of alkali there is a rapid initial change as the monomeric hydroxide complex is formed followed by a slow drift in pH to lower values as the polymer is formed. On acidification the polymer is only slowly destroyed though this can be hastened by heating the solution. The solution of the polymer has a characteristic absorption spectrum different from that of monomeric plutonium(1V). On further addition of alkali to a polymeric plutonium(1V) solution a polymeric hydroxide is precipitated different from that obtained by rapid addition of excess of alkali to monomeric plutonium(1V). The composition of these polymeric precipitates is often approximately (I%( OH)3.85X0.15)ra where X is the anion initially present.27* 34 Uraniuni(1V) also polymerises. It is not clear a t present whether the polymeric plutonium(1V) is simply a mixture of a number of ions contain- ing various numbers of Pu atoms combined by Pu-0-Pu links and with varying numbers of hydroxyl groups attached or whether it is a definite compound but in spite of the relative constancy of the amount of associated anion it is probably the former. Ions containing more than one metal atom occur also in somewhat basic uranyl and probably plutonyl solutions e.g. TJ2052+,xHz0. Analogous to the hydrolysis of the heavy-metal ions is their combina- tion with anions other than hydroxyl in the solution. Some evidence of such combination comes from the formation of double salts such as (NH,),Pu(NO,), but this is not by itself a proof of complexity as X-ray examination has shown many double salts e.g.RbUO,(NO,), not to con- tain complex anions. More definite evidence of complexity comes from electrical migration experiments and from observed changes in absorption spectra solubilities and cell e.m.f.s when various anions are added. Quali- tatively i t is found that sulphate acetate oxalate and probably fluoride combine strongly with Pu4+ the nitrate moderately and the chloride and perchlorate still less. 38 In perchloric acid solution plutonium(1V) and (VI) show no signs of migration as an anion even a t l0M-concentration. In hydrochloric acid plutonium(II1) migrates chiefly as a cation even in 86 H. Guiter Bull.SOC. chim. 1947 64. 37 M. S. Sherrill C. B. King and R. C. Spooner J. Amer. Chem. Soc. 1943 65 170. C. K. McLme J. S. Dixon and J. C. Hindma MDDC 1215. LISTER CHEMISTRY OF THE TRANSURANIC ELEMENTS 31 IOM-acid; plutonium(1V) begins to migrate chiefly to the anode when the acid concentration reaches about 5-6M. and the same is found for plutonium(V1). In nitric acid plutonium(1V) migrates as an anion a t concentrations above about 6~-acid and plutonium(V1) does so above about lOM-acid. In sulphuric acid plutonium(II1) migrates as an anion at concentrations above about 4M- plutoniurn(1V) above about O - ~ M - and plutonium(V1) above about M-acid. The behaviour of neptunium(V1) is very like that of plutonium(VI) though anionic complexes are slightly more easily formed. Acetate and oxalate both readily give anionic com- plexes with plutonium(IV) but fluoride does not do so even at 10M-con- centration in transference experiments although there is other evidence showing that a positively charged plutonium( IV) fluoride complex is formed.Since plutonium(II1) combines less readily with these anions than does plutonium(IV) the potential of the Pu(III/IV) couple should be lowered by complex formation and this i s found to occur. The extent of this effect is shown by the following values 39 E.m.f. v. - 0.966 - 0.953 - 0.92 - 0.74 - 0.50 Acid x. HCl HClO HNO H,SO HF It is interesting to note that both thorium and cerium(1V) have been shown to form complex anions with nitrate and this has been rarely found else- where. These results and the absorption 40 *l can be inter- preted by assuming that at least initially a complex of the general formula PuX3+ is formed by Pu(1V) with an anion X- and the value of its associa- tion constant K = [PuX3+]/([Pu4+].[X-I) is about 3 (mole/I.)-l for X = nitrate and about 0.2 for chloride. For sulphate the value is prob- ably about 2000 and for fluoride 42 6 x lo8 (mole/I.)-l. These figures apply only to the first complex formed by association with an anion; it was shown by the transference measurements described above that in many cases further combination occurs leading presumably to ions such as PU(NO,),~-. Compounds of the Transmanic Elements From the solutions of these ions of the transuranic elements various solid salts have been obtained by the usual methods of evaporation and precipitation. In certain cases solutions have been described (e.g.in work on absorption spectra) without an actual separation of a solid salt having been made the nature of the salt present being inferred from the method of preparation of the solution. In this section an account will be given of the compounds actually isolated from solution. In addition to preparation from solution many compounds have been obtained by dry methods and these will also be described here. The analogous thorium uranium or rare-earth compounds will be briefly noted. Metals.-Neptunium and plutonium metal have both been prepared by methods which show that these elements like uranium and unlike the 39 J. J. Howland J. C. Hindman and K. A. Kraus MDDC 1260. 40 J. C. Hindman MDDC 1256. 4% C. K. McLane MDDC 1147. 41 Idem MDDC 1257.32 QUARTERLY REVIEWS platinum metals are base metals and vigorous methods have to be used for their isolation. Neptunium l4 is prepared by the reduction of the fluoride by barium vapour at 1200". Plutonium uranium and americium 43 can be obtained similarly. The heats of formation (given in the table below in kcals./g.-mol.) of the ions of these elements from the metal 29 also show that we are not dealing with noble metals [compare e.g. Pt + 2C1 + PtCI (solid) - 56 kcals./g.-mol.] Ion formed. Th. U. _________ M3+ . . . . - - 123.6 - 159.8 M4+ . . . . - 185.5 - 147.2 - 165.7 Pu. - 141.9 - 129.0 I I Curium metal has not been described. The density of americium metal is given as 10-11 which makes the atomic volumes in the series roughly Pu Am - 22-24 Th Pa U NP 20.9 - 12.8 13.5 Ce Pr Nd - Sm Eu 20.6 20.8 20.6 - 21.7 29.0 It is interesting to notice that a sudden increase occurs a t europium and (presumably) americium an increase which is attributed in the case of europium to packing of essentially Eu2+ with two valency electrons as compared with say Ce3+ with three such electrons.Thorium does not fit the expected trend but this may be due to electrons in 6d orbits. Oxides.-The transuranic elements like uranium form a number of oxides though it has not always proved possible to obtain all the oxides corresponding to all the valency states known in solution. If we exclude peroxides the following oxides have been described The oxides UO NpO and PuO have been obtained usually in relatively small amounts by vigorous reduction of the higher oxides-for example when traces of oxygen are present as an impurity during reduction of the fluoride to meta1,44 or by reactions such as U + TJO -+ 2UO a t tempera- tures above 2000".Uranium mono-oxide is a brittle high-melting substance with a metallic appearance ; the monoxides of neptunium and plutonium are apparently-similar. They all have a crystal structure of the sodium chloride type. These oxides correspond to valency states that are not known in solution ; they are presumably semi-metallic compounds. Americium monoxide is obtained from the dioxide and hydrogen a t 800" ; 43 S . Fried AECD 1930. 4 4 R. C. L. Mooney and W. H. Zachariasen AECD 1787. LISTER CHEMISTRY OF THE TRANSURANIC ELEMENTS 33 it can be sublimed to give black crystals isomorphous with uranium or plutonium mono-oxide and is probably a normal oxide though it has not been obtained directly from the americious salts.The cornpourid Pu,O was obtained by reduction of the dioxide by atomic hydrogen or by heating the dioxide to 1700° It forms body- centred cubic crystals isomorphous with the C modification of the rare- earth sesquioxides. The lattice constant is somewhat variable in different preparations and this is attributed to variable composition over the range P U O ~ . ~ to P U O ~ . ~ ~ but the system does not seem to have been examined in detail. Its direct preparation from tervalent plutonium solutions has not been reported presumably because plutonium trihydroxide is too readily oxidised to be dehydrated to the sesquioxide Pu,O The same applies to the preparation of sesquioxides from uranium(II1) or neptunium( 111) solutions.It is however remarkable that americium trihydroxide on ignition in air gives americium dioxide and this on reduction in hydrogen gives americium monoxide and not the sesquioxide. Curium so far as is known forms only the sesquioxide Cm,O,. The dioxides Tho, UO, NpO, PuO, and AmO all have the fluorite- type structure with lattice constants (A,) as follows PUO Amo ::% 5.386 5-372 Tho uos 5.586 6.457 All except uranium dioxide are the normal oxides formed by ignition of their other oxides or decomposable salts such as nitrates in air at not too high a temperature. Plutonium dioxide is the highest plutonium oxide 45 known and attempts to oxidise it further by nitrogen dioxide or atomic oxygen were unsuccessful. Neptunium dioxide can however be oxidised under these conditions to give a compound of composition about NpO,.,, presumably Np,O with a slight deficiency of oxygen.-It is isomorphous with the uranium analogue U,08. Uranium is remarkable for the range of oxides intermediate between UO and UO that it forms.4* Triuranium octaoxide U,O is of course the oxide formed by heating other oxides in air but this can be converted into uranium trioxide UO by heating it in oxygen under pressure. Low-temperature oxidation of uranium dioxide goes about as far as UO,. with small change in the crystal structure and magnetic measurements 47 show that the U(1V) oxidised in this range becomes sexivalent uranium. From U,O to UO we have another range of stability with progressive change of crystal structure and magnetic measurements can best be interpreted in terms of quinquevalent uranium being p r e ~ e n t .~ ~ ~ 48 Hydroxides.-These are formed by the addition of aqueous ammonia or other alkali to solutions of the transuranic elements. Tri- and tetra- hydroxides of plutonium neptunium tetrahydroxide and americium tri- raD. M. amen and J. J. Katz AECD 1892. 46 E.g. K. B. Alberman and J. S. Anderson Chemical Society Symposium Oxford 47 J. K. Dawson and M. W. Lister unpublished. 48 H. Haraldsen and R. Bakken Natzlr&s. 1940 28 127. April 1949; J. 1949 s303. 34 QUARTERLY REVIEWS hydroxide are all obtained as insoluble precipitates redissolving in acids to give the corresponding salts and insoluble in excess of aqueous ammonia. There is no sign of ammine formation. The trihydroxides of the elements before plutonium are too easily oxidised to be isolated ; Pu(OH) is easily oxidised whilst americium trihydroxide is quite stable.The latter is a pink gelatinous material. Plutonium tetrahydroxide has been examined in some detail by K r a ~ s * ~ who finds that the dark-brown precipitate formed by addition of alkali to aqueous plutonium tetranitrate contains after thorough washing nitrate in the proportion Pu(OH),.~~(NO~)~.~~. It will be remembered that polymeric plutonium(1V) precipitates contain anions in roughly this proportion so that polymerisation is presumably occurring. Ksaus by following the pH changes on precipitation and washing also obtained evidence of basic nitrates. In its sexivalent state plutonium like uranium has acidic properties. Barium hydroxide precipitates a compound of composition about BaPu30, -at least this is the Ba Pu ratio.27 This is no doubt a polyplutonate analogous to the polyuranates.A similar precipitate can be obtained with sodium hydroxide which is considerably more soluble ; the precise com- position is uncertain. There is evidence from the pH titration curves of M0,2+ solutions with alkali where M is uranium neptunium or plutonium that polymerisation of all these ions occurs in slightly alkaline solution and on addition of acid again depolymerisation occurs only slowly. Peroxides.-Various peroxides may be mentioned. Uranyl solutions with hydrogen peroxide precipitake a uranium(V1) peroxide hydrated UO,. Plutonyl solutions on the other hand are reduced by hydrogen peroxide rapidly to plutonium(V) and then more slowly to plutonium(IV).50 Plu- tonium(1V) with amounts of hydrogen peroxide comparable to the plutonium present gives two soluble peroxido-complexes which can be distinguished by their absorption spectra.The brown complex which is formed first contains two plutonium atoms and one peroxido-group -0-O- per molecule; the red compound contains one more peroxido-group. It can be shown that the dissociation constant of the brown compound K = [brown compound]/([Pu*+]~ [H,02]) = 7 x lo7 (moles/l.)-l in 0 . 5 ~ - hydrochloric acid at 25" whilst for the red compound the constant K = [red compound]/([brown compound]. [H202]) = 145 (moles/l.)-l. It is thus evident that most of the plutonium in these solutions will be com- bined with the hydrogen peroxide. The structure of these complexes has not been elucidated in detail.If an excess of hydrogen peroxide is added a green precipitate is obtained 5 1 9 52 which contains some of the anion present particularly if this is sulphate. Its composition varies somewhat owing to decomposition but approximates to Pu207,xH,0 or Pu,06,S0,,xH,0 ; somewhat different forms are precipitated depending on the relative amounts of plutonium and hydrogen peroxide. These compounds con- 4* K. A. Kraus MDDC 1377. E. Connick and W. H. McVey MDDC 619. 61 E. I). Koshland J. C. Kromer and L. Spector MDDC 1263. 62H. H. Hopkins MDDC 1334. LISTER CHEMISTRY OF THE TRANSURANIC ELEMENTS 35 tain quadrivalent plutonium so we must write formuls for them such as O\ I Pu-0-0-Pu / O 1 . They have been examined by X-ray diffraction 44 l o OH o/ I OH and shown in one case to resemble closely thorium peroxide which forms a compound of empirical formula Th,O,,SO ; but detailed structures have not been determined.Salts,-Hatides and oxy-htides. The following simple halides of the heavy elements have been described (in this table X = any halogen) (PuF,) - NpF6 - NpF, NpCI NpBr, PuF NPX PuX Valency. - - I - - - - AmX CmX 6 . . . 6 . . . 4 . . . 3 . . . 2 . . Th. j u _. ThX ThI ThI Ny. 1 Pu. 1 Am. 1 Cm. 1 The volatile easily hydrolysed uranium hexafluoride has long been known; it is made from uranium tetrafluoride and fluorine. Neptunium trifluoride with fluorine a t red heat gives neptunium hexafluoride which melts at 53" and is easily volatile; this is isomorphous with uranium hexafluoride and is a white solid easily hydrolysed; its stability is less than that of uranium hexafluoride.Uranium hexachloride made by the reaction ZUCI -+ UCI + UCI, is much less stable than the hexafluoride ; as would be expected therefore neptunium hexachloride is unknown. Plutonium hexafluoride is not known for certain though traces of it may have been obtained. The uranyl halides are well-known stable com- pounds UO,X,. The corresponding neptunyl and plutonyl fluorides chlorides and bromides must all have been obtained in solution; iodide reduces these cations. PuO,F,,xH,O is a white fairly soluble solid. Quinquevalent uranium fluoride and chloride are known and U2F9 but no corresponding transuranic compounds. Quinquevalent salts of the type Np0,Cl must occur in solution but the solids have not been described. Neptunium tetrafluoride has been made as a light-green solid on the micro-scale by the reaction 4NpF3 + 0 + 4HF -+ 4NpF4 + 2H,O at about 500".Neptunium tetra- chloride and tetrabromide can be obtained by interaction of the dioxide with carbon tetrachloride and aluminium tribromide respectively. The tetrachloride is a yellow solid volatile at high temperatures ; the tetra- bromide is a reddish-brown solid volatile a t about 500". No tetraiodide could be made. Plutonium tetrafluoride is the only tetrahalide of plutonium that can be isolated though the tetrachloride must exist in solution but attempts to separate it have led only to the trichloride. Plutonium tri- The tetrahalides of uranium are all stable. m S. Fried and H R. Davidson J . Amer. Chem. SOC. 1948 70 3539 ; MDDC 1332. 36 QUARTERLY REVIEWS chloride does not react with chlorine at 170°.54 The hydrate PuF4,2H,0 is precipitated from aqueous solution; 55 the tetrafluorides of all of this group of elements are insoluble but the other halides are soluble.No tetrahalides of americium are known so that we have a regular decrease in the stability of the tetrahalides in passing from uranium to americium. The oxyhalides UOCI and NpOC1 are known. The series of the trihalides of these elements is known more or less completely except of course with thorium. They are obtained in the earlier elements (uranium and nep- tunium) by heating the higher halides in hydrogen or in the later elements by heating the oxides with an aluminium or carbon halide. Neptunium trifluoride has been made by heating the dioxide with hydrogen and hydrogen fluoride and americium trifluoride has similarly been made by heating the trihydroxide in gaseous hydrogen fluoride.Plutonium tri- fluoride can be precipitated from aqueous solution. The fluorides are in general insoluble but the other halides dissolve to give M3+ solutions. Their relative stability like that of the 3 + ions rises in passing from uranium to americium and curium. It is interesting to note 56 that although plutonium tetrachloride cannot be obtained the tetrafluoride can be made by use of the equilibrium QPUF $0 + BPUF +PuO Plutonium trichloride with 6 3 or 1 molecule of water of crystallisation can be obtained from aqueous solution. On heating it gives the oxy- chloride PuOC1 not the anhydrous trichloride. The oxybromide can similarly be prepared from the tribromide hexahydrate or by the following reaction at high temperatures 57 PuO + #Ha + HBr + PuOBr + H,O The equilibria PuX3 + H80 + PuOX + 2HX (where X = C1 or Br) have been measured a t 500-700".A number of complex halides 59 has been prepared from solution. These consist of a variety of fluorides and one series of double chlorides ; the fluorides are as follows The oxyiodide behaves ~imilarly.~8 Complex halides. C,H,NH,UO,F,,H,O C,H,NH,PuO,F ,H,O Cs,( Pu0,),F8,28H,0 M,PuF6 (M = K or NH,) KPu,F ; CsPu,Fg,3H,0 NaPuF KThE' KUF MPuF (M = Na K or Rb) K2UF6 KNPZF9 KTh,F KU,F9 Other complex thorium and uranium fluorides are known but the table shows only those analogous to the transuranic compounds. It is uncertain whether these plutonyl fluorides contain true complex ions ; and the same is true of the lower complex fluorides since though combination of plu- tonium and fluoride certainly must occur in solution as was mentioned (NaLaF is known) 64 B.M. Abraham MDDC 1574. H. H. Anderson MDDC 1130. 56 S. Fried and N. R. Davidson MDDC 1250. 68 I. Sheft and N. R. Davidson MDDC 1712 1713. N. R. Davidson MDDC 1578. El[. H. Anderson MDDC 1129 1130 1362. ** E. S. Maxwell AECD 2134. LISTER CHEMISTRY OF THE TRANSURANIC ELEMENTS 37 above there is no evidence from electric migration experiments of complex anions. A small number of complex chlorides has been prepared of the general formula M,PuCl, where M was casium tetramethylammonium pyridinium or quinolinium (cf. Cs2ThC1,,8H,0 and Cs,UC1,). No double chlorides could be isolated with potassium rubidium or zinc. The complex anion Puc162- has been shown to be present in the crystal by X-ray diffraction.Transuranic sulphides have been prepared by dry methods ; they are all lower-valency compounds The compounds KMF and KM,F form isomorphous series.,l Xulphides. ThS us (PUS) NPaSa NpOS Triuranium octaoxide when heated in hydrogen sulphide at 1300-1400" gives uranium disulphide US, and some of the sesquisulphide U2S3. The disulphides ThS and US are normal covalent compounds but the sesqui- sulphides Th,S and U2S3 are semi-metallic. 639 64 Neptunium dioxide on heating to 1000" in hydrogen sulphide gives first an oxysulphide NpOS and a t 1200" a sesquisulphide Np2S3 which is isomorphous with U,S and also apparently semi-metallic. Plutonium dioxide and hydrogen sulphide at 1300" give a compound Pu,O,S with a metallic lustre.On further treat- ment a t 1340" a black compound with a composition intermediate between Pu,S and Pu,S is obtained which gives a different X-ray difiaction pat- tern from the sesquisulphide obtained from plutonium trichloride and hydrogen sulphide at 800-1000". Plutonium sesquisulphide is not semi- metallic in character so that as usual we have a progressive increase in the stability of the lower valencies in passing from uranium to plutonium. There are some indications of a lower sulphide probably PUS obtained when plutonium trifluoride and calcium vapour react in a barium sulphide crucible.65 Nitrates. A nitrate Pu(NO,),,xH,O has been prepared 66 as light- green crystals. No double salts could be obtained with nickel cobalt or manganese nitrates but the salt (NH4),Pu(N03) has been prepared,40 isomorphous with the corresponding thorium and ceric compounds.Ths compounds M,Pu(NO,) are also known where M is Cs Rb T1 K or quinolinium. These probably contain the complex anion PU(NO,),~- as is shown by the uniformity of their formula by the electric-migration experiments and by analogy with the corresponding thorium compounds Rb,Th(NO ,) and Cs,Th( NO ,) whose diamagnetic susceptibilities are much less than the sum of those of their components suggesting that new bonds are formed in the double salts. $1 W. 33. Zachariasen J. Amer. C h m . Soc. 1948 70 2147. esE. F. Strotzer and M. Zumbusch 2. anorg. Chem. 1941 247 215. **E. D. Eastman MDDC 193. *sB. M. Abraham N. R. Davidson and E. F. Westrum AECD 1788. 66 H. H. Anderson MDDC 1130. *' C. Braselitin Cbmpt.r e d . 1941 212 193. 38 QUARTERLY REVIEWS Sulphates. Quadrivalent plutonium gives the reddish-brown sulphate Pu(SO4),,4H,O which on heating loses water to 3H20. H. H. Anderson 68 could not obtain the anhydrous salt without decomposition. Uranium disulphate also gives a tetrahydrate which loses water on heating to give a hemihydrate which cannot be completely dehydrated without hydrolysis. Plutonium gives a greyish-green basic sulphate Pu,0(S0,),,8H20. Double sulphates M,Pu(SO,) + 1 or 2H20 with M = NH, K or Rb have been prepared and there are thorium and uranium salts of the same composi- tion. The electric-migration experiments make it probable that these are true complex salts. Tervalent plutonium gives light-blue crystals of composition Pu,(SO,),,xH,O. A number of double sulphates are known MPu(S0,),,4H20 where M is TI Na Rb Cs or "H,; potassium gives a pentahydrate.Double salts K,Pu(SO,) and T1,Pu(S04) can be made but no compounds of the type M,Pu(SO,) known. The rare earths of course give numerous double sulphates of the type M(R.E.)(SO,), but of somewhat variable hydration. Iodates. The light-brown iodate plutonium tri-iodate and the light-pink tetraiodate are both insoluble. Plutonyl iodate is somewhat more soluble,~Q Oxuhtes. Diplutonium trioxalate is a slightly soluble salt,70 and the compound Pu(C204),,6H,0 is also insoluble (about 10-4 g.-mol./l. at 25"). 71 With excess of oxalate it gives a more soluble complex ion Pu(C,O,),~- and it is found that the constant of the equilibrium Pu(C~O,) 3- H2c204 + Pu(C,O4),'- + 2H+ g.-mols./l.). Thorium oxalate is similarly soluble in excess of oxalate.Neptunium dioxalate is also Acetates. The sexivalent ions of uranium neptunium and plutonium form isomorphous easily-crystallised compounds NaM02( OAc) ,. This is a relatively insoluble salt of these ions whose compounds are generally easily soluble. The solubility of sodium plutonyl acetate is 19.5 g./L in water at 25" and is less in the presence of excess of sodium acetate.60 The tetra-acetylacetonate of plutonium has been pre- pared and is isomorphous with the thorium compound. It is soluble in benzene or chloroform and can be sublimed under reduced pressure.72 The plutonyl oxine compound PuO,(oxine),,H(oxine) analogous to the uranyl compound,16 and the dark- red Pu(oxine) are Plutonyl nitrate gives a reddish-brown precipitate with potassium ferricyanide probably (PUO,),[F~(CN)~],,~H,O.Potassium ferrocyanide reduces plutonium(V1) rapidly but reduces plu- Acetyhcetonutes. S - ~ ~ d r o x y q ~ ~ n o Z ~ n e (oxine) derivatives. Terri- and ferro-cyanides.74 $8 H. H. Anderson MDDC 1127. loD. F. Maatick and A. C. Wahl MDDC 1761. 'a J. S. Dixon and C. Smith MDDC 1205. 7 s R. L. Patton MDDC 1386. ** Idem MDDC 1407. 71 W. H. Reas MDDC 1218. H. H. Anderson MDDC 1434. LISTER CHEMISTRY OF THE TRANSURANIC ELEMENTS 39 tonium( IV) only very slowly. Aqueous plutonium(1V) and potassium ferricyanide give a greyish-brown precipitate Pu3[ Fe( CN),], 15H,O. Plu- tonium( IV) and potassium ferrocyanide or plutonium(II1) and potassium ferricyanide give black precipitates of apparently different solubility and water content ; they are both PuFe(CN),,xH,O.The individuality of these two compounds is said to be uncertain and indeed it would seem probable that they are the same compound with the resonance forms PU(III)Fe(III)(CN) and Pu(IV)Fe(II)(CN), analogous to the Prussian blues NaFe(II)Fe(III)(CN),. This is supported by the black colour of the compound. Plutonium( 111) and potassium ferrocyanide gave a pre- cipitate from acid solution of HPuE'e(CN),. Hydrides.-Uranium and plutonium metal both combine with hydrogen to give solid hydrides whose formule approximate to 'U-H and PuH,. The hydrogen content is frequently lower than that required by these formule but there is no evidence of a definite lower hydride. The absorp- tion isotherms and kinetics of the reaction 2Pu + 3H2 -+ 2PuH3 have been examined.75~ 7 It is remarkable that the decomposition pressure of plu- tonium(II1) deuteride may be 1.4 to 1.5 times that of the hydride at cer- tain temperatures.The heat of reaction calculated from the pressure- temperature curve is 4.4 kcals./Pu atom at moderate temperatures. The structure of uranium(II1) hydride has been examined by X-ray diffraction ; the uranium atoms have a cubic (@-tungsten) arrangement ; the hydrogen atoms cannot of course be located directly. R. E. Rundle '7 believes that U-H-U bridges occur with one pair of valency electrons available in each bridge i.e. to the two U-H bonds. This conclusion is supported by L. Pauling and F. J. Ewing '* who calculate that such a structure would agree with the observed cell dimensions. Nitrides.-Plutonium nitride PUN can be made by the action of ammonia on the trichloride trihydride or metal at high te~nperatures.~~ Under these conditions no other nitride is formed.gives a nitride UN and also U,N3 and UN,. Carbides.-Carbides PuC and UC are known. All these mono-nitrides carbides and oxides have the sodium chloride type of structure 43 81 with the following lattice dimensions (in A.) Uranium ..~___..____ _____._ MC . . . . MN . . . . MO . . . . U. NP. Pu. Am. 4.951 - 4.910 - 4.880 - 4.895 - 4.92 5-00 4.948 4.95 _ _ ~ _ _ ___ - 751. B. Johns MDDC 717. 77 J . Amer. Chem. SOC. 1947 69 1719. 79 B. M. Abraham N. R. Davidson and E. F. Westrum MDDC 1640. 76 J. E. Burke .4ECD 2124. 78iZbid. 1948 70 1660. R. E. Rundle N. C. Baenziger and A. S. Wilson J . Amer. Chem. Soc. 1948 W. H. Zachariasen AECD 2196.70 3299. 40 QUARTERLY REVIEWS Borohydrides.-Aluminium borohydride reacts with heavy-metal tetra- halides to give volatile borohydrides of which the following have been identified These borohydrides are stable in the cold and in air but are hydrolysed by water with evolution of hydrogen When heated they decompose to give borides. Their vapour pressures are of the order of 0.1 mm. Hg at room temperature. The composition of the plutonium compound is un- certain ; it was prepared from plutonium tetrafluoride and aluminium borohydride but its formula has not been definitely established. Structure of the Heavy-metal Atoms As was mentioned in the introduction the heavy elements from actinium onwards might follow the sequence of the second long period owing to filling of 6d orbits or the sequence of the third long period (rare earths) owing to filling of the 5f shell or some modification of the sequence of the third long period owing to the filling of the 5f shell starting at some rela- tively later element.In the chemistry of these elements it is evident that thorium behaves as a group 4 element protoactinium (probably) as a group 5 element and uranium predominantly as a member of group 6. After this the sequence differs entirely from the second long period and neptunium to curium are not analogues of rhenium and the platinum metals but instead resemble uranium and to some extent the rare earths. There is a strong similarity between these elements when they are in the same valency state ; thus uranium(VI) neptunium(VI) and plutonium( VI) are similar ; so are thorium(IV) uranium(IV) neptunium(IV) and pluton- ium(1V) ; and actiniurn(III) plutoniumfIII) americium(III) curium(III) and to a lesser extent the easily oxidised uranium(II1) and neptunium(II1).Thus the properties of for instance uranium(IV) apart from its oxidation to wcanium(VI) closely resemble those of thorium(IV) and are not very like those of tungsten(1V). We have good chemical evidence for the presence of 5f electrons in uranium and the succeeding elements. The oxidation-reduction relations as was explained above are also consistent with the presence of 5f electrons if these are assumed to be less firmly held than 4f electrons (at least at first) and to provide relatively less screening of the nuclear charge so that the oxidation-reduction relations alter more rapidly along the series.Finally we may review briefly the physical evidence that bears on this point. The atomic spectra of thorium and uranium have been examined. Thorium s2 has its four valency electrons in 6d27s2 ; uranium 83 has its six valency electrons in 5f36d17s2. Thus in uranium and presumably in suc- ceeding elements 5f levels are in fact occupied. The fact that uranium 82 W. F. Meggers Science 1947 105 514 ; quoting P. Schuurman Thesis Amster- dam 1946. 8a 0. C. Kiess C. J. Humphreys and D. D. Law J . Res. hTat. Bur. Stand. 1946 37 57. LISTER CHEMISTRY OF THE TRANSURANIC ELEMENTS 41 has three 5f electrons suggests that the starting place of the series is two places earlier namely at thorium ; but this is largely an artificial point of view since it assumes a regularity that need not and in fact does not exist.Further evidence for 5f structures comes from the absorption spectra of the heavy-element compounds and particularly from their resemblances therein to the rare earths. It is well known that the colours of the ter- valent rare-earth ions are due to transitions amongst 4f levels; this is shown by the great sharpness of the absorption bands since the 4f levels are shielded by the 5s and 5p electrons from disturbance by the fields of surrounding molecules ; and by their relatively low intensity having regard to the narrowness of the bands since these are " forbidden " transitions in the sense that the 1 quantum numbers do not change. The absorption spectra of the heavy-element ions also give narrow absorption bands which are to be interpreted as transitions between 5f levels.(See for instance plut~nium,~ neptunium,13 or americium spectra. 31) The spectra of the tervalent ions in some cases somewhat resemble those of the " correspond- ing " rare-earth ions i.e. uranium(II1) and neodymium(II1) ; plutonium(II1) and samarium(II1) ; and particularly americium(II1) and europium(III).84 The resemblance of the spectra of americium and europium trichloride which were examined in the crystal is evidence of six 5f electrons in the former but no detailed analysis of the spectra has yet been published. The ions of the heavy elements like those of the rare earths are fre- quently paramagneti~.~~~ 8 6 ~ 8' Thorium(1V) ions having a rare-gas struc- ture are of course diamagnetic and the uranyl ion has a small temperature- independent paramagnetism.As a result of measurements in solution the following values of the magnetic moments in Bohr rnagnetons were obtained Np0,2+ f f 1 NpO,+ . . u4+ * . . I No. of Ion. I electrons ~ Moment. above Rn core. ~~ Ion. .__ 2.40 u3+ . . 3 3.20 3-03 3.086 Pu4+ . 4 1.85 2.95 0.90 2.91 Am3+. . . 6 0-8 6 I above En core. These results are mostly from measurements at room temperature only and the calculated values of the moment assume that A in the Weiss- Curie law is zero. Analogous measurements on the rare-earth ions show 8c S. Fried and F. J. Leitz AECD 1890. 86 J. J. Howland and M. Calvin AECD 1895. 86 C. A. Hutchison and N. Elliot AECD 1896. 87 R. W. Lawrence J . Am?. Chem. Soc. 1934 S6 776. 42 QUARTERLY REVIEWS that A may be appreciable even in magnetically-dilute materials so that the values of some of these moments may need some modification prob- ably upwards.The general trend of these values closely follows that of the rare-earth ions Ion (all 3 f ) ce Pr Nd - Sm Eu Electrons above xenon cor0 1 2 3 4 6 6 Moment 2.39 3.47 3.52 - 1.58 3-54 In addition to these results in solution various solid compounds have been measured e.g. uranous oxalate trihydrate 88 3-66 ; uranous sulphate 86 3.46 ; uranous acetylacetonate 86 3.39 ; UF4 89 3.30 ; UO 47 3-19. These moments are a11 given in Bohr magnetons; the susceptibilities were measured over a range of temperature and obeyed the Weiss-Curie law x(T + A) = C where x is the magnetic susceptibility. The moments of the rare-earth ions have been well explained by H. J. Van Vleck90 on the assumption that all electrons above the inert-gas core are in 4j orbits and that the spin-orbital coupling is not disturbed.The agreement of the moments of the ions Np02,+ and Ce3+ makes it probable that neptunium here has one 5f electron (ground state 2P5,2). The moments of the U4+ NpO,+ and PuO,~+ compounds differ somewhat but in general they are lower than that of Pr3+ compounds. This may be due to the strong interaction of neighbouring atoms suppressing the contribution of the orbital momentum so that only the spin is effective. In such a case an atom containing n unpaired electrons will have a moment of dn(n + 2) which is 2-83 in these ions and the moments of various uranium(IV) nep- tunium(V) and plutonium(V1) compounds are not much above this value. However such interaction would be more probable for 6d than for 5f elec- trons and these low values provide some support therefore for 6d structures.On the other hand if the uranous ion had a 6d2 structure in which L S coupling were preserved ground state 3F2 the calculated moment would be 1.63 much lower than any of the observed values. The observed values are indeed usually somewhat higher particularly for the uranous com- pounds than the " spin only " moment ; and also the A values where these are known are large and positive so that unless this constant is also measured the observed moments will really be too low. Moments of uranium tetrafluoride and dioxide are undoubtedly nearer the spin-only value but these are of course the most magnetically concentrated substances. The uranous sulphate and oxalate moments are in agreement with a 5f2 structure.The values for the Np*+ and U3+ ions are also lower than that calculated for three 5f electrons and L-S coupling ground state 419,2 namely 3.62 ; and here the spin-only formula gives a higher value of 3.87 whilst structures involving 6d electrons would give values much lower than those observed. The values for Pu3+ and Am3+ ions fall much below those of Sm3+ and Eu3+; but it will be This is also true of the Pu4+ ion. 88 C. A. Hutchison and N. Elliot. Php. Reviews 1948 73 1229. a@ N. Elliot &id. 1949 '76 431. $0 '' Theory of Electric and Magnetic Susceptibilities " Oxford 1932. LISTER CHEMISTRY OF THE TRANSURANIC ELEMENTS 43 remembered that Van Vleck and Frank in their calculation of the moments of latter ions had to assume that the multiplet intervals were comparable with kT so that states above the ground state are appreciably contribut- ing.The other moments are calculated assuming multiplet inteivals large compared with k17 this would lead to a value of 0434 for Sm3+ and 0.0 for Eu3+. Consequently the moment of Pu3+ agrees with a 5f5 structure (ground state 6H5,z) with wider multiplet spacing ; and the value for Am3+ is probably also best explained as a 5fs structure (ground state 'FO) with multiplet intervals somewhat wider than those of the europium ion; but detailed calculations have not yet been published. Thus the magnetic evidence taken as a whole undoubtedly supports the 5f structures for the transuranic compounds. The crystal structures of a number of series of analogous heavy-element compounds have been determined by X-ray diffraction and from these radii for the heavy-element ions have been deduced assuming values for the radii of the associated ions.21v 91 The series most fully investigated are the trifluorides trichlorides tribromides and dioxides.The radii (A.) so obtained are as follows 3+ ion . . Pu. Am. __ _-_____ M-0 distance in MO . . . La. Ce. Pr. Nd. - I sm. Ell. 1.04 1.02 1.00 0.99 - 1 0.98 0.97 l ipp -~~~~ Whilst these absolute radii depend on the values chosen for the radii of the halide ions their relative values are unambiguously determined by the X-ray data. It will be seen that a steady fall takes place with rising atomic number and this of course parallels the behaviour of the rare-earth compounds. Corresponding figures for the rare-earth 3+ ions using the same halide radii are In addition the cell dimensions have been found to show a similar contraction in other series of heavy-metal compounds such as NaM02( OAc), KM2Fs where M is the heavy element ; and indeed this contraction seems to be quite general.The cause of this phenomenon is no doubt the same as in the rare-earth group vix. that the ionic size is determined by the quantum numbers of the outermost electrons and by the effective nuclear charge (ie. the nuclear charge minus the screening effect of the other @1 W. H. Zachariasen MDDC 1572. D 44 QUARTERLY REVIEWS electrons) in which they are held. In the heavy-metal ions if we assume that their structure is similar to that of the rare-earth ions the outermost electrons are always in the completed 6p sub-shell and the effective nuclear charge rises with the atomic number since the screening effect of the extra electrons in the 5f shell does not entirely compensate for the increased nuclear charge.Hence this contraction is in fact evidence for the 5f theory of the structure of the heavy-metal compounds. It will also be seen that the contraction is somewhat more rapid in the heavy metals than in the rare earths which is at least consistent with the more rapid alteration of the oxidation-reduction potentials. The author thanks the Director Atomic Energy Research Establish- ment for permission to publish this review.