A spectrophotometric study of trivalent actinide complexes in solution—II

J. inorg, nucl. Chem., 1966,Vol. 28, pp. 2725 to 2732. PergamonPre,m Ltd. Printedin Northern Ireland

A SPECTROPHOTOMETRIC ACTINIDE COMPLEXES

STUDY OF TRIVALENT I N S O L U T I O N - - I I *c1~

NEPTUNIUM AND PLUTONIUM M. SHILOH~"and Y. MARCUS* Radiochemistry Department, Soreq Nuclear Research Center, Israel Atomic Energy Commission, Yavne, Israel

(Received 5 October 1965; in revised form 21 March 1966) Abstract--Spectra of neptunium (Ill) and plutonium (HI) were studied in aqueous halide solutions as functions of the halide concentration. Each element was found to exhibit strong absorption at a characteristic wavelength in concentrated lithium chloride or bromide solutions. In chloride solutions neptunium and plutonium have intense absorption bands at 26,050 and 32,050 em -x respectively, and in bromide solutions the bands are shifted about 250 cm -x towards lower energies. These bands are attributed to 5f ~--+ 5f ~-1 6d 1 transitions. The stability of the complexes MXI + could be estimated, being the same, within experimental error for both M = Np and M = Pu, the values for the effective constants fls* being (1.0 -4- 0.1) x 10-s M -~ for X = CI and (3.1 ± 0.4) × 10-7 M -2 for X = Br. At intermediate halide concentrations, the species MX 2+ are formed. The stability of these complexes are much lower than those of solvent shared ion pairs of these elements, as is the case with uranium

(III). INTRODUCTION

THE trivalent state of neptunium and plutoniumin aqueous solutions is more stable than that of uranium studied previously.(11 However,chemicalinformationon their state in solution and on their behaviour towards complexing agents is not abundant, especially for neptunium (III). The absorption spectra of neptunium (III) and plutonium (III) in dilute aqueous solutions have been published, c2'z) The spectra published for 1M perchloric acid solutions of these elements may be taken to represent those of the free (uncomplexed, unhydrolysed, hydrated) ions Np a+ and Pu a+, since it is generally accepted that no complex formation takes place in such solutions. Dilute hydrochloric or hydrobromic acid solutions show little or no change in the spectra relative to perchloric acid solutions. There is no information concerning neptunium (III) in more concentrated halide solutions and only qualitative information concerning the changes of the visible spectrum of plutonium (III) as the concentration of hydrochloric acid varies.C4) These changes were found to be small, and no extensive complex formation is expected. Still, even in relatively dilute solutions, plutonium (III) was found to form ionpairs with chloride ions, and the association constant has been estimated as K1 ----0.70, * Presented in part at the 149th A.C.S. Meeting, Detroit, April 1965. ? Part of a Ph.D. thesis submitted to the Hebrew University, Jerusalem. :~ Present address: Department of Inorganic and Analytical Chemistry, The Hebrew University, Jerusalem. ,x~ Part I. Uranium, M. SnmoH and Y. MARCUS,Israel J. Chem. 3, 123 (1965). '~ J, J. KATZ and G. T. S~A_nORG, The Chemistry of the Actinide Elements pp. 223 (Np), 295, 296 (Pu). Methuen, London (1957). ,3) W. C. W A O O ~ R , J. phys. Chem. 62, 382 (1958). ,4~ G. T. S~nORG, J. J. KATZ and W. M. MANNING(Editors), The TransuraniumElementspp. 372, 373. McGraw-Hill, New York (1949). 2725

2726

M. SI-nLOHand Y. MAgCOS

3"7 and 14.7 M -1 at ionic strengths 1.0, 0.1 and zero respectively. Since only small spectral changes are observed even in solutions which should contain a high fraction of the ion-pairs, it is concluded that they are of the solvent-separated type. 650) appears, having a peak at 26,050 cm -1 (384 m/z) for chloride and at 25,830 cm -x (387 m/z) for bromide. It has a shoulder, indicating another intense peak, at 2400 c m -1 lower energy. This intense b a n d covers three low intensity (e -~ 25) bands neptunium (III) has at approximately 24,000, 25,000 and 26,500 c m -x. In the ~s~L. G. SILL~Nand A. E. MARTELL,Stability Constants of Metal-Ion Complexes p. 268. Special Publication 17, Chemical Society, London (1964). c6~S. R. CottoN, J.phys Chem. 61, 1670 (1957); J. M. SMrrnsoN and R. J. P. WmHAMS,J. chem. Soc. 457 (1958). ~7~G. R. HALLand P. D. I-I~RNIMAN,ar. chem. Soc. 2214 (1954).

A spectrophotometric study of trivalent actinide complexes in solution--It

2727

visible region the changes are much smaller, the absorption band at 18,100 cm -1 (553 m#) is hardly changed at all, but the sharp peak at 12,700 cm-1 (785 m/~) decreases in intensity down to 40 per cent as the halide concentration is increased. The characteristic band of neptunium (III) at 7350 cm -1 (1360 m/~)3, is changed by less than 10 per cent as the lithium chloride concentration in D20 (necessary in order to avoid intense absorption by H~O in this region), is increased from 1.0 to 12.9 M. i

I

i

i

I

_~

i

800

/

B0 60 T

600 -

T 400

-

, , i

~v

200

0

,

I

,

I

I

'

~ '"

t

J 400

500

E

i

I

/

"'

¢0

,

tu

II

'

'

20

"P" 600

700

k

(m F )

800

FIe. 1 . - - A b s o r p t i o n spectra o f n e p t u n i u m ( I I I ) in dilute H C I . . . . LiCI -and 11.5 M L i B r . . . . . . .

, ll'7M

It should be noted that solutions of neptunium (III) in concentrated aqueous lithium halide are green, similar to dilute aqueous neptunium (IV), rather than the bluish-violet of dilute neptunium (III) solutions. A detailed spectrophotometric examination is necessary in order not to be misled by the colour. In saturated magnesium iodide solutions (8"2 N), which has a mean ionic activity of 2000/8) which is higher than those of concentrated lithium iodide solutions, no change of the absorption spectrum, even in the 26,000 cm -1 region, compared with dilute aqueous solutions could be observed. The absorption spectrum in 5 M sulphuric acid in the visible region is similar to that of dilute aqueous solutions, but above 25,000 cm -1 the absorption intensity increases markedly. The liquid zinc amalgam method could not be used to prepare neptunium (III) in nitrate solution, since nitrate is reduced preferentially. More noble metals, such as tin or lead, which do not reduce nitrate, did not reduce neptunium to the + 3 stage at a measurable rate. When neptunium solutions were passed through a Jones reductor, acidity had to be kept above 0.5 M in order to retain neptunium (IV) unhydrolysed in solution. When the acid solution was run into a nitrate solution, immediate oxidation of the neptunium (III) occurred. Thus no spectrum of neptunium (III) in nitrate solutions could be obtained.

Plutonium (III) The absorption spectra of plutonium (III) in lithium chloride and bromide solutions are shown in Fig. 2. In concentrated halide solutions, an intense band appears, as is the case with neptunium (III) and uranium (III). (1) The peak of this band occurs at 32,050 cm-1 (312 m/~) in chloride solutions, and at 310 cm -1 lower energy (compared (a) R. A. ROB~SON and R. H. STOKES,Electrolyte Solutions (2nd Ed.) Butterworths, London (1959).

2728

M. SmLOl-i and Y. MARCUS

with 220 cm -1 for neptunium and 100 cm -t for uranium) for bromide solutions, with r > 900 M -~ cm -i. A shoulder, indicating the presence of another peak, occurs around 29,600 cm -1 (348 m#), and a less pronounced one at 27,800 cm -1 (360 m/z). The intense band covers completely the broad band at 33,400 cm -i (299 m/z, ~ -- 120), but causes little change in the absorption of the band having a shoulder (r = 480), at 36,900 cm -1 (271 in#) and three peaks at 40,810, 41,230 and 42,010 cm -1 (245, 243 and 238 m/z, ~ = 1700, 1600 and 1600 respectively), recently observed (9) in dilute I

T

I

I

[

I

800

8O

6oo

60

E

o 400

-40 t~ ~

WJ

200 0

FIO.

i

-20 ....

300

___..~m_o

350

LJ

¢00 600 X (rnH.)

800

I00(

o

l--Absorption spectra of plutonium (III) in dilute HC1 . . . . LiCI .. and 10"2M LiBr . . . . . . .

, 10-2 M

solutions. The bands tg) at 44,150cm -1 (227mg, e = 1100) and at 48,420cm -1 (207 In# e = 3000) could not be observed in concentrated chloride solutions, because of absorption by the halide ions. In the visible region, the change in the absorption is small, the peaks at 17,850 cm -i (560 m/z) and at 16,700 cm -i (600 m/z) decreasing somewhat in intensity, the latter more so than the former, on increasing the halide concentration. Plutonium (III) behaves similarly to neptunium (III) in saturated magnesium iodide solutions and in sulphuric acid solutions. In the former, no spectral changes could be observed, nor in the latter in the visible region. In the ultra-violet, intense absorption was found for plutonium (III) in sulphuric acid solutions. Nitrate solutions of plutonium (III) are stable, and can be prepared by reduction with hydrazine nitrate. Because of the high absorptivity of the nitrate ion above 28,000 cm -x, the characteristic peak of plutonium (III) found at 32,000 cm -i in halide solutions cannot be observed. In the visible region, spectral changes in concentrated lithium nitrate (8 M) solutions are somewhat more pronounced than in lithium halide solutions. Again, the peaks at 17,850 and 16,700 cm -1 are mainly affected (MARCUS and GIVON).(1°) In experiments to extract plutonium (III) from concentrated lithium chloride solutions into a 20 ~o triisooctylammonium chloride solution in xylene, immediate oxidation to plutonium (IV) occurred and no spectrum of plutonium (III) in the (o~ D. COItXN,d. inorg, nueL Chem. 18, 211 (1961); R.E. ELSON,Israel A.E.C. Semi-AnnualReport, IA-775, p. 69. Jan.-June (1962). it0) y. MARCUSand M. GtVON,Israel A.E.C. Semi-AnnualReport, IA-726, p. 69. July-Dec. (1961); I. A B ~ R , M. GIVONand Y. MARCUS. To be published.

A spectrophotometric study of trivalent actinide complexes in solution--II

2729

organic phase could be recorded. This was observed even with carefully purified amine and diluent, at macro concentrations of plutonium, and in the presence of a holding reductant. When plutonium (III) was extracted from 8 M lithium nitrate, however, oxidation occurred with a half-time of about 5 min. The spectrum of the organic phase could be recorded at intervals, and from it the spectrum of plutonium (III) could be calculated by subtraction of the ingrowing known spectrum of plutonium (IV), and extrapolation to zero time. The spectrum so obtained (1°) shows again effects mainly on the 17,850 and 16,700 cm -x peaks, however they are in the opposite direction, i.e. the absorptivity is increased, in particular for the 17,500 cm -x (572 mff) peak.

ESTIMATION OF THE COMPLEX STABILITIES The change of the molar absorbance at the characteristic peaks is shown in Fig. 3 as a function of the halide concentration for both neptunium (III) and plutonium (III). It is seen that higher concentrations of bromide than of chloride are required to yield the same relative change in absorbance. Examination of these curves showed that preliminary estimates of the stability of the complex species formed could be made by assuming that at the highest halide concentrations only the species MX2+ exists along with uncomplexed metal ions M 3+. The molar absorptivity of this species at any wavelength is given by (m e2 = e0 + Aez = e0 + lim (e -- e0) l/a ~ 0

(1)

where e is the measured, average molar absorbance of the solution, e~ is the molar absorptivity of species M X and a is the mean molar activity of the lithium halide: i a = C~ixyzi x. The effective stability constants (~) for the species MX~ + are given by #o* = (~ - ~0)/(~0 - ~)a2

(2)

1600 I

Pu

i

1200 T

,~,/v oN P

T

.4 :/

E

800

~=;' /

o v

?'/~b¢

400 02

Pu,

i -F

;J

-i

t

T

J ,~Np ~

-

//
i 8 CLici,

I I0 M

', 12

14

4

6

8

I0

12

CLi Br ' M

F I G . 3.--The molar absorptivities of neptunium (III) and plutonium (HI) at the characteristic wavelengths(~7 312 m/~, A 315 m,u, O 384 m/~, [] 387 m/~)as functions of the LiC1(left) and LiBr (fight) molafities.

(xx) K. B. YATSIMIRSKII,Zh.fiZ. Khim. 30, 28 (1956); of. F. J. C. ROSSOTTIand H. S. ROSSOTT[,Deterraination of Stability Constants p. 281. McGraw-Hill, New York (1961).

2730

M. Snmori and Y. MARCUS

The quantity fa* will be a constant, provided other complex species besides MX~+ can be neglected in the ligand range studied, and provided certain assumptions concerning activity coefficient ratios tl) are valid. The data in Table 1 show that this is the case, and that only r a n d o m errors occur. It was attempted to refine the values of the constants, using YATSI~IRSKn'S method, m) and utilizing the whole ligand concentration range. This method, or any equivalent method, requires a number of extrapolations, involving the data and four unknown parameters: e1, e~, fix* and f~*. Using the absorption data at the characteristic peaks, only e2 and f~* cSuld be obtained with a reasonable degree o f TABLE 1.--EFr~crIw rOgMnTtONOF CONSTANTSMXz+ fOR Np(III) AND Pu(l'lI) NpC1,+ 105fla* (M) (M -a)

Cr, tc1

9.50 9.96 10.80 11.04 11.12 1i. 17 11.43 11.87 12.36 12.66 Average

1.25 1.I0 0.99 0.89 1.05 1.23 0.98 0.83 1.03 1.02 1.04 4- 0.13

NpBra+ CLmr 10¢fla* (M) (M-a)

PuCla+ CLio1 10st, * (M) (M-a)

PuBra+ CLmr 107/~a* (M) (M -a)

9.29 9.67 9.69 9.96 10.27 10.50 10.71 11.14 11.44 11.52

10.40 10.85 10.85 11.30 11.45 11.95 11.95 12.95 13.65 13-90

8.75 9.65 10.10 10.45 10.75 11.10 11.30 11.45 11.65 11.75

3.40 2.88 2.61 3.14 3.25 3.76 3.59 3.29 3.65 3.45 3.3 q- 0.4

0.98 1.19 1.26 0.87 1.00 0.99 1.18 0.71 0.89 0.91 1.00 :k 0.16

2.85 3.52 3.20 3.17 2.99 2.32 2.90 2.60 2.96 2.99 2-9 -r- 0.4

certainty, and the value of flz* could be confirmed using absorption data at other wavelengths [e.g. the decrease of e at 12,700 cm -1 for neptunium (III)]. As regards the species M X ~+, it was certainly found to be of importance at the lower halide concentrations (-~. 7-9 M); but only rough estimates of its stability constant could be made. This is because the constant for its formation from uncomplexed metal ions, KI* = f i * , nearly equals the constant for the formation of MX2 + from M X 2+, K2* = f~*[fl*. The results of these calculations are shown in Table 2. TABLE 2.--STABILITIES AND MOLAR ABSORPTIVlTIES OF HALIDES

Chloride Np(III) log flx*(M-x) --2"4 -4- 0.1 log fla*(M-2) --4.96 ± 0"04 Wavelength of characteristic peak (A) 3840 Wave number of characteristic peak (cm-1) 26,050 Czat characteristic peak (M-x em-1) 1170 '

Pu(III)

Np(III) AND Pu(III)

COMPLEX

Bromide Np(III)

Pu(III)

--2"4 -4- 0'1 5.00 4- 0"06

--3'4 -4- 0"1 --6-54 -4- 0"06

--3"5 -4- 0"1 --6"48q- 0"05

3120

3870

3150

32,050

25,830

31,740

1590

830

1590

A spectrophotometric study of trivalent actinide complexes in solution--II

2731

DISCUSSION

Examination of the spectra in Figs. 1 and 2 reveals that the characteristic new bands for complexed neptunium (III) and plutonium (III) are due to allowed 5fn-~ 5f *-1 6d transitions. The molar absorptivities of the complex species at the peaks, e2 ,~ 1000 for neptunium and e2 ,~ 1600 for plutonium are much higher than those of the uncomplexed ions, e0 = 40 for neptunium and e0 = 100 for plutonium, which are due to forbidden 5f ~ -~ 5f ~ transitions. Also the bands for the complexes MX, + are much broader, width at half-height being 3000 cm -1 for both actinides, compared with widths of <100 cm -1 for single 5f~---~ 5f n transitions. Finally, the direction and magnitude of the shift of the peaks from chloride to bromide complexes (220 cm -1 for neptunium and 310 cm -1 for plutonium towards lower energies) is what is expected for a 5f ~ -+ 5f ~-1 6d transition, and much lower than that expected for electron-transfer-from-ligand transitions ~m (6000 cm-X). Despite recent arguments to the contrary, <13) there is little reason to believe that the assignment by STEWART(14) and JORGENSEN{15) of the intense bands in the ultraviolet of the uncomplexed hydrated ions M a+ to 5f ~ -+ 5fn-16d transitions is incorrect. The energy of these transitions is correlated empirically t15) with the nuclear charge Z and the oxidation number z of the actinides by the relationship AE(in 10a cm -1) = 7(Z -- 96) -}- 18z

(3)

The Lapporte-allowed 5f" -+ 5f "-x d transition appears in the complex species (MX2 +, although MX 2+ appears to absorb at the same wavelength) probably because of degeneracy removal due to the lower symmetry, due to (partially) covalent bonding to the ligands, replacing water molecules. The transition energies for these complexes, 18,330 for uranium, tx) 25,940 for neptunium, 31,900 for plutonium and 42,550 for americium *C16) (all in cm -1, and all, except for americium, averages between values for chloride and bromide ligands), correlate well with Equation (3) with z ---- 2.6. If curium behaves like the other trivalent actinides, it would have a band at approximately 47,000 cm -1 (213 m/z), possibly just discernable above the absorption edge of chloride ions in the concentrated solutions that must be used. The fractional value of z can be explained by the (partially) covalent nature of the bonding, reducing the charge of the central ion. A fractional charge between two and three was also obtained for the lanthanide ions in ethanolic bromide solutions, from observations of shifts of 4f'~--+4f n-a 5d transitions, t17) The shift for the lanthanides is about 2000 cm -1, compared with a shift of around 8000 cm -~ for the complex band from the uncomplexed ion band in the actinides. The difference is due to the more elongated shape of 5forbitals compared with 4forbitals, hence their greater sensitivity to the chemical environment. A further indication that halide complex formation, rather than differences in solvation, is responsible for the spectral changes, may be seen in the fact that 10 M * See footnote t on p. 2725. ~12)C. K. JORGENSEN,Absorption Spectra and Chemical Bonding in Complexes pp. 146, 166. AddisonWesley, Reading, Mass. (1962). ~lZ~j. H. MILES, J. inorg, nucL Chem. 27, 1595 (1965). c14~D. C. STEWART,U.S.A.E.C. Reports ANL-4812, AECD-3351 (1952). txs~ C. K. J~ROENSEN,J. inorg, nucL Chem. I, 301 (1955). ~t6~y . MARCUSand M. SHIrOH. To be published. ~xT~C. K. J~RGENSEN,Molec. Phys., 5, 271 (1962).

2732

M. SHILOH and Y. MARCUS

perclaloric acid, which should have approximately the same dehydrative properties as concentrated lithium chloride solutions has almost no effect on the spectrum of neptunium (III)3 is) Decreased positive charge on the uranium (III) and neptunium (III) ions, produced by complex formation with chloride ions, has also been used by G&umq et aL (19) to explain the spectral features of solutions of these ions in fusedpotassium chloride euteetic. It has already been noted by DIAMOND e t aL (~°) that the complex formation of the lighter trivalent actinides with chloride cannot be explained in terms of ionic complexes, but that complex formation of a (partially) covalent nature must be considered. It is not surprising, therefore, that covalent complex stability as measured spectrophotometricaUy in this work, should give results quite different from those given by thermodynamic methods. Thus the estimated stability constants given in Table 2, 10-2,4 and 10-3,5 for the first chloride and bromide complexes, contrast sharply with estimates for the solvent shared ion-pairs formed in relatively dilute solution, e.g. 10+l'z at zero ionic strength. (5) The lower stability of the bromide, compared with the chloride, complexes is what is expected for typical class-A cations such as the trivalent actinides. That the stability of the iodide complexes is still so much lower that they cannot be detected in aqueous solutions is somewhat surprising. Sulphate complexes seem to be more stable than the halide ones, since lower ligand concentrations are required to bring about similar spectral changes. Nitrate complexes seem also to be relatively stable. The ability of a long-chain amine salt to extract the nitrate but not the chloride complex of plutonium (III) is related to the relative stabilities of the plutonium (III) and (IV) complexes, and therefore not a direct measure of the stability of the plutonium (III) species. The stability of the neptunium (III) and plutonium (III) complexes is very similar; the slightly higher stability of the former noted in Table 2 is barely Significant statistically. The stability of the MX 2+ species, although only roughly estimated, is however definitely higher than for uranium (III) (1) (log fll * = --2"9 for chloride, --3.9 for bromide), but lower than for americium (III) tie) (log i l l * : - - 2 . 2 for chloride and --3.3 for bromide). The lighter trivalent actinides thus form a more or less smooth series of the stability of their chloride and bromide complexes. The gradation in complex stability is seen not only in the values of the stability constants, but also, more directly, in the concentration of lithium halide required to produce the same relative increase in intensity of the characteristic 5f" ~ 5f "-t 6d peak. Acknowledgements--We would like to acknowledge helpful discussions with Dr. M. GIVONand the technical assistance of Miss N. ABEL. Thanks are due to the Nuclear Chemistry Division, Lawrence Radiation Laboratory, University of California, Berkeley, for its hospitality towards both authors during the writing of this paper. (ls) R. SJOBLOMand J. C. HINDMAN, f. Am. chem. Soc. 73, 1744 (1951). (1,) D. M. GRUEN, S. FRIED, P. GRAF and R. L. MCBETH, Proc. Int. Conf. Peaceful Uses Atomic Energy, Geneva, 1958, p. 940. United Nations (1959). is0) R. M. DXAMONO,K. STREETand G. T. SEABORO,J. Am. chem. Soc. 76, 1461 (1954).