Diffuse reflectance spectra of some uranium (IV) complexes

Spectrochimica Acts, 1966,vol. 21, pp. 1589to 1595. Pergsmon PressLtd. Printed in NorthernIreland

DiBuse reflectance spectra of some uranium (IV) complexes P. GANS,*~

B. J. HATHAWAY~

and B. C. SMITH*

(Received 2 January 1965) Abstract-The diffuse reflectance spectra (25000-4000 cm-l) of some uranium (IV) complexes involving different co-ordination numbers of the central uranium ion are reported. The spectra can be divided into three classes on the basis of the band intensities. The “weak” intensity spectra occur when the co-ordination number of uranium is six and the environment is centrosymmetrical. The “strong” intensity spectra occur when the co-ordination number is higher than six and a centre of symmetry is absent. The frequencies of the “medium” intensity spectra can be correlated more closely with those of the “strong” than the “weak” intensity spectra. This indicates the absence of a centre of symmetry, and it, is suggested that “medium” intensity spectra occur when octahedral or cubic symmetry is destroyed by the presence of of non-equivalent, ligands. The use of the three classes of spectra in predicting the environment uranium (IV) ions in complexes is discussed.

THE absorption spectra of uranium (IV) complexes in the solid state [l-4] and in The spectroscopic solution [5-141 are described in a number of recent publications. properties of the uranium (IV) ion in an octahedral environment-e.g. in hexahalogenouranates (IV) [ 1,7-l l]-have been discussed. Detailed treatments are given of the polarised single crystal spectrum of uranium tetrachloride [2] and of the spectrum of uranium tetrafluoride measured in potassium bromide discs [3]. In all cases the origins of the bands are considered to be Laporte-forbidden f -+ f transitions which appear weakly via a vibronic mechanism in the spectrum of the octahedral hexachlorouranate (IV) anion [I], and more strongly via an electronic mechanism [2] in

* Department w.c.1. t Present

address:

Leeds 2. $ Department [l] [2] [3] [4] [5] [6] [7] [S] [9] [lOI [Ill [12] [13] [14]

of Chemistry,

Birkbeck

Department

of Chemistry,

College

(University

of Inorganic

The University,

of London),

and Structural

Malet

Chemistry,

Street,

London

The University,

Hull.

R. A. SATTEN, D. YOUNU and D. M. GRUEN, J. Chern. Phys. 33,114O (1960); S. A. POLUCK and R. A. SATTEN, J. Chem. Phys. 36, 804 (1962). R. MCLAU~EIN, J. Chem. Phys. 36, 2699 (1962). J. G. CONWAY, J. Chem. Phys. 31, 1002 (1959). D. M. GRUEN and M. FRED, J. Am. Chem. Sot. 76, 3850 (1954). B. JEZOWSEA-TR~EBIATOWSKA and K. BWIETYNSKA, J. Inorg. Nucl. Chm. 19, 38 (1961). W. E. KEDER, J. L. RYAN and A. S. WILSON, J. Inorg. Nucl. Chem. 20, 131 (1961). J. L. RYAN and C. K. JORGENSEN, Mol. Phya. 7, 17 (1963). J. L. RYAN, J. Phye. Chm. 85, 1856 (1961). C. K. JORGENSEN, Acta C%em. Scam?. 1’7, 251 (1963). J. L. RYAN, Imorg. Chem. 3, 211 (1964). R. PAPPALARDO and C. K. JBRGENSEN, He&u. Phys. Acta 39, 79 (1964). D. G. KARRAKER, J. Inorg. NucZ. Chem. 26, 751 (1964). H. A. C. MCKAY and J. L. WoODHEAD, J. ChemSoc. 717 (1964). K. W. BAGNALL, D. BROWN, P. J. JONES and P. S. ROBINSON, J. Chem. SOC. 2527 (1964). 1589

P. GANS, B. J. HATHAWAYand B. C. S~H

1590

the spectra of the less symmetrical uranium tetrachloride and uranium tetrafluoride. Irrespective of whether a vibronic or electronic mechanism is involved, the spectra are interpreted by means of an intermediate coupling scheme for the f2 configuration [15] followed by a crystal field splitting of the J multiplets [16]. No clear attempt [f&-11] has been made to relate the differences in intensity of the spectra to the crystal environment of the uranium (IV) ion. The diffuse reflectance spectra in the visible and near infra-red regions of a series of twenty compounds of uranium (IV) at room temperature are reported here. Absolute values of extinction coefficients are not available, but it is found in practice that the spectra of uranium (IV) complexes can be divided into three classes on the basis of the intensities of the bands on the arbitrary Beckman scale of o-2. “WEAK” INTENSITY SPECTRA The reflectance spectra of five hexachlorouranate (IV) complexes, M,UC’,(M = are similar in the observed frequencies Cs, Et,P, MePPh,, Pr’,PH, and C,H,NH),

0

z 0.2 iz = a co jj Q4 E 0

0.2

25000

20000

15000 FREQUENCY

10000

5000

4000

(CM-‘)

Fig. 1. “Weak” intensity reflectance spectra of: (A) (Et,P),UCl,; (C) UCI,,BEt,PO.

(B) Cs,UCl,;

and relative intensities of the bands. The intensities are less than O-5 on the Beckman scale. The spectra of dicaesium hexachlorouranate (IV) and his (tetraethylphosphonium) hexachlorouranate (IV) are illustrated in Fig. 1. This type of spectrum is characteristic of the hexachlorouranate (IV) anion which has a regular octahedral arrangement of chloride ions surrounding the uranium (IV) ion [17]. The spectrum of dicaesium hexachlorouranate (IV) agrees closely with the data of SATTEN, YOUNG and GRUEN [l], although details of the individual levels are less F. H. SPEDDING, Phye. Rev. 58, 255 (1940). [16] W. Low, Solid State Physic& (Supplement No. 2) (1960). [17] A. SIEQEL, Acta Cryat. 9, 827 (1956). [15]

Diffuse reflectance spectra of some uranium (IV) complexes

1591

clearly resolved.

The uranium environment has a centre of symmetry, electronic are Laporte-forbidden, and the spectra are weak because the bands are vibronic in origin. The three phosphine oxide complexes, UCl,, 2Et,PO, UCl,, 2Et,PhPO andUCl,, 2Ph,PO have almost identical reflectance spectra. The relative intensities of the bands resemble those of the hexachlorouranates (IV) but there is a shift to higher The spectrum of uranium tetrachloride-his (triethylfrequencies of cu. 500 cm-l. phosphine oxide) is illustrated in Fig. 1. The general similarity to the spectra of the hexachlorouranates (IV) indicates a similar octahedral configuration involving four chloride ions and two oxygen atoms from phosphine oxide molecules. The weak intensities of the spectra show that a centre of symmetry is retained, and a tram configuration for the two phosphine oxide groups is indicated. Certain other types of structure can be eliminated. For example, the spectrum of a complex of the type [(R3PO)BU][UCl,J2 would incorporate the hexachlorouranate (IV) spectrum which differs significantly from, and which could be resolved from, the observed spectrum. The maxima of the bands of the observed spectra correspond approximately to the positions of the underlying electronic transitions [l]. It is significant that all the bands of the phosphine oxide complexes are shifted to higher frequencies, as this is consistent with a general shift of the free ion terms of the f” configuration, and suggests that the chloride ion has a greater nephelauxetic effect than the oxygen of Combination of this result with results for bromide [9] and phosphine oxides. iodide [7] ions in hexahalogenouranates (IV) gives the following order of increasing nephelauxetic effect: R,PO < Cl- < Br- < I-. These shifts are unlikely to arise from differences in the crystal fields of phosphine oxide and chloride, which are unlikely to be large and which would probably produce equal numbers of increases and decreases in the observed frequencies.

f -+ f transitions

“STRONG” INTENSITY SPECTRA The observed frequencies and relative intensities of the reflectance spectra of seven compounds, UP,, UCI,, UC1,,6Me,SO, 2UC1,,5HCONMe,, U(SCN),, 4MeCONMe,, U(SO& 4H,O and U(C,O,),, 6H,O, are recorded in Table 1. The intensities of the bands are in the region of 1-O. The empirical formulae do not suggest any obvious co-ordination number for the uranium ion, but the great similarity of the spectra indicates that the compounds are structurally quite similar. The crystal structures of uranium tetrafluoride [Is], uranium tetrachloride [19], and uranium (IV) sulphste tetrahydrate [20] are known, and in each of these compounds the uranium ion exhibits the co-ordination number eight. The origin of the “strong” intensity spectra is still Laporte-forbidden f +f transitions. The environment of the uranium ion lacks a centre of symmetry, and the purely electronic transitions are partly allowed by the mixing-in of orbitals of even symmetry which over-rides the purely vibronic mechanism. The spectra are more intense than those of the centrosymmetric hexachlorouranates (IV), and the splitting of the various J multiplets is determined by the symmetry of the crystal [18] A. C. LARSON, R. B. ROOF and D. T. CROMER, Acta Cry&. 17, 655 (1964). [I91 R. C. L. MOONEY, Acta Cryst. 2, 189 (1949). [20] P. KIERKEGAARD, Acta Chem. &and. 10, 699 (1956).

(1.0)

9.22

(0.7) (0.6)

(0.7) (0.7) (0.5)

(0.6)

11.01 sl& 11.90 15.4 ah 15.7 16.1 sh 17.5 #h 18.47 t8.73 sh

20.14 20.1 21.9

23.7

(0.3) (@4) (0.95) (1.0) (0.8)

(0.3) (0.5) (0.8)

sh 6.34 sh 6-99

5.96

KW

4.45

UF,

22.4

20.3 20.8

(1.0) (0.9) (0.8) (l-1) (0.75) (0.75) (0.75) (0.70)

(0.7) (1.1) (1.15) (0.4)

(1.15)

(1.2)

(1.0) Pw

(0.95)

VW

scale me shown in brackets.

17.7 1802 sh 19.45 eh (0.75) 20.04 (0.75) 21.8 22.2 22.7 (0.9) 23.15

(0.9) (0.7) (0.7) (0.8) (0.9)

14.5 16.3 sh 17.4 sh 17.8 19.3

11.1 14.7 sh 15.2 16.7 sh

8.62 9.09 sh

(1.0) (0.9) (0.8)

8.62 9.09 sh 9.43 sh (0.7)

6.01 sh 6.35 7.20 sh

(0.7) (0.8) (0.8)

5.78 6.45 7.02

11.0

4.28

UCl,,BMe.$O

(0.9)

UCI,

4.46

Intensities on the &2 Beckman rrhrepresenta shoulder.

Term

Table 1. “Strong”

(1.1) (0.8) (0.9) (0.6)

22.7 24.7

(0.9) (0.9)

17.9

20.2 21.0

(0.7) (0.6) (1.2) (0.75)

11.4 13.7 sh 15.15 16.7 sh

(1.1)

(1.0)

6.45

8.85

(0.85) (0.9) (0.7)

4.14 ah 4.46 5.0

2UC1,,5HCONMe,

-

(0.85) (0.8)

8.48 8.86 sh

(0.7)

(0.8) (0.4) (0.4)

19.9 21.0 sh 22.0 sh 22.35

(0.7) (0.7)

17.6 19.3 sh

(0.4) (0.85) (0.8) (0.7)

(0.6) (0.7)

5.97 sh 6.31

10.8 14.5 15.0 15.62

(0.7)

4.35

-

U(SCN),,4MeCONMe,

intensity reflectance spectra ( lo3 cm-‘)

20.2 20.7 21.8 sh 22.8 sh 23.2

18.2

15.5

11.6

9.36

8.80

6.08 sh 6.53

4.21

(0.9) (0.85) (0.5) (0.7) (0.8)

(0.8)

(1.0)

(0.6)

(1.0)

(1.0)

(0.75) (0.9)

(0.9)

U(SO,),,4H,O

(0.9) (0.3) (0.7) (0.75) (0.9) (0.76) (0.5) (0.4)

18.18

20.1 sh 20.95 21.5 sh 22.6 23.65

(1.5) (0.5) (0.4) 15.04 16.4 sh

9.05 11.04 11.9 sh

(1.25) 8.51 sh

(0.7)

(1.3) (1.1)

sh

6.06 6.87

4.09

U(C,O,),,6H,O

Diffuse reflectance spectra of some uranium (IV) complexes

1593

field about the uranium (IV) ion. This type of argument is used to interpret the polarised single crystal spectrum of uranium tetrachloride [a]. The resolution of the reflectance spectrum of uranium tetrachloride allows only approximate assignments of the bands to the various J levels. There is little indication of the splitting of the J levels by the crystal field, except possibly in the case of the 3H3c 3H, transition. This band was outside the range of MCLAUGHLIN'S measurements, but he calculated that this transition would occur at 5079 cm-l and the 3Fzc 3H, transition at 3795 cm-’ [2]. The bands at CCL.6500 and 4400 cm-l respectively in the reflectance spectra are assigned tentatively to these transitions. The spectra of uranium tetrachloride and uranium tetrachloride-hexakis(dimethyl sulphoxide) are illustrated in Fig. 2. The 3H,t 3H, transition in the

25000

20000

15000 FREQUENCY

Fig. 2. “Strong”

10000

5000

4000

(CM”)

intensity reflectance spectra of:

(D) UCl,;

(E) UC1,,6Me,SO.

spectrum of uranium tetrachloride should be split into five crystal field levels, which because of overlap of the transitions or poor resolution, appear as three bands in the reflectance spectrum. Consequently the spectra cannot be used to provide detailed information about crystal structures although the lower the number of bands or shoulders observed in the transition the higher is likely to be the symmetry of the uranium ion, or the weaker the crystal field splitting. It is significant that the

P. GANS, B. J. HATEJAWAY and B. C. SRIITH

1694

8H5 c 8H4 transition in uranium tetrachloride-hexakis(dimethy1 sulphoxide) is resolved into a peak and two shoulders. This implies a similar eight-co-ordinate environment for uranium. A six co-ordinate structure of the type [U(Me,SO),]Cl, can be rejected. The intensities of the spectra of all the compounds in this group suggest eight-co-ordination and the absence of a centre of symmetry. The most probable structure for bis(uranium tetrachloride)-pentakis(dimethylformamide) consists of two square antiprisms joined by three bridging groups across a shared triangular face. “MEDIUM” INTENSITY SPECTRA The observed frequencies and relative intensities of the reflectance spectra of five compounds, UCl,,phen,, UC14,2MeCN, UCl,, 3C4H802, UCl4,3Me,PO and UCl,, 4POC18, are recorded in Table 2. The intensities of the bands are in the region of Table 2. “Medium” Term

UCl,,phen2

UC1,,2MeCN UCI,,IC,H,O,

31”,

4.08

SH,

5.94sh (0.35) 5.93 6.45 (0.50) 6.29

sp, 8.82 8F, 9H,

10.82

JP,W,

14.70

'%

1563sh

17.24 19.60 20.00sh 20.50sh 21.90

intensity reflectance spectra (1 O3cm-l)

(0.50) 4.17

4.35 4.73 (0.35) 5.82 (0.45) 6.67 6.95sh (0.30)

(0.45) (0.25) (0.20) (0.40)

UC1,3Me,PO 4.35 5.04 6.08 6.71sh

(0.40) (0.30) (0.35) (0.45) (0.35)

(0.70) 9.05 (0.50) 9.61& (0.30) (0.25) 11.40 (0.55)

(0.65) 8.70 (0.50) 9.09sh

(0.60) (0.45)

(0.30) 11.11 14.99

(0.30) (0.60)

(0.55) 16.10

(0.50) 15.88 16.95sh

(0.60) (0.25)

(0.35) 7.07

(0.30)

(0.35) 17.70 (0.50) 19.46 CO.451 (0.4Oj 20.33 (0.50) 22.57

(0.30) 18.60 (0.30) (0.40) 20.20.d (0.35) 21.20 22.90 (0.40) 23.80sh

(0.55) 23.75

(0.50) (0.35) (0.35) (0.50)

4.42 4.65sh 6.06sh 6.45 (0.55) 7.02sh

8.34ah (0.45) (0.60) 8.81 (0.60) 8.62 9.308h (0'50) 9.36sh 10.81 (0.03) 11.11 (0.45) 11.36sh 14.93 (0.60) 15.46 (0.55) lb.15 (0.60) 16.05 (0.24) 16.26sh (0.40) 16.45 17.24sh (0.45) 18.69 (0.45) 19.88 21.14 (0.55) 21.55sh

UCl,,4POCI,

(0.50)

0.35 (0.30) (0.45) (0.35) (0.25)

18.35 18.45

(0.40) (0.40)

20.0sh 20.49 21.98sh 23.10

(0.35) (0.50) (0.40) (0.40)

Intensities on the O-2 Beckman scaleare shown in brackets. ah

representsshoulder.

0.3-0.6, intermediate between the “strong” and “weak” intensity spectra discussed earlier. The spectra of uranium tetrachloride-tris(trimethylphosphine oxide) and uranium tetrachloride-bis(o-phenanthroline) are illustrated in Fig. 3. The frequencies can be correlated more closely with those of the “strong” than the “weak” intensity spectra, which shows that the spectra are not only vibronic in origin, but that some electronic intensity is present. The empirical formula of uranium tetrachloride-bis(methy1 cyanide) suggests six-co-ordination. A trans octahedral environment with a centre of symmetry for the uranium ion can be ruled out, but a cis octahedral environment, which lacks a centre of symmetry, is probable. Higher co-ordination numbers, such as eight, are also intensity spectra, although the possible for compounds which exhibit “medium” overall symmetry is likely to be higher than for compounds with “strong” intensity spectra. A cubic configuration in which the centre of symmetry is destroyed by

1695

Diffuse reflectancespectra of some uranium (IV) complexes

25 000

20 000

15 000 FREOUENCY

5000

10 000

4000

(CM-‘)

Fig. 3. “Medium” intensity reflectance spectra of: UC&phen,.

(F) UC1,,3Me,PO;

(G)

ligands would be possible for compounds such as uranium tetrachloride-tetrakis(phosphorus oxychloride) and uranium tetrachloride-bis(o-phenanthroline). EXPERIMENTAL

non-equivalent

The complexes of uranium (IV) were prepared by standard methods or by methods described previously [21]. A standard grinding technique was used, and the samples were pressed flat and covered by a silica plate during measurement of the spectra. The grinding and mounting of hygroscopic samples was carried out in a dry box. Spectra were measured on a Beckman DK2A recording spectrophotometer with a diffuse reflectance attachment. The intensities of the bands on the O-2 Beckman scale were reproducible to &O-l units for the “weak” spectra and &@2 units for the “strong” spectra. Acknowledgement.+-!t’heauthors are indebted to Miss B. LO~ELANIJfor the spectral measurements, the D.S.I.R. for a grant for Special Research (Beckman spectrophotometer), and Dr. H. BA~NALL for gifts of uranium tetrafluoride and uranium tetrathiocyanate-tetrakis(dimethyla&amide). Part of this work was supported by the Atomic Energy Research Establishment, Harwell, and the authors thank Mr. F. Hunsw~u for his interest and encouragement. [21] P. GANS and B. C. SMITH,J. Chem. Sot. 4172, 4177 (1904).