2845
ABSORPTION SPECTRA OF OCTAHEDRAL LANTHANIDE HEXAHALIDES
Absorption Spectra of Octahedral Lanthanide Hexahalides
by Jack L. Ryan Battelle Pacific Northwest Laboratory, Richland, Washington1
and Chr. Klixbull Jpgensen Cyanamid European Research Institute, Cologny (Geneva), Switzerland
(Received February 18, 1966)
Hexahalide 4f group complexes can be prepared in aprotic solvents such as nitriles and in triphenylphosphonium and pyridinium salts. The internal transitions in the partly filled 4f shell correspond to absorption bands much weaker than for the corresponding aqua ions (except the hypersenitive pseudoquadrupolar transitions) and show the vibrational structure characteristic for octahedral complexes with a center of inversion. The nephelauxetic effect is much stronger than for the same central ions in L a c 4 but not as pronounced as for the oxides. The electron-transfer spectra observed in Ce(IV), Sm(III), Eu(III), Tm(III), and Yb(II1) hexahalides are discussed; the apparent optical electronegativities are somewhat higher than for the analogous halide complexes in ethanolic solution. The 4f + 5d transitions observed in Ce(II1) hexahalides show that the subshell energy difference A is larger than 15 kK. The similar transitions in Tb(II1) hex% halides seem to involve a spin-forbidden absorption band at relatively low wavenumber and indicate slightly smaller exchange integrals K(4f, 5d) than in the isoelectronic Gd+2.
Introduction
6 but may contain bidentate nitrate groups. Thus, The octahedral symmetry of hexahalides RiIx6+2-6 the crystal structure* of [hiIg(H20)6]3[Ce(N03)B]z.6H20 confirmed Judd’s spectroscopic predictions of somewhat allow many group-theoretical arguments to be applied distorted, icosahedral groups C e ( N 0 ~ ) 6 -with ~ N = 12. to the energy levels. I n particular, the excited levels X-Ray diffraction studies of aqueous solutions of Ceof the detailed electron-transfer spectra have been (ref 10) seem to show N = 12. As seen in the classified, using inductive RiIO theory, in a large number Experimental Section, the organic cation P(CaH&JI+ of 4d and 5d group Because of chemical problems, the study of the hexahalides of the other transition groups is far more difficult. Thus, the 5f group hexahalides cannot normally be studied in aqueous (1) The work done at this laboratory was performed under Contract solution. However, using anhydrous acetonitrile CHINo. AT(45-1)-1830for the U.8. Atomic Energy Commission. CN as solvent, we obtained the absorption spectra of (2) C. K. Jplrgensen, Mol. Phys., 2, 309 (1959). various U(IV), Np(IV), and Pu(1V) hexahalides and (3) C. K.JGrgensen, Acta Chem. Scand., 17, 1034 (1963). identified internal transitions in the partly filled 5f (4)C. K. Jplrgensen and K. Schwochau, 2. Naturforsch., 20a, 65 (1968). shell, 5f --t 6d transitions, and electron transfer r --t (5) J. L.Ryan and C. K. Jplrgensen, Mol. Phys., 7, 17 (1963). 5f.6 Similar techniques allow the study of CeC16-2 C. K.Jplrgensen, “Inorganic Complexes,” Academic Press Inc., (6) and the much less stable The situation is London, 1963. even more difficult in the case of trivalent lanthanides. (7) W. E. Keder, J. L. Ryan, and A. 9.Wilson, J . Inorg. Nucl. Chem., Until recently, many scientists believed that 6 is a 20, 131 (1961). (8) A. Zalkin, J. D.Forrester, and D. H. Templeton, J . Chem. Phys., common coordination number N for lanthanides. 39, 2881 (1963). Actually, N = 8, 9, 10, or even 12 seems to be far more (9) B. R. Judd, Proc. Roy. SOC.(London), A241, 122 (1957). typicaL6 For instance, hexanitrates such as U(N03)6-2 (10)R. D. Larsen and G. H. Brown, J . Phys. Chem., 68, 3060 or Pu(N03)e-2 (ref 7) may not necessarily have N = (1964). Volume 70,Number 9 September 1966
JACKL. RYANAND CHR.KLIXBULL J~RGENSEN
2846
(ref 11, 12) has once more shown its suitability for precipitating salts of i\IC16-3 and h!tB~-g-~,under nearly anhydrous circumstances. However, the only appropriate solvent we have been able to find in which these salts are sufficiently soluble for spectral studies of the internal 4f transitions is a mixture of acetonitrile and succinonitrile. The reasons why halide complexes are so relatively much more stable in nitriles than, say in alcohols, may be connected with the slightly decreased internuclear distances 11-X suggested by the solvent shifts of the electron-transfer spectra of various RTX6-? 3 , 1 3 ]+om considerations of the ionic radii, one would expect R/I16-3 to be stereochemically more favorable. We have not been able to obtain spectra of such species. However, one should not despair; recently, salts of Th16-2 and UIe-2 have been reported.14
Experimental Section Preparation of Compounds. Compounds [ (C6H5)3PH]3i\/ICle,where 31 is Ce, Pr, Nd, Sm, Gd, Dy, Ho, Er, Tm, and Yb, were prepared. These compounds were precipitated by mixing the rare earth chloride in HC1 saturated or nearly saturated ethanol with a small excess of (C6Hs)3PHC1in the same solvent. The compounds tended to supersaturate, and precipitation could be initiated by passing HC1 gas through the solution or by seeding with a minute crystal of a salt of another of the lanthanides. The heaviest rare earths were the worst in this regard and precipitated very slowly even after seeding. The lighter rare earth salts tended to precipitate much more readily and usually precipitated as larger crystals. The light rare earth salts precipitated readily from hot solutions, but with the heavy rare earths it appeared necessary to cool the solutions unless the concentration of the reactants was quite high. In order to obtain high yields the water concentration in the ethanol was kept as low as possible. With the light rare earths, the oxides were dissolved directly in absolute ethanol by addition of HC1 gas. The heavy rare earth oxides (above Eu) did not dissolve readily in hot HC1-ethanol. In these cases the oxides were dissolved in hot aqueous HCl and taken to dryness on a hot plate. The residue was then dissolved in hot HCl saturated ethanol. This procedure was also used with P r which was obtained as the higher oxide Pr6OI1. All rare earth oxides were from either American Potash Lindsay Division or Michigan Chemical and were of 99.9% or greater purity. The Tb407 used contains less than 0.005% lead (hence, the band observed for TbC16-3 at the same position as a much stronger band of PbC14-2 cannot have this origin). Triphenylphosphonium chloride solutions were prepared by adding HC1 gas to Eastman The Journal of Physical Chemistry
triphenylphosphine in absolute ethanol. The salts were washed with HC1 saturated or nearly saturated absolute ethanol. If the salts are washed with pure ethanol, acetone, or similar solvents not containing HC1, decomposition occurs. The light rare earth salts can be readily recrystallized by dissolving them in hot ethanol followed by the addition of HC1 gas. With the heavy rare earths (at least above Ho), this resulted in impure products containing less than stoichiometric amounts of triphenylphosphonium chloride. With these an excess of (C6Hs)3PHC1in solution is apparently necessary. The purpose of the large concentrations of HC1 in all the solutions is to react with water and ethanol to form oxonium ion and ethyloxonium ion and thereby markedly to decrease the coordinating power of the water and ethanol and a t the same time to increase chloride activity. The salts were dried with a heat lamp and were stored over anhydrous Rlg(C104)2as they appear to be somewhat deliquescent with decomposition to MC13.sH20. Anal. Calcd for [ ( C ~ H ~ ) I P H ] ~ PC,~ C56.7; ~ ~ : H, 4.24. Found: C, 55.8; H, 4.74. Anal. Calcd for [ ( C G H J ~ P H I ~ N ~Nd, C ~ ~12.6; : C, 56.5; H, 4.23. Found: Nd, 12.6; C, 55.8; H, 4.25. Anal. Calcd for [(C6H5)3PH]3SmC16: Sm, 13.0. Found: Sm, 13.0. Anal. Calcd for [ ( C ~ H ~ ) ~ P H ] ~ D C, Y C 55.7; ~~: H,4.16. Found: C, 57.0; H,4.3. Anal. Calcd for [(CeH6)3PHI3ErC16: Er, 14.3; C, 55.4; H,4.15. Found: Er, 14.1; C, 55.6; H, 4.35. Anal. Calcd for [ ( C B H S ) ~ P H ] ~ YYb, ~ C ~14.7. ~: Found: Yb, 14.6. The compounds (CsH5NH)sNdCla and (C5H5NH)3YbCb were prepared in the same manner as the triphenylphosphonium salts. The pyridine hydrochloride solution in ethanol was prepared by adding HC1 gas to CP pyridine in absolute ethanol. The compounds precipitate readily in good yield. The pyridinium salts, although easily prepared, are very deliquescent and are therefore less useful than the triphenylphosphonium salts. Preparation of other pyridinium rare earth chlorides was not attempted, but they are undoubtedly easily prepared. Anal. Calcd for (C5H5NH)3NdC16: Nd, 24.1; C1, 35.6. Found: Nd, 24.2; C1,35.1. ~
(11) P. Day and L. Venanzi, private communication, and J . Chem. Soc., Sect. A , 197 (1966). (12) C. K.J@rgensen,Acta Chem. Scand., 17, 251 (1963). (13) C.K.JZrgensen, J . Inorg. Nucl. Chem., 24, 1587 (1962). (14) K.W.Bagnall, D. Brown, P. J. Jones, and J. G . H. Dupreez, J . Chem. Soc., 350 (1965).
ABSORPTION SPECTRA OF OCTAHEDRAL LANTHANIDE HEXAHALIDES
2847
for several hours) and were stored over anhydrous Anal. Calcd for (CJ&NH),YbCl6: Yb, 27.6. Found: Yb, 27.9. Mg (C104)2. Anal. Calcd for [(C6H5)3PH]3PrBr6 (C6H5)sPHBr: Attempts were made by two methods to prepare C, 49.3. Found: C, 49.6. compounds of the type [(C2H5)4N]3MC16 where M is Anal. Calcd for [(C6H5)3PH]sNdBr6(CeH5)3PHBr: a trivalent rare earth. The first of these consisted of Nd, 8.21; C, 49.2; H, 3.64; Br, 31.8. Found: Nd, adding first ethanol and then acetone to a very con8.18; C,49.1; H,3.62; Br,30.7. centrated aqueous HC1 solution of the rare earth (Nd) (CeH5)3PHBr: and (C2H5)4NC1as in the preparation of [(CZHS)~N]Z- Anal. Calcd for [(C6H5)3PH]3SmBr6. Sm, 8.53; C, 49.0; H, 3.66. Found: Sm, 8.55; C, U B k 5 This results in the precipitation of MCL.zH20. 48.1; H,3.4. The second method was that used in preparing the Anal. Calcd for [(C6H5)3PH]3E~Br6 (C6H5)3PHBr: triphenylphosphonium and pyridinium salts. With Eu, 8.61; C, 49.0; H, 3.66. Found: Eu, 8.92; C, Nd no precipitation occurred. With Er, using very 48.9; H, 3.80. concentrated reactants, some precipitate formed after The salt (C5H5NH)3NdBr6was prepared in the same initially cooling to -70" and then allowing the reaction manner as the corresponding chloride salt. The same mixture to stand at 25" for 18 hr. This material problem of moderately low solubility of the reactants did not correspond to any simple stoichiometry but in HBr saturated ethanol was encountered as in the contained about 25% carbon. It is possible that this and ErC13.zsolvate. preparation of the triphenylphosphonium lanthanide was a mixture of [(C2H5)4N]3-ErC16 bromides. Although this salt precipitated readily No further attempt was made to resolve this situation. The compounds [ (C6H5)3PH]314Br6. (C6H5)3PHBr in good yield, an attempt at preparation of the corwere prepared where M was La, Pr, Nd, Sm, and Eu. responding E r salt was unsuccessful. (C6H5NH)3The formula of the compound is shown as containing NdBr6, like the corresponding chloride salt, was found the MBr6-s complexes rather than M B ~ . I because -~ of to be very deliquescent. Anal. Calcd for (C5H5NH)3NdBr6: Nd, 16.7; Br, the nature of the absorption spectra of the salts as discussed elsewhere in the paper and because the spec55.5. Found: Nd, 16.8; Br, 56.1. trum of the Nd salt is essentially identical with that of Attempts a t preparation of triphenylphosphonium (C5H5NH)3NdBr6. These salts were made in the same rare earth iodides by methods similar to those used manner as the corresponding chloro salts. With the for the preparation of the chlorides and bromides were bromide systems, somewhat more care is necessary in unsuccessful. The solubility of the reactants (MX3. the preparation than in the case of the chlorides since solvate and (C6H5)3PHX) in HX saturated ethanol the solubilities of the reactants, MBr3 ezsolvate and decrease with increasing size of the halide, X. This becomes a minor problem with the bromides but is a (C6H5)3PHBr,are much less in HBr saturated ethanol than their chloride analogs are in HC1 saturated major problem with the iodides. Also H I reacts with ethanol. Because of this, there is a possibility of conthe ethanol rather rapidly. taminating the product with excess of one of the All rare earth analyses were by spectrophotometry. reactants. At first, this was thought to be the reason Samples were all dissolved in constant-boiling hydroM B ~ ~ - chloric acid (the organic portion of triphenylphosphofor the analysis indicating [ ( C O H S ) ~ P H ] ~ (C6H5)3The Nd comPHBr instead of [(C~H~)BPH]SMB~.B. nium salts is not appreciably soluble in dilute acid). pound was recrystallized up to three times by dissolving Standards were prepared by dissolving the dried rare in hot ethanol and then adding HBr gas. It was also earth oxides in constant-boiling hydrochloric acid. recrystallized from methanol and butanol in the Carbon and hydrogen analyses were performed with same manner. In all cases the analysis corresponded an F & M Co. chromatographic carbon, hydrogen, exactly to [ ( C ~ H S ) ~ P H ] ~(C6H5)3PHBr. N~B~GThe erand nitrogen analyzer. Carbon and hydrogen analyses bium salt (and presumably other of the heavy rare on several of the compounds were also performed by earth salts) could not be prepared although the light Schwarzkopf Microanalytical Laboratory, Woodside, rare earths were prepared easily in good yield. Several N. Y. The two sets of carbon and hydrogen numbers attempts to prepare the Er salt were made. Even were in close agreement. Chloride and bromide though the Er solution was seeded with [(CeH6)3- analyses were by controlled-potential coulometric PH]3EuBr6 (C6H5)3PHBr, no precipitate could be t i t r a t i ~ n . ' ~The accuracy of halide analyses of the obtained other than the reactants (ErBr3-zsolvate, triphenylphosphonium salts is probably not so good as (CsH&PHBr, or mixtures of these). The salts were for the pyridinium salts since hydrolysis of triphenylsomewhat deliquescent (the orange-yellow europium salt changes to white upon exposure to laboratory air (15) L. R. Duncan, unpublished analytical method. +
-
-
Volume 70,Number 9 September 1966
2848
phosphonium salts (dissolved initially in ethanol) in dilute acid produces insoluble triphenylphosphine. [(CzHs)4N]zCeClewas prepared in the same manner as was the previously reported tetramethylammonium salt.16 Freshly prepared Ce(OH), was dissolved in cold (below 0"), concentrated HC1, and a solution of (C2H6)4NC1in cold, concentrated HC1 was added immediately. The precipitate was washed with cold, concentrated HC1 and then acetone and dried. Molar extinction coefficients of CeC16-2 in acetonitrile were obtained using this salt as a primary standard. Excess C1- was not required to maintain the CeC16-' complex in this solvent. The CeBre-' complex can be obtained in acetonitrile or nitromethane by two methods. If a solution of c e B r ~ -(containing ~ anhydrous HBr) is allowed to air oxidize or a small amount of Br2 is added, this Ce(IV) species forms slowly to a very small fraction of the total Ce. Since its charge-transfer band is well above (in wavelength) the absorption region of c e B r ~ -and ~ its molar extinction coefficient is quite large, its spectrum (above about 400 mp) can be obtained in the presence of a large excess of Ce(II1). If a quaternary ammonium bromide and HBr are added to an acetonitrile or nitromethane solution of CeC16-2, the CeCis-' is converted immediately to CeBrc-' apparently because of the much weaker acidity of HC1 than HBr in these solvents. The CeBr6-2 in these solutions decomposes rather rapidly (half-time about 70 sec in nitromethane) at 25" but is much more stable a t -30". The molar extinction coefficient of CeBra-' was obtained from a solution prepared by adding a cold nitromethane solution of [(C2H5)4N]2CeC16 to a frozen solution of tetrabutylammonium bromide and anhydrous HBr in nitromethane a t Dry Ice temperature. The mixture was warmed just enough to melt and was mixed and made up to volume a t the freezing point of the solution (about -30"). The CeBro-' solution was kept frozen in a Dry Ice bath. An absorbance scan was obtained by barely melting this solution, pouring it into a 1.0-mm cell, and recording the absorbance a t 522 mp as a function of time. The absorbance was extrapolated back to the time the solution entered the thin cell and thus was warmed to room temperature (about 15 sec before the absorbance trace started in most runs). A correction was made for the expansion of the solution between the temperature at which it was prepared and 25". Solid [(C2H6)4N]2CeC16 can also be converted to [(C2Hs)4N]zCeBr6 by passing a stream of anhydrous HBr over it a t 25 to 100". The time required for the reaction to go to completion has not been measured. The [(CzH6)qN]zCeBr6 is stable in dry air. The Journal of Physical Chemistry
JACKL. RYANAND CHR.KLIXBULLJ~~RGENSEN
Spectrophotometric Measurements. Spectrophotometric measurements were made with a Cary Rilodel 14 recording spectrophotometer. Absorption spectra of solids were obtained by preparing a thick paste of the crystalline salts in Kel-F brand chlorofluorocarbon grease and placing this between glass or fused-silica plates. The reference was CaCOa in Kel-F grease plus in some cases aqueous starch solution. The purpose of the reference mull is to flatten the base line by producing an equivalent wavelength dependence for the scatter. The absolute value of the base line is better controlled by neutral filters (wire screens) in the reference beam. Since the molar extinction coefficients are very low for the M X G - ~ internal 4f transitions, very thick mull samples were required. In the visible region, the Cary Model 1471200 high-intensity source was used at full brightness and the spectra were run on the 1.0-2.0 absorbance scale with a reference about 1 absorbance unit less opaque than the sample to minimize slit width and give maximum resolution. Slit widths ran typically 0.04-0.15 mm over the visible region. Spectra of solids at liquid nitrogen temperature were obtained with a liquid helium dewar designed for use with the Cary Model 14. Both dewar compartments were filled with liquid nitrogen. The sample consisted of a typical mull in Kel-F grease as discussed above between thin strips of glass and sealed in 1-cm2 fused-silica tubing so that the region around the sample could be evacuated without disruption of the mull. A special beam-condensing system was used in conjunction with the dewar, and slit widths of 0.020.15 mm were maintained. Scatter and base line appeared to be better than at 25" on the same sample. In choosing a solvent for spectrophotometric measurements of the salts of the h!tX6-a complexes in solution two criteria are of importance. The solvent must be sufficiently noncomplexing so that the iLIX6-' complex can be maintained in the solution and the salt must be sufficiently soluble to obtain useful spectra of the MXS-~complexes having internal 4f transitions (with the exception of hypersensitive transitions) with molar extinction coefficients generally less than 1. Acetonitrile6 and nitr~methanel~have been shown to be satisfactory solvents for the 5f group MX6-2 complexes but do not adequately meet the second criteria mentioned above. Liquid succinonitrile has a dielectric canstant of 56.5 at 57.4", and the solid a t 25.7" has a dielectric constant of 65.5.18 (16) C.K.J@rgensen,Mol. Phys., 5, 271 (1962). (17)J. L.Ryan, J. Phys. Chem., 65, 1856 (1961). (18)A. H.White and 9. 0. Morgan, J. Chem. Phys., 5, 655 (1937).
ABSORPTION SPECTRA OF OCTAHEDRAL LANTHANIDE HEXAHALIDES
Also the dielectric constant as a function of temperature shows no discontinuity a t the freezing point (54.5”).18 If it is assumed that the dielectric constants of these very similar solvents are additive, a mixednitrile solvent containing 85% succinonitrile and 15% acetonitrile would have a dielectric constant of about 62 at 25”. This is a considerably higher dielectric constant than those for nitromethane or acetonitrile (-38). This mixture has been found to be a very effective noncomplexing solvent for salts such as cs2uc16, CsZUOzCl4,and [ (CzH&N]*UOz(S04)3 which are not appreciably soluble in acetonitrile or nitromethane. l9 The triphenylphosphonium salts used in this work were more soluble in this solvent than in acetonitrile. Acetonitrile was obtained as Spectrograde from Eastman. The succinonitrile (also Eastman), although inherently colorless, was not Spectrograde and was found to be light yellow. The mixed solvent (85 vol. % succinonitrile-15 vol. % acetonitrile) was dried and purified by passing it through a bed of neutral-type activated alumina. This treatment removed the color sufficiently that the mixed-nitrile solvent was usable down to 220 mp in 0.1-cm cells and to 260 mp in 1.O-cm cells. The solubilities of the triphenylphosphonium salts in the mixed-nitrile solvent were sufficient to obtain good absorption spectra in the region of the internal 4f transitions (e (C6Hj)3H]~PfBrg.P(CoH&HBr, 300°K 20,350
20,442
20,509
20,584
20,661
taining PrC16-3 show the vibrational wavenumber differences 108 and 225 K, whereas the solution shows 98 and 212 K. As expected, PrBr6-3 has lower w a v e number differences, 70 and 155 K. These values are quite plausible when compared to other h e x a h a l i d e ~ . ~ ~ ~ ~ The two lowest frequencies corresponding to normal modes of odd parity probably nearly coincide. The ratio between the wavenumbers corresponding to bending and stretching normal modes tends to be relatively large in “compressed” octahedral molecules with strong ligand-ligand interactions, such as SFS.PrCls-3, particularly in the salt, seems to belong to this category. A closer analysis of the vibrational and J-sublevel structure of the spectra would need very extensive (21) R.A. Satten, C. L. Schreiber, and E. Y. Wong, J . Chem. Phys., 42, 162 (1965). (22) R. Pappalardo and C. K. Jgkgensen, Helv. Phys. Acta, 37, 79
(1964).
ABSORPTIONSPECTRA OF OCTAHEDRAL LANTHANIDE HEXAHALIDES
work and comparison of spectra taken at different temperatures. Hence, we restrict ourselves in this paper to consider the baricenter of each J level. We, as well as Dr. Romano Fappalardo, intend to return to the other much more complicated problems later. Perhaps an even more striking indication of the oct& hedral symmetry of our hexahalides is the very low intensity of most of the internal 4f transitions, usually some ten times weaker than the aqua ions (Figure 2). This is the first quantitative evidence from solutions of the strong influence on intensities of the absence of a center of inversion in the aqua ions, though some qualitative observations previously have been made on crystalline samples. Actually, in late years, the general opinion has been that all lanthanide complexes have 4fP transitions which are about as intense as or slightly more intense than for the aqua ions, with exception of the hypersensitive psedoquadrupolar transit i o n ~ . In ~ ~ Russell-Saunders coupling, these transitions are characterized by the selection rules J 4 J - 2, S --+ S, and L + L - 2, behaving as the strongest electric quadrupole transitions would behave. In our hexahalides, the hypersensitive pseudoquadrupolar transitions are very pronounced; the corresponding absorption bands are at least an order of magnitude stronger than the other bands. The reflection spectra of mixed oxidesz4suggest low intensities, and perhaps coexcited vibrations, of 4fq transitions of lanthanides in sites having a center of inversion such as the eightcoordinated position in pyrochlores of the type ErTi03.5 or the six-coordinated position in perovskites such as high-temperature LaEr03, when compared to the spectra of disordered fluorites or C oxides. Thus, Kisliuk, Krupke, and Gruberz5 could not detect any absorption lines from that quarter of the erbium(II1) ions which are present in C-type Yz03at sites presenting a center of inversion. However, in these cases, no clear evidence for hypersensitivity is found. It may be remarked that the center of inversion must be present with a large precision before the spectrum is dominated by the weak, coexcited vibrations. The approximate octahedral symmetry of Er(II1) in YC13 studied by Rakestraw and Dieke26 is not sufficient, and we must admit that the electronic lines still are quite prominent in our spectra, suggesting weak distortions away from cubic symmetry. Tables I-VI1 give the baricenters of excited J levels of our hexahalides and, for comparison, also those for the aqua ions, the oxides, and various results for M(II1) in LaCI3 and YC13. The nephelauxetic effect can be evaluated from the data by the linear relation24
2851
I
420
440
I
I
460
480
I
500
Wavelength ( m v l
Figure 2. Absorption spectra of praseodymium(II1): (1) Pr(II1) in 1 M HClOa and ( 2 ) [(C&L)3PH],PrCle in 85% succinonitrile-15% acetonitrile containing (C2Hs)4NCl.
where dp is the relative decrease in per cent of the interelectronic repulsion parameters when compared with the aqua ion, a and asquaare the observed wavenumbers of the baricenters, and da represents the stabilization of the lowest sublevel of the ground J level of the compound considered minus the similar stabilisation of the aqua ion. Actually, the linear relation is obeyed fairly well by most, but not all, transitions in a given central atom. Thus, zHII,, of Nd(II1) and of Pr(II1) (cf. also ref 27) move less than Er(II1) or *I6 expected.24 It is seen in Table VI11 that dp is 2.5 to 4.5 times larger in our hexahalides than in M(II1) substituted in LaCl3, and db is somewhat smaller in hexahalides than in the oxides. Since the oxides are thoughtz4 to have an anomalously large nephelauxetic effect compared with M(III)LaC13, our results might perhaps be taken as an argument that it is rather the latter host lattice which produces an anomalously low nephelauxetic effect. The explanation is probably connected with the strong dependence of the overlap integrals between the 4f shell and the ligand orbitals on small variations of the internuclear distances.28 Hence, we cannot establish a unique nephelauxetic series of ligands, which would be a fairly good approximation ~
~
~~~
~~~
(23) C. K.JZrgensen and B. R. Judd, Mol. Phys., 8,281 (1964). (24) C.K.JZrgensen, R. Pappalardo, and E. Rittershaus, Z . N ~ u T jorsch., 20a, 54 (1965). (25) P. Kisliuk, W.F. Krupke, and J. B. Gruber, J . Chem. Phys., 40,3606 (1964). (26) J. W.Rakestraw and G. H. Dieke, ibid., 42,873 (1965). (27) J. Sugar, Phys. Rev. Letters, 14, 731 (1965); J. Opt. SOC.Am., 55, 1058 (1965). (28) C. K. JZrgensen, R. Pappalardo, and H. H. Schmidtke, J . Chem. Phys., 39, 1422 (1963).
Volume 70,Number 9
September 1966
JACK L. RYANAND CHR.KLIXBULL J~RGENSEN
2852
Table I : Internal 4fa Transitions in Praseodymium(II1) Complexes" P r C l s - W
'Dz 3P~ 3P1 3P~
-Pr(HnO)n++
PrBr6--
I
XJ
UJ
hax
XJ
UJ
emax
SJ
592 485 475 450
16.89 20.61 21.05 22.22
0.09 0.94 0.40 0.34
595 487.6 477 455.5
16.81 20.50 20.96 21.95
0.14 1.0 0.59 0.45
16.78 20.69 21.29 22.43
CmpX
1.9 4.0 4.6 10.5
prmLaCls
PrIIIGdCis
PrIIILaBrs
SJ
SJ
SJ
16.73 20.47 21.08 22.23
16.69 20.41 21.01 22.16
16.67 20.37 20.98 22.13
XJ is the wavelength in mp and CJ the wavenumber in kK of the baricenter of the band group corresponding to the excited J level indicated. emax is the molar extinction coefficient of the highest band of each group in the solution absorption spectra; the wavenumber of this band does not necessarily coincide with CJ.
Table I1: Internal 4f3 Transitions in Neodymium(II1) Complexeso
881 808 747 -686 590 534 517 434 358
" Notation
11.35 12.38 13.39 14.58 16.95 18.73 19.34 23.04 27.93
0.31 0.65 0.56 0.05 5.6 0.72 0.27 0.11 -1 .o
878 803 748 685 587 533 520 434
...
11.39 12.45 13.37 14.56 17.04 18.76 19.23 23.04
...
0.38 0.94 0.68 0.07 10.0 1.5 0.40 0.12
...
11.58 12.62 13.58 14.84 17.40 19.18 19.63 23.40 28.28
3.6 11.8 7.2 0.4 7.0 4.4 1.7 0.6 5.2
11.44 12.48 13.44 14.72 17.21 19.03 19.44 23.21 27.97
11.19 12.27 13.25 14.47 16.72 18.60 19.17 22.84 27.20
as in Table I.
Table III: Internal 4f6Transitions in Samarium( 111) Complexes"
-1420 1255 1090 562 531 491 478 465 422 410
7.04 7.97 9.17 17.79 18.83 20.37 20.92 21.51 23.70 24.39
Strong Very weak Very weak Weak Weak Weak
... ... 0.02 0.01 0.03 0.04 0.025 0.15 0.67
7.15 8.0 9.25 17.9 18.6 20.02 20.88 21.55 24.0 24.9
1.6 2.1 1.8 0.04 0.02 0.08 0.6 0.5 0.5 3.3
7.05 8.00 9.08 17.86 18.86
7.34 8.05 9.22 17.57 18.87
...
...
20.60 21.56 23.78 24.54
20.75 21.41 23.64 24.39
Notation as in Table I.
in the 3d, 4d, and 5d transition groups.29 Keating and Drickamerao studied the nephelauxetic effect of high pressures applied to 4f group compounds. A more chemical technique for modifying the internuclear distances was by McLaughlin and Conwaya' studying Pr(II1) in Lacla, CeCla, NdCla, SmCla, and GdCla. The gradudy decreasing Pr-Cl distances The Joumal of Physieal Chemhtry
produce a strongly increasing nephelauxetic effect, d/3 in Table VI11 going from 0.8 to 1.2%. On the other (29) C. K. JZrgensen, "Orbitals in Atoms and Molecules," Academic Press Inc., London, 1962. (30) K. B. Keating and H. G. Drickamer, J . C h m . Phys., 34, 143 (1961). (31) R. D. McLaughlin and J. G. Conway, ibid., 38, 1037 (1963).
2853
ABSORPTIONSPECTRA OF OCTAHEDRAL LANTHANIDE HEXAHALIDES
Table IV : Internal 4fQTransitions in Dysprosium(II1) Complexes" [ P(CaHs)8H IaDy-C l
BHe/z,' F I ~ / ~ 'H7/Z, 'Fe/z
'F?/z 'FVZ
XJ
'JJ
1300 1100 910 808
7.69 9.09 10.99 12.38
-Dy(HzO)e
Strong
C-DyzOs
DyIIILaCls
t-
'JJ
&&H&O&and Gd(HtO)e(CzHsSO& showing that this lattice has sufficiently many degrees of freedom
dr
dP
du
dP
+0.25 f0.20 -0.04 0 -0.05 $0.05 +0.05 -0.06 f0.20
1.9 2.3 0.8 1.2 1.3 2.2 2.3 0.6 3.6
$0.15 +0.45 +O,lO -0.03 0
1.1 2.5 1.2 0.3 0.9
+0.20
1.6
$0.10 $0.30
1.3 1.5
a The relative "ligand-field" stabilization du in kK and the nephelauxetic ratio dp (relative to the aqua ions) in per cent. ~~
~~
Volume 70, Number 9 September 1966
JACK L. RYANAND CHR.K L I X B ~J$RGENSEN L
2854
to allow the Pr-0 distances to achieve the same, most favored, values. A similar effect was found for Er(111)26 having dp 3 times larger in the six-coordinated Yc13 with relatively short Er-C1 distances than in the nine-coordinated LaCb. There is little doubt that our TLIXS-~exemplify a similar behavior. In particular, the M-X distances are probably even smaller in organic solvent^'^ such as acetonitrile than they would be in aqueous solution if the complexes did not immediately exchange their halide ligands for water. The molar extinction coefficients of the normal, not hypersensitive, transitions increase from some 0.04 times the intensities of the aqua ion in PrCla-3 to about one-tenth of This may suggest that the cubic symmetry of ErCls-3 is perhaps slightly less nearly perfect. Recently, Sugar2' (who also was so kind as to supply this information at an early stage) found 12 of the 13 levels of the configuration 4f2 of gaseous Prfa. The four levels of Table I are situated a t 17.33, 21.39, 22.01, and 23.16 kK. The blue shift of these J levels relative to the aqua ion would indicate that d/3 of Pr+3 is some -4.3%, though the relative shifts are smaller than expected in the infrared. As also discussed by Sinha and S ~ h m i d t k e this , ~ ~ effect may be connected with a relatively smaller variation of the Land6 parameter 14f,say 1.5%, than the variation of the parameters of interelectronic repulsion. Electron-Transfer Spectra. I n the ultraviolet, the hexahalide complexes of the reducible central ions Sm(III), Eu(III), Tm(III), and Yb(II1) show broad, moderately strong, absorption bands (Figure 3) which can be ascribed to electron transfer from the highest filled N O , mainly localized on the halide ligands, to the partly lilled 4f shell.'" Table IX gives data for those absorption bands and for the only two lanthanide(IV) hexahalides we have been able to study, CeCla-2 and CeBrs-'. The bands are much more intense in the latter case, the empty 4f orbitals probably being somewhat more delocalized out on the ligands. Surprisingly enough, the intensities of CeCla-2 and CeBr6-' are even larger than those of UIe-2 and the Np(1V) and Pu(1V) hexahalides previously studied.6 Since the theory for the variation of the optical electronegativity xoptof 4f and 5f group elements has been discussed e l ~ e w h e r e , we ~ ~ 'are ~ ~here ~ ~ restricting ourselves to the much simpler equation gobed = [ x O P t ( ~ )
-
2 u n c o r ( ~ ) 1.30
kK
(3)
where the wavenumber Uobsd of the first electron-transfer band is related to the optical electronegativity of the ligand and the uncorrected not taking spin-pairing energy or other forms of interelecThe Journal of Physical Chemistry
350 450 Wavelength ( m p l
250
550
Figure 3. Electron-transfer spectra of europium(II1) hexahalides: (1) 0.114 M EuBr6-S in acetonitrile containing excess (C4H9),NBrand HBr in 0.0108-cm cell (absorbance scale displaced 0.4) and (2) 0.0125 M EuC16-S in (C2H5)4NC1-saturatedacetonitrile in O.lO-cm cell.
Table IX : Electron-Transfer Spectra of Hexahalides in Nitrile Solution"
x
CeCla-*
232 286 301 234.5 409 (309) 270 -260 272.5 342 ( 240 1 376 255 522
U
43.1 35.0 33.2 42.6 24.5 (32.4) 37.0 -38.6 36.7 29.2 (41.7) 26.6 39.2 19.2
t
930 1,050 400 640 250 (340) 540 -300 160 105 (450) 5,200 13,800 N5,700
a(-)
2.3 2.4 2.1 (3.8) 2.0
... ... 1.7 2.4
... 2.9 3.2 2.5
" The wavelengths X in mp, wavenumbers u in kK, and molar extinction coefficients 6 are given for maxima (shoulders in parentheses). 6(-) is the half-width in kK toward smaller wavenumben. tronic repulsion effects nor relativistic effects into account, zuncor(M)for the central atom in a definite oxidation state. Table X gives the values of zuncor(M) obtained in a variety of cases. It is seen that the hexabromides (1965). (32) s* Sinha and H*H. Schmidtke, Mol. Phy8., (33) C. g, J@reensen,'
