1238
S. LINDENBAUM AND G. E. BOYD
edge the assistance of Mr. L. L. Egan and Mr. B. E. Gran, and, in particular, the suggestions of Professor
Vol. 67
R. S. Haiisen of Iowa State University that initiated this investigation.
SPECTROPHOTOAIETRIC INVESTIGATION OF THE EXTRACTION OF TRANSITION METAL HALO-COMPLEX IONS BY AMINE EXTRACTANTS BYS. LINDENRAUM AND G. E. BOYD Chemistry Divasion, Oak Ridge National Laboratory, Oak Ridge, Tennessee Received November 19, 1969 The spectra of the extracted species in the distribution of the halo-complex ions of Fe(III), Co(II), Cu(II), Mn( II), and Ni(I1) between organic amine solutions and aqueous chloride and bromide solutions were studied. In each instance the only species observed in the organic phase waR the four-coordinated complex ion. In several cases (Fe(III), Co(II), Cu(II), and Ni(I1)) it was shown that even when the extraction was from aqueous solutions whose spectra indicated the complete absence of the tetrahedrally coordinated species, the spectra observed in the organic phase were in excellent agreement with spectra shown t o be representative of the MCla-2 and MClr- ions. It was not possible t o prepare an aqueous solution of Ni(I1) which showed any indication of the spectrum characteristic of the NiClr-2 ion. The spectrum of Xi(I1) extracted by a toluene solution of a tertiary amine hydrochloride from 13 M LiCl was, however, identical t o spectra of compounds containing NiCl,-2.
Introduction The generally large affinity of quaternary ammonium anion-exchange resins for the transition metal chlorocomplex ions, and the dependence of this specificity on the ligand concentration in aqueous solution, has been the basis for many anion-exchange column separations. It has been shown by Smith and Page2 and others3-6 that a large selectivity also exists for “liquid ion exchange” systems prepared by dissolving tertiary amine hydrochlorides in water-immiscible solvents. Further, a remarkable similarity in the relative orders of selectivity between liquid and resinous anion exchangers may be demonstrated by comparing published data for a liquid amine extraction system6 with that for an anion-exchange resin’ in a plot of the distribution coefficients us. the aqueous hydrochloric acid concentration. It was the objective of this study to determine the nature of the species involved in the ion-exchange extraction of the halo-complex ions of some of the 3d transition metals and to obtain an understanding of the origins of the large selectivities frequently observed. A liquid amine system was chosen for study because it lends itself more readily to spectrophotometric investigation. Spectra of ions absorbed onto ionexchange resins have been measured recently.8-12 In general, it has been found that the species on the anion-exchange resin was the same as that in the liquid amine extractant. In one case, however, Ryan1’ observed that for the anion-exchange resin absorption of hexavalent uranium from nitrate solution, both tri(1) K. A. Kraus and F. Neleon, Proc. Intern. Conf. Peaceful Use8 of Atomic Energy, Geneua, 7 , 113 (1955). (2) E . L. Smith and J. E. Page, J. Soc. C h e m Ind., 67, 48 (1948). (3) G. W. Leddicotte and F. L. Moore, J. Am. Chem. Soc., 7 4 , 1618 (1952). (4) F. L. Moore, “Liquid-Liquid Extraction with High Molecular Weight Aminea.” National Acad. of Sei. Series on Nucl. Sci., Deo. 15, 1960. (5) K. B. Brown, C. F. Coleman, D. J. Crouse, C. A. Blake, and A. D. Ryon, Proc. Intern. Conf. Peaceful Uses of Atomic Energy, I n d , Geneva, 8, 472 (1958). (6) H. A. Mahlman, G. W. Leddiootte, and F . L. Moore, Anal. Chem., 26, 1939 (1954). (7) K. A. Kraus and G. E. Moore, J. Am. Chem. Soc., 75, 1460 (1953). (8) D. K. Atwood and T. DeVries, ibid., 83, 1509 (1961). (9) E. Rutner, J . Phys. Chem., 65, 1027 (1961). (10) J. L. R y a n , ibid., 64, 1375 (1960). (11) J. L. R y a n , ibid., 66, I099 (1961). (12) J. L. Ryan, ibid., 65, 1856 (1961).
nitrato and tetranitrato complex ions occurred in the resin phase. The liquid amine system, however, extracts only the trinitrato complex ion UOr(N03)3-.1a Spectral measurements on ion-exchange resins, however, are beset by difficulties involving scattering of light, obtaining a reproducible blank, and adjusting the concentration of the extracted species. Furthermore, for the extraction of ions from concentrated aqueous solution not all of the ionic species taken up by the resin are necessarily associated with the exchanger sites; some tire present in the electrolyte solution imbibed by the resin phase as the result of Donnan “invasion.” Measurements have shownl4 that the solubility of electrolytes in solutions of tri-n-octyl amine salts in toluene is quite small. Experimental Materials.-Triisooctylamine (TIOA) as obtained from Union Carbide Chemicals Company was pale yellow in color, and its neutral equivalent was 375.5 (theor. 357.7). Solutions of TIOB in toluene were shaken with excess aqueous HCI or HBr to form the hydrochloride or hydrobromide. Tri-n-octylamine hydrochloride, TOA.HC1, was obtained as a white crystalline solid from Enstman Kodak Co., Rochester, Kew York. TOA. HBr was prepared from redistilled tri-n-octylamine (Eastman) by equilibrating an ether solution of the amine with aqueous HBr. The ethereal solution was dried and the ether evaporated to obtain the pure solid TOA.HBr. Cupric bromide and ferric bromide were prepared by the repeated addition of HBr to the nitrate salt and evaporating t o dryness. All other chemicals used were reagent grade. Spectrophotometric Measurements.-Spectral measurements were performed with a Cary Model 14 recording spectrophotometer. Equal volumes of aqueous HCl or HBr solutions containing known amounts of transition metal chloride or bromide were equilibrated with amine-toluene solutions. Absorption spectra of the organic phase and the original aqueous phase were determined, using for the reference cell a solution identical with that in the sample cell except for the presence of the transition metal compound. All measurements were made with 1-cm. path length cells.
Results Extraction of Fe(II1) from HCl and HBr Solutions.Trivalent iron was extracted from 12 M HC1 and 1 M HC1 by 0.225 M TIOA.HC1 in toluene. The spectra taken on the aqueous and organic phases are shown in (13) W. F. Keder, J. L. Ryan, and A. S. Wilson, J. Inorg. Nucl. Chem., 20, 131 (1961). (14) S. Lindenbaum and G. E. Boyd, J. Phys. Chem., 66, 1383 (1962).
June, 1963
EXTRACTION OF T R A N S I T I O N
Fig. 1. The spectra for Fe(II1) in the organic phase are identical with those reported for KFeC14 and HFeC14 in ether by Friedman15 and Metzler aiid Myers,16which Friedman ascribes to the FeC14- ion. The spectrum shown for Fe(II1) in aqueous 12 M HC1 solution is similar to the organic phase speetra, except for the apparent absence of a peak a t 532 mp. Metzler and Myers16 have suggested that this peak is overshadowed in HC1 solutions by the leading edge of the very intense FeC13 ultraviolet absorption bands. Good and Bryan1’ have found also that the species extracted by organic amine solutions is the same as the one for which Friedman assigned the FeC14- structure. More recently, Raman spectral8 and X-raylg evidence, consistent with the tetrahedral FeC14- structure for the extracted species, has been presented. Spectra are shown (Fig. 1) for the extraction of Fe(111) from aqueous 4.5 M and from 9 M HBr solutions. Identical species appear to be extracted. The spectra of the organic phase species are identical with the spectrum of (C2HS)4NFeBr4 in dimethylformamide obtained by Gi1lZ0and ascribed by her to the FeBr4- ion. The spectrum of Fe(II1) in 9 M HBr shows a reversal in relative heights of the two prominent peaks from that observed iii the organic phase; the molar absorbancy indices are also very much larger. Apparently here, also, the leading edge of an ultraviolet band of one or more complexes less saturated than FeBr4- is superimposed on the FeBr4- spectrum. Extraction of Co (11) from HC1 Solutions.-Aqueous solutions of CoClz exhibit colors ranging from pink in pure water and dilute HC1 to blue in concentrated HC1 solutions. There is evidence2122that the pink solution contains six-coordinated cobalt and the blue solution the four-coordinated C O C I ~ -species. ~ The spectrum (Fig. 2) of a solution of CoCl2 in concentrated HCl is recognized to be very similar to that obtained by Gill and iSyholm22for [ P ~ ~ C H J2CoC14 ~ A S dissolved nitromethane; it also resembles the spectra of solid Cs+20C1423,24 and C S ~ C O C (within ~ ~ ~ the ~ resolution obtainable by reflectance methods) in which the presence of the tetrahedral CoC14-2 ion has been demonstrated ~rystallographically.~5 The spectra (Fig. 2) of Co(I1) in the organic phases are characteristic of the four-coordinated CoCl4-Z species also. The spectrum of Co(T1) in 1 ill €IC1 is that due to the aquo ion, C O ( H ~ O ) ~ + +The . species extracted from both dilute and concentrated acid is largely CoC14-2, even though the spectrum of the dilute HC1 aqueous phase shows no indication of the presence of coc14-2. Extraction of Cu(I1) from HC1 and HBr Solutions.-Solutions of CuClz aiid CuBrz in water have identical spectra characteristic of the blue, octahedral Cu(HzO)6++species. In concentrated aqueous HC1 and HBr solutions the spectra differ considerably, the (15) H. L. Friedman, J. Am. Chem. Soc., 74, 5 (1952). (16) D. E. Metzler and R. C . Myers, %had.,7a, 3776 (1950). (17) M. L. Good and S. E. Bryan, zhzd., 82, 5636 (1960). (18) L. A. Woodward and hi. J . Taylor, J . Chem. Soc., 4473 (1960). (19) C. L. Standley and R. F. Kruh, J. Chem. Phgs., 34, 1450 (1961). ( 2 0 ) N. S.Gill, J. Chem. Soc., 3512 (1961). (21) L. I. I h t z i n and E. Gebert, J. Am. Chem. Soe., 7a, 5464 (1960). ( 2 2 ) N S Gill and R. 8. Nyholm, J . Chem. Soc., 3997 (1959). (23) L. I. Icatzin and E. Gebert, J . Am. Chem. Xoe., 75, 2830 (1953). (24) L. I. K a h n , z b d , 76, 3089 (1954). (25) H. M. Powell and A. F. Wells, J. Chem. Soc., 359 (1936).
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M E T A L H.4LO-COMlJLEX I O K S
Fig. 1.-Absorption spectra of aqueous and organic phases for the extraction of Fe(II1) by TIQA in toluene from aqueous HCX and HBr solutions: A, 1 M FeC13 in 1 M HC1; B, 0.225 M TIOA.HC1 equilibrated with 1 M FeC13 in 1 M HCl; C, 1 M FeCb in 12 M HCl; D, 0.225 M TIOA.HC1 equilibrated with 1 M FeC13 in 12 iM HC1; E, 0.05 M FeBr, in 4.5 M HBr; F, 0.05 M TIQAHBr equilibrated with 0.05 iM F e B s in 4.5 M HBr; G, 0.1 M FeBra in 9 M HBr; H, 0.05 M TIQA HBI, equilibrated with 0.05 M FeBrt in 9 M HBr. Absorbance = optical density = log I/Ie.
0 6 ~
Organic 0 1 M TIOA HGI E q u i l ibroted w i t h 0002MGoG12in IZMHGI
Equilibrated w i t h
0 2I
Aqueous 000.1M G 0 G I Z in I Z M H C I
I
Fig. 2.-Absorption spectra of aqueous and organic phases for the extraction of Co(I1) by TIOA.HC1 in toluene from aqueous HCI solutions.
predominant species being the yellow CuC14-2 22.p6 and purple CuBr*+ 22 complex ions, respectively. The spectra of the organic and aqueous phases for the extraction of Cu(I1) from HC1 and HBr solutions are shown in Fig. 3. The organic phase spectra are identical, independent of the concentration of acid in the equilibrium aqueous phase; they differ from those for the concentrated HCl and HBr solutions in that the peak positions are shifted to longer wave lengths. A similar shift in spectra was observed by Moeller upon increasing the chloride to copper ratio in aqueous Cu(T1) nitrate solutions. Ehidently, Cu(I1) in concentrated HCl and HBr solutions is not present exclusively as (26) T. Moeller, J . Phys. Chem., 48, 111 (1944).
S. LINDENBAUM AND G. E. BOYD
1240
WAVELENGTH, angstroms.
Fig. 3.-Absorption spectra of aqueous and organic phases for the extraction of Cu(I1) by TIOA in toluene from aqueous HCI and HBr solutions: A, 0.04 M CuClz in HzO; B, 0.1 M TIOA, HCI equilibrated with 0.1 144 CuClz in HzO; C, 0.0001 M CuClz in 12 M HCl; D, 0.1 M TIOA.HC1 equilibrated with 0.002 M CuClz in 12 M HCI; E, 0.05 M CuBrz in HzO; F, 0.05 M TIOA. HBr equilibrated with 0.2 M CuBrz in HzO; G, 0.00025M CuBrz in 9 M HBr; H, 0.05 n/f TIOA.HBr equilibrated with 0.0005 M CuBrz in 9 M HBr. I
1
1
4 1
1.0
0.9 0.8
0.7
ORGANIC
-
WAVELENGTH, ongstroms.
Fig. 4.-Absorption spectra of aqueous and organic phases for the extraction of Mn(I1) by: A, 1 M TOASHBr in toluene from 0.36 M MnBrz in 11.4 M LiBr; B, 1 M TOA.HC1 in toluene from 0.60 M MnC12 in 11.4 M LiC1.
the CuC14-2 and C u B r P species, and the presence of less saturated (e.g., CuC13-, CuC12, aiid CuCl+) species causes a shift in the absorption maximum to shorter wave lengths. Further evidence that the organic phase contains the fully saturated chloro-complex CuC14-2 is that the peak positions in the spectrum of crystalline C S ~ C U C Iare ~ ~very ~ . ~close * to those observed in the liquid amine phase spectrum. The presence of the CUCL-~ion in Cs2CuC14has been demonstrated by X-ray diffraction.2732* Extraction of Mn(I1) from LiCl and LiBr Solutions.Divalent manganese is only weakly extracted from (27) L. Helmholz a n d R. F. Kruh, J . A m . Chem. Soc., 74, 1176 (1952). (28) B. Morosin and E. C. Lingafelter, J . Phys. Chem., 66, 50 (1961).
Vol. 67
concentrated HCl or HBr solutions by anion-exchange resins or organic amine solutions. To obtain measurable quantities of Mn(I1) in the organic phase, it was necessary to employ concentrated aqueous LiCl or LiBr solutions of Mn(I1). The spectra of the aqueous and organic phases for the extraction of Mn(1I) from 11.4 M LiBr solution by 1 M TOA.HBr are shown in Fig. 4A, and from 11.4 ill LiCl solution by 1 214 TOA. HC1 in toluene in Fig. 4B. The organic phase spectra are nearly identical with the spectra obtained by Gill and l;yholmZ2for MnC14-2and ;IlnBr4-2 with solutions of [Et4N]zMnBr4 and [Et4N]zMnC14 (or the corresponding triphenylmethylarsonium salts) in nitroobserved in Fig. 4A (361, methane. Values of, , ,A 375, 437, and 453 mp) may be compared with the values of Gill and Nyholm (371, 437, and 452 mp) for Mi1Br4-~. Similarly, in Fig. 4B the values of A,, (357,433, and 446 mp) are in good agreement with the values reported (430 and 445 mp) for i\/InC14-2. The lowest wave length absorption bands (361 mp for MnBr4-2 and 357 mp for MnC14-2) were not observed by Gill and Kyholm, however. These bands cannot be measured in nitromethane because this solvent is opaque in this wave length region. The aqueous phase spectra (Fig. 4A and 4B) show some of the same features observed with the organic phase, but a t lower intensities and also somewhat distorted. These spectra are probably due to the presence of a small amount of MnBr4+ and MnC14-2 and other species intermediate between the hexaaquo and the fully halogensaturated complex. Extraction of Ni(I1) from LiCl Solutions.-It is well k n o m that divalent nickel is not absorbed appreciably from hydrochloric acid solutions either by ionexchange resins or liquid anion exchangers; in contrast, divalent cobalt and copper which are adjacent elements to nickel in the periodic table are well absorbed. It is interesting to note that Ki(I1) does not form T\'iC14-2 complex ions in aqueous solutions, whereas Co(J.1) and Cu(I1) do. Herber and IrvineZ9in fact cite evidence that the only nickel species present even in concentrated HC1 are Ni++ and IViCl+. It was pointed out by Keder,30however, that Ni(I1) can be extracted into an amine solution if the aqueous chloride ion activity is made high enough (or the water activity low enough). I n Fig. 5 are shown the aqueous and organic phase spectra for the extraction of Ni(1I) from 13.0 M LiCl solutions into 0.1 M TOA.HC1 in toluene. The organic phase spectrum is in excellent agreement with that reported by Gill and Nyholm for NiC14-2 in nitromethane, and also with the spectra of Gruen and McBeth31of NiClz dissolved in molten pyridinium chloride aiid CsZXiCl4in solid solution with Cs2ZnCL. In the latter case it was showii that the solid solution containing nickel is isomorphous with pure tetrahedral cs2znc14. The aqueous phase spectrum for X(I1) in 13.0 M LiCl (Fig. 5) showed all the bands reported by Gill and Kyholm for T\'i(Hz0)6++except that the entire spectrum was shifted to longer wave lengths. Interestingly, Gruen and McBeth have shown that the spectrum of (29) R. H. Herber and J. W. Irvine, J . A m . Chem. Soc., 78, 905 (1966). (30) W. E. Keder, Abstracts of Papers, Division of Physical Chemistry. 140th National Meetmg, American Chemical Society, Chicago, Ill., September, 1961. (31) D. M. Gruen and R. L. McBeth, J . Phys. Chem., 68, 393 (1959).
June, 1963
EXTRACTIOX OF TRANSITIOS METALHALO-COMPLEX IONS
Ki(I1) in LiN03-KK03 eutectic melt could be shifted toward the red end of the spectrum by the addition of chloride. This shift was interpreted as the consequence of the stepwise formation of chloro-complexes of Ni(I1). It is likewise suggested here that the aqueous phase spectrum in Fig. 5 represents Si(I1) partially complexed with chloride ions. The molar absorbancy index for the NiC14-2 ion is much lasger than that for S(H20)6++; hence, the fact that there is no indication of the NiC14-2 spectrum in the aqueous phase suggests that even in 13.0 M LiCl very little KiC1,-2 can be present.
Discussion It has been shown for the extraction of Fe(III), Co(II), Cu(II), Mn(II), and Ni(II), by solutions of tertiary amines in toluene from aqueous chloride ,and bromide solutions that in each case a strong preference for the four-coordinated species exists. This preference is so specific that even when the amine solution is equilibrated with an aqueous phase containing the transition metal almost exclusively in the hexa aquo form (e.g., Cu(H20)a++) the visible spectrum of the organic phase shows only the bands associated with the four-coordinated tetrahedral species. However, k~all the cases studied the spectral bands for the tetraNedra1 species are much more intense than those for the corresponding octahedral species. Therefore, small amounts of the metal ion in the organic phase in the octahedral form might not be observed. The absorbance of those bands shown to be representative of the tetrahedral species is in all cases far greater in the organic phase than in the aqueous.32 In fact,, it has been shown for Fe(III), Co(II), Cu(II), and Si(I1) that the tetrahedral species can be observed in an organic phase which has been equilibrated with an aqueous solution showing no sign in its spectrum of bands due to the tetrahedral ion. It has therefore been demonstrated that the “liquid ion-exchange” system has an extremely strongpreference for the fully saturated halo-complex ion. Certain M X P species ( e . g . , TcC16-2, ReC1a-2)33are known to be strongly absorbed by anion exchangers so (32) It also has been observed b y Murrell, Katzin, and Davies, Nature, 188, 459 (1959), t h a t two phases are obtained in the CoClz-HzO-acetone system and t h a t the tetrahedral species also predominates in the acetonerich (organic) phase. (33) G. E. Boyd and Q. V. Larson, unpublished results of this Laboratory (1961).
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0.6
0.5 0.4
0.3 W
y
0.2
m a 0.t 0)
:0.0 O.’ 0.0
0
s
8
s
8
g
8
e
8
g
8
w
8
;
WAVELENGTH, angstroms.
Fig. 5.-Absorption spectra of aqueous and organic phases for the extraction of Ni(I1) by 0.1 M TOA.HC1 in toluene from 0.073 M NiClz in 13.0 M LiCl. (Organic phase diluted 1:5 with 0.1 M TOA.HCI in toluene, aqueous phase diluted 1:5 with 13.0 M LEI.)
that the absence of octahedral species in liquid exchangers [for example, RIIC13(H20)3-]probably cannot be attributed solely to the coordination number but rather to the greater stability of tetrahedral species such as MX4+ in a solvent where excess halide ions are available. In a paper which appeared while this report was in p r e p a r a t i ~ nKatain , ~ ~ reported the spectra of the species obtained on dissolving transitioii element chlorides in dimethylformamide. In such a medium of high dielectric constant and low base strength the formation of the tetrahedral configuration was promoted. It was suggested that in this medium the tetrahedral configuration is more stable since the average ligand bond strength is greater than in the octahedral configuratio11.36 It was possible, therefore, by using dimethylformamide to study the octahedral-tetrahedral equilibrium in one phase. Our work indicates that in a medium of low dielectric constant and lorn base strength, the halide complexes of the first transition series assume the tetrahedral configuration to the exclusion (of octahedral species within the limits of detection. (34) L. I. Katzin, J. Chem. P h y s . , 36, 3034 (1962). (35) L. I. Katzin, ibid., SS, 467 (1961).
