Oct. 5, 1960 [CONTRIBUTION FROM
PREPARATION OF SALTS O F COBALT(II1) HEXAFLUORIDE I O N THE
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DEPARTMENT OF CHEMISTRY, MASSACHUSETTS INSTITUTE OF TECHNOLOGY, CAMBRIDGE 39, MASSACHUSETTS]
The Preparation, Structures and Infrared Absorption of Salts of Cobalt (111) Hexafluoride Ion BY M. D. MEYERSAND F. A. COTTON RECEIVED MARCH9. 1960 The preparation of salts of C O F S -by ~ fluorination of the corresponding C O ( C N ) ~salts - ~ (Klemm and Huss’ method) is described in detail. X-Ray and infrared studies of I(&aCoF6, KtCoFa, Na8CoF6, Li8CoF6and Bas [CoF611lead to the conclusion t h a t only the first is rigorously cubic, whereas the remaining ones show increasing distortion of t h e lattice and t h e CoFe+ octahedra in the order listed. Comparisons of t h e structures and infrared spectra with those of the corresponding aluminum compounds are made.
Introduction The spin-free 3d6 system occurs, in stable compounds, only in spin-free Fe(I1) compounds, which are relatively numerous, and in the [ C O F ~ ]ion.’ -~ In carrying out an investigation of the dynamic Jahn-Teller effect in such systems, which is reported separately,2we had occasion to prepare some salts of the [ C O F ~ ] anion -~ with various cations. Several of those described here have been reported before while two are new. In all cases we have examined (or reexamined) the X-ray powder pictures and the infrared spectra, since these data give information on structure and symmetry vital to the interpretation of data on the electronic spectra and magnetic properties of the substances. We also report our preparative and analytical data for the known as well as the new compounds in some detail for two reasons. While the preparative method is not original with us, it has not, so far as we know, been described in any detail before, nor have analytical details indicating the purity of samples previously subjected to certain physical measurements been reported. Since we shall report magnetic results and certain X-ray data somewhat a t variance with those previously given, as well as new infrared data, we feel obliged to justify our confidence in our own results. K3CoF6 has been prepared previously by several methods. There is a “wet” method3which involves electrolytic oxidation of CoFz in concd. aqueous H F to give CoF3-7/2KZOfollowed by treatment of this with solid K F a t -10’. This same method failed4to give (N&)&oF6 when NHIF was used in place of KF. Klemm and co-workers have devised several sorts of “dry” methods. One general method5 involves fluorinating a physical mixture of CoClz with some salt containing the cation desired in the final product, e. g., 3KC1 CoCIZ to get K,CoF6. A still better method involves fluorinating a complex salt differing from the desired product only in the ligands in the anion, e. g., K Q C O ( C Nto )~ get K3CoF6. This method has the advantage of being more likely to produce a homogeneous sample of a true chemical compound.
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Discussion Preparations.-Hoppe and Klemm6s7 have reported the preparation of K ~ C O Fas B well as the corresponding Li, Na, R b and Cs compounds by this method but only in review articles where virtually no experimental details were provided. We have used this method to prepare Li3CoF6, Na3CoF6 and K3CoF6 from the corresponding &CO(CN)6 salts. In addition, Ba3(CoF6)zhas been prepared from Ba3[Co(CN)6]2 and KzNaCoF6 from a substance of empirical composition K2NaCo(CN)6. While we shall present evidence that K2NaCoF6is a true compound, we do not know for certain whether the starting material is a compound or a mixture of K&o(CN)6 and hTa3Co(CN)6. Experimental details of the preparations and analyses are summarized in Table I. An attempt to prepare La[CoFG] by fluorination of L ~ C O ( C Nfailed. )~ The evidence indicates that we obtained LaF3 COFQinstead. It seems reasonable to explain this on the basis of lattice energy differences; that is, we assume that the sum of the lattice energies of CoF3 and LaF3exceeds the lattice energy of La [CoF,] (ignoring entropy differences which are unlikely to be large). Since La+3has a very high charge-to-radius ratio, the occurrence of phase separation in this system but not in many others with cations of smaller charge-to-radius ratio appears reasonable. X-Ray Studies.-X-Ray powder studies have been made of all compounds prepared. Hoppe6 and Klemm’ have stated that NasCoF6 and K3CoF6are cubic with unit cell edges, UO,of 8.13 and 8.55 A., respectively, a t room temperature. They also state that the compounds were isomorphous with K3FeF6.’ Structures of this general typeg are derived from the cubic CaFZ (fluorite) structure. In KzSiF6,” X:+ ilons occupy the F- sites of CaFz while [SiFs]-’ ions occupy the Ca+2 positions of CaFz. This arrangement produces a cubic unit cell with complex ions a t each corner and a t each face center. The K + ions occupy the centers of the eight quadrants of the unit cube. To increase the number of cations per formula unit to three, as in K3MF8,a potassium ion is placed a t the center of the unit cell
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(1) It also occurs in Co(II1) ions in certain oxide systems but these a r e not suited t o the objectives of our work.
(2) F. A. Cotton and M. D. Meyers, THISJOURNAL, 82, 5023 (1960). (3) R . L. hlitchell, Thesis, University of Buffalo, 1938 (4) B. Cox and A. G. Sharpe, J Chem. Soc., 1798 (1954). ( 5 ) W. Klemm a n d E. Huss, 2. anoig. u. allgem. Chem., 2 5 8 , 2 2 1 (1949).
(6) R. Hoppe, Rec. Irav. chem., 1 5 , 569 (1956). (7) W. Klemm, Bull. soc. chim. France, 1323 (1956). (8) L. Pauling, THISJ O U R N A L 46, , 2738 (1924). (9) R. W. G. Wyckoff, “Crystal Structures,’’ Vol. 111, Interscience Publishers, Inc., New York, N. Y., 1953, Chap. IX,text page 27. (10) A. F. Wells, “Structural Inorganic Chemistry,” Oxford Press, London, 1945, p. 289.
M. D. MEYERS AND F. A. COTTON
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and twelve others are placed a t the mid-points of each unit cube edge. The discussion of our X-ray data will be based upon analogies with the data for M3AlF6 and similar systems and especially with the results of Steward and Rooksby. l i These workers established that there is a general pattern of behavior in compounds of this class. All of them are cubic (space group Fm3m-0h6) a t a sufficiently high temperature. On cooling, the lattice contracts until a t a temperature characteristic of each substance, distortion sets in. For Na3A1F6 (cryolite, either natural or synthetic) the structure is cubic above 550' but becomes distorted to monoclinic, increasingly so as the temperature falls, below 550'. With K3L41F6 the critical temperature is below 300'; the lattice is tetragonal, with a. = 8.40 and co = 8.46 a t 20'. This distortion is manifested as very slight line splittings in the powder picture. (NH4)3FeF6 and (NH4)3AlF6 were found to be cubic a t 20' but tetragonally distorted a t - 180'. KzNaAlF6 was found to be cubic a t 20' and still cubic a t - lSOo. The stability of the cubic structure of K2WaA%1F6 in a temperature range where both K3A1F6and Xa3AlF6 are distorted was attributed by Steward and Rooksby to the placement of the K ions a t the centers of the octants and the Na ions a t the other cation sites, sites of these two sorts occurring in a 2 : l ratio. This proposal of site occupancy had been substantiated by Manzer.12 Our powder pictures of K3CoF6 confirm the conclusion of Klemm and Hoppe. All lines present have h k , la 1 and k I even, as required by a face-centered lattice and all lines can be satisf actorily indexed assuming the cubic structure described above with a. = 8.55 A. a t 25'. l v e should like to note, however, that small distortions of 0.01-0.1 A. to give perhaps a tetragonal unit cell probably would escape detection. Since refined workI3 on K3AlF6has reyaled a slight tetragonal distortion (co - a. = 0.06 A,) a t 20' i t would not be unlikely that a similar distortion might occur a t room temperature in K3CoF6where the 51F6 octahedrals are somewhat larger. Considering that Na3AlF6has a greater tendency to distortion than does KaAlF6,Klemm and Hoppe's report that Na3CoF6is cubic a t room temperature seemed surprising. We find that the powder pictures of xa3CoF6 and Na3AlF6 a t room temperatures show considerable resemblance, although intensities vary as would be expected. This suggests that there may be a monoclinic distortion of xa3CoF6similar to that in T\;a3A1F6. At the very least it may be said that the pattern for Na3CoF6 definitely indicates distortion of some kind for it is more complex than those for K3CoFtjr KsFeF6 or K3A1Fs. K2NaCoF6 was found to have the d-spacings given in Table 11. These results indicate that K2KaCoF6 like K2NaA1F6is truly cubic a t room temperature. Moreover, the powder picture clearly shows that K2T\TaCoF6is a true compound and not a mixture of K3CoF6 with Na3CoF6.
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(11) E. G. Steward and H. P. Rooksby, A c f u Crysi., 6 , 49 (195:{). (12) G. Manzer, F o i t s r k r . . M i ~ w ~ n l17, . , 61 ( 1 9 3 2 ) . ( 1 3 ) C. Rrosszt. :lvk K t ~ w il , f i i z r r a i G P ~21A, , Xu !I (1940).
Oct. 5, 1960
PREPARATION OF SALTS O F
TABLE I1 D-SPACINGS FOR K2KaCoFG Index ( h k l )
111 200
Calcd."
Found b
4.74 4.11
4.73 w 4 . 0 9 vvw .. .. 3.81 vwe .. .. 3 . 4 4 vvwc 220 2.91 2 . 9 1 vs 311 2.48 2 . 4 8 vvw 222 %.3i 2.37 m 400 2.06 2 . 0 6 vs. 422 1.68 1 . 6 8 ms 511,333 1.58 1.59 vvm 440 1.45 1 . 4 6 ms 620 1.30 1.31 w 444 1.19 1.19 w Calculated for cubic structure using a = 8.22 A. * w = weak, s = strong, m = medium, v = very. These lines cannot be indexed a n d are believed due t o impurities, although they do not correspond t o any of the three strongest lines of NaF, KF, COO or C0203, 3.81 may be due t o r';a3CoFs where it occurs strongly.
No satisfactory powder photograph was obtained for Ba3(CoF& in several attempts. The sample in the capillary tube was dark after the X-ray exposure. A few strong lines, in addition to many weak and diffuse lines, were observed, suggestive of decomposition and/or a unit cell of low symmetry. A powder photograph of Li3CoF6 which had not been pulverized showed only discrete spots. Grinding the sample caused decomposition within about 10 minutes. Infrared Spectra.-Until the recent work of Peacock and SharpL4(who cite the limited earlier literature) little attention had been given to the infrared spectra and Raman spectra of salts containing MF6 anions, although there has been considerable study of gaseous, neutral hexafluorides. l6 MFe species will, in the absence of distortions of a regular octahedral structure, belong to the point group oh and have therefore only two infraredactive fundamental modes of vibration. One (USUally designated v3) is principally an M-F stretching motion, and the other (v4) is mainly an F-M-F angle bending motion. Peacock and Sharp studied the variation in the frequencies of these modes, especially v3, as a function of the metal atom M and the charge, but there has not been any systematic study of the effect of changing crystal environment upon the spectrum of one anion. We have examined the effects of environment upon both v3 and v4 in the anions AlF6-3 and C O F ~ - ~The . main results are presented in Table 111. It will be observed that the positions of the bands are essentially insensitive to the lattice in which the AlFsV3and CoF6+ anions find themselves. There are, however, significant variations in the widths of the bands which appear to be related in a qualitative way to the existence of or degree of departure from strict octahedral site symmetry in the various lattices. The spectrum of K&F6 is given for comparison since it has been foundl6.l7that the SiF6+ octa(14) R. D. Peacock and D. W. A. Sharp, J . Chem. SOL.,2762 (1959). (15) See ref. 14 for literature citations. (16) J. A. Ketelaar, 2 . Krisl., 92, 155 (1935). (17) R. M . Bozorth, THIS JOURNAL. 4 4 , 1060 (1922).
COBALT(II1) HEXAFLUORIDE ION
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hedra are perfectly regular in this compound and so oriented that their site symmetry is o h . Recent studies have confirmed these results a t and below room temperature.la Hence the very symmetrical envelopes and small widths of the bands for this substance may be considered representative of what can be expected for a completely symmetrical MF6 anion. In agreement with this hypothesis, the bands of AlF6-3 in K2NaA1FB, already known to have a strictly cubic structure, are similarly symmetrical and narrow. The very close similarity of the bands in KzNaCoF6 to those in KzNaAlFe and K2SiF6 then provides excellent confirmation for the conclusion, reached above on the basis of the X-ray data, that KzNaCoF2 is strictly cubic with undistorted CoF6+ octahedra. It also will be seen that in the sodium and potassium salts of AlF6-3 and C0F6+, for which X-ray data indicate some departure from perfect cubic symmetry of the lattice, the widths of v3 are significantly greater than in the perfectly cubic lattices. Since the X-ray data suggest that lattice distortion is greater in the sodium salts than in the potassium salts, i t might have been anticipated that the band widths would be greater for the sodium than for the potassium salts. ils Table I11 shows, the band widths differ by an amount scarcely outside of experimental error but apparently in the opposite sense. If this inverse variation is real, it is not necessarily enigmatic, for it is possible to imagine that the slightly more distorted lattice might nevertheless contain the slightly less distorted octahedra. For Li3CoF6, where the lattice is known to be far from cubic, there is a pronounced broadening of both bands, especially v3, and for B a ~ [ c o F g which ]~ is also not cubic it appears that there is actually a splitting of v 3 into two bands. Thus taken qualitatively, the infrared spectra corroborate the conclusions previously reached as to the varying degrees of distortion of the hTF6 octahedra in the compounds examined. Experimental Preparation of CO(CN),-~ Salts.-.Inhydrous K3Co( C S ) , was prepared according to Bigelow.'S For the preparation of the other salts, H X o ( CN)6,xH20was required. This was prepared by precipitating Ag,Co(Ch-)s, a slurry of which was then treated with HzS. T h e 4g2S was filtered off and the clear yellow filtrate evaporated in vacuum t o a white residue. Continued pumping of this residue for 24 hr. a t 60-70" gave a white product with sternutatory properties. Determination of acid equivalents aiid ?lemental analyses (Co, N ) showed that the prciducts so obtained in various preparations were H3CO(CN),~.vHrO with x varying from 0.3 to 0.5. Baa[ C O ( C N ) ~ ] ~ , H ~ O . - T substance ~~S has been mentioned in the literature before but in highly hydrated form.2'J,21 LVe desired as anhydrous a product as possible. H3Co( C X ) ~ , O . ~ H ZinO water was carefully neutralized with a stoichiometric quantity of Ba(OH)2,8H20. Upon or shortly before the addition of all of t h e Ba(OH)?.8H20, the p H reached a value of 6.0 and the solution became slightly cloudy (probably BaC03). After filtering, the clear, light yellow solution was evaporated in vacuum. Crystals of the (18) R . C. L. Slater, Dept. of Physics, Xl.I.T., personal communication. (19) J. H. Bigelow, "Inorganic Syntheses," Vol. 11, McCraw-Hill Book Co., New York, S . Y . , 1946, p. 223. (20) A . Ferrari and L. Coghi, Gazz. chiim ital., 69, 3 (1939). (21) R. W. G , Wyckoff, ''Crystal Structures," Vol. 111, Interscience Publishers, I n c . , New York, S . Y . , 1953, C h a p TX, text page 27.
hI. D. MEYERS AND F. A. COTTON
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TABLE I11 KUMERICAL PARAMETERS OF INFRARED SPECTRA OF S ~ F G AlFe-8 -~, VJ
7
Compound
S l u x . , cm.-'
--a3
AND
C O F ~SALTS -~
v4
Width a t ' / n lit., cm. - 1
Width a t N a x . cm.
-1
'/P ht., cm.-'
Crystal structure a t 23'
KzSiF6 I 75 f 5 485 i 5 13 i 2 Cubic KzSaA1 Fa 000 5 10 90 =t21) 405 i 5 17 f 2 Cubic K2NaCoFa ;ilO i 10 75 i 10 310 f 5 25 i 5 Cti b ic KaAlfle 595 =k 10 130 f 10 392 i 5 32 i 5 Slight tetrag. KsCoFti 4i5" 140 i 15 310 i 10 25 k 10 Slight tetrag. GO0 i 13 100 f 10 (416, 400) f 5b 40 f loh Monoclinic SasAlFs SaaCoFs 6UO f 10 110 i 30 (34O(sh), 320) i sb 35 f lob Monoclinic 481) f 10'; -150 320 i 5 40 f 10 Unknown, not cubic Ba:j[CoFs]2 LisCoF5 -500 \'ery broadd S o t observed Unknown, not cubic a There is considerable uncertainty in locating "the" band maximum, submaxima which are always very poorly resolved ])ut apparently repruducible appear a t -505, -480 arid -450 cm.-I. * ~g is often broad on t h e high frequency side, b u t in these coinpounds a second I ~ i is d clearly discernible; t h e "width" is the combined widths of both builds. There is also a weaker band a t -620 c m - l with some absorption between it and the main band a t -480 ern.-'. Tlie adsorption begins a t about 7lJO ciii.-l aiid has a value of 50',6 a t 400 cm.-'; the width is >300 cm. pruduct bcgdi I f , appear 'I\ hen tlie concentration reached 0.3 -0.4 iiiohr. Ilva!)oration under vitcuuin then was contiiiued to dryiiesr while riiaiiitainirig tlie solution ;it 60-70". Tlie solid was further dried under v:icuurii over P,Oa in a drying pistol a t 1-40' lur 24 Iir. .ltiul. Calctl. for Uap[Co(CX),,;2: BL, 48.9; for EaJ[ c , J \ C ~ ) t i ] ? ~ l h, i 2 ~ .I:.g. : ~ ' O U l l d : Ha, 47.8 (1st batch), 47.6 (2lld batch). ,UarCu( C N IVLLS prepared by neutralizing H3Co(CN)e with the stoichiometric quantity of NazCOJ( p H ,7). Filtration and evapuratioii followed by dryiiig overnight a t 145' iri vacuum afforded t h e anhydrous compound. K2NaCO( CN)6 was prepared by inixing solutims containing NaaCo( C S ) &and K4Co(CS)ti iii 1 : 2 inole ratio, evaporating to dryiiess and dryiiig i i i vacuuiii 24 hr. a t 70'. 0 1the basis ol yield (103'h) calculated as anhydrous R,SaCo(CN)e i t was concluded that tlie product was essentially anhydrous. I,iaCo(CS),, was prepared by evaporatiou uf a solution prepared by mixing stoichiometric aniouiits of H3CO( C N j o and LiOH, evaporating to dryness and drying in vacuum a t 140'. This material doubtless was a hydrate, b u t by this point iri the work it had been concluded t h a t use ( J f anhydrous cobalt cyanides prohahly was not essential : ~ n dit was therefore used Without further drying. Preparation of [CoFs] --B Salts.-Tlie fiuorinatioii apparatus used was very siiiiild' t o th&t described iii det;.il by Priest .22 I~luurine\vas purcliused Eroiri the I'ennsalt Cliemical Co. ;riitl used without further purification. T h e elevated temperatures required fur tlie fluorinatiuiis were provided by a tubular furnace and the tenlixrzturcs were measured by a theriiiiicouple juiictioii iii ii well p1:iced withiii :r ceiitinleter of the nickel bciuts containing tlie samples. The general procedure was the w i n e for all compounds. A weighed quantity of tlic sippropridte [Cu(CS),] --3 salt iii a weighed S i boat \vets I J I U X ~ i i i the apparatus \vhicli Tlie apparutus then was usually was preheated to -100". swept for 1-2 hr. with dry iiitrogeii. Sweeping with fluorine was then begun arid contiiiued throughout tlie run at a rate of about 30 cc. per inin. AAfter-4.5 h r . of sweeping with fluorine the temperature I\ ;LS quickly raised t o operating teinperature. After fluorinatioii was coiiiplete the oven was cooled wliile displaciiig the fluorine with nitrogen. Tlie boats were reiiii~vedfr(,iii tlie iiitriigen atmosphere at temperatures of 100-150" ~ t i i c i iiiiiiicdi.itrly p1:iced in a v:tcuuin desiccator uver P,Oa. \\-lien tlie sample was completely cool it was weighed quickly. Speciiic details fur each riiii are given iii Table I . Lye attempted the preparation oi L:L[COF~]..%nhydr(~us L a C o ( C x ) s , made b y heat~ C~U~ U I I I :rt 110' for 12 hr., ing L ~ C O ( C S ) , ; . F ~ / in ~ Ha ~VO T h e prodiict was was fluoriiiated for G hr. a t 300-330". light brown i i i color indicating tlie presence of CoF3. An X-ray powder picture definitely showed the presence of LaF3. Fluorination t o CoFR LaFl was shown t o be complete by analysis (calcd.: F, 36.55; foun K;.IAIFs _was _ prep.irerl b y the nietliod o f Pain
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(22) H. F. I'riest, "Inorganic Syntheses," Vol. 111, hfcGran-HiI! Bun1.: C u . , New York, S . Y . , 154.50, 1) 171. ( 2 . ; ) C . Tames and P. s. \Villund. T H I S TOURNAT-. 38, 1497 (191G). (24) 1'. A. Paine a n d j.I'enrsoo. J . C i i i , i r i Sot., 1172 (194i).
An X-ray powder photograph of the product showed the nine strongest lines reported in the X S T N index25 for Ka241F6. Two very weak lines due t o K F also were found. KlNaAlFs was prepared by dissolving the correct amount of aluminum metal in a sulution containing KOH and NaOH in 2: 1 iiiole ratio. The solution was then neutralized with aqueous H F and evaporated t o dryness. An X-ray powder diffraction pattern confirmed the identity of tlie material.26 T h e sample of K2SiFewas quite pure niaterial prepared for X-ray work and kindly given t o us by I)r. R. C. L. Slater. Analytical Methods.--Fluorine was separated b y volatilization26 from concd. €IC104 iii a glass apparatus. Samples of -100 mg. mere used with 80-100 nil. of distillate being collected. The fluoride was weighed as PbC1F.27 Trial runs on known substances, e.g., cryolite of 99.997, purity (kindly given by the Pennsalt Co.) indicated t h a t we would obtain results correct t o within -02(;c (low). Cobalt was determined by EDTA titration .2* Calibration runs indicated results should be reliable t o within i 0 . 3 7 , . Potassium was precipitated with sodium tetraphenylboron2g; test runs indicated our results should be accurate t o within 10.57,. Barium was precipitated as sulfate in t h e standard way. X-Ray Studies.-X-Ray powder diflr:rction patterns of the IiesafluorocobaILates were obtained 011 photographic film with a General Electric XKD3 X-ray diflraction unit and a North American Phillips powder camera of 57.3 mm. radius. Samples were loaded into 0.3 mm. diameter Pyrex capillaries and irradiated with CuKo radiation using a nickel filter. The pictures were indexed in t h e standard manner," and a film shrinkage correction, applied t o each arc length measured, was obtained from an S a C l calibration pattern. Infrared Spectra.-These mere obtained using a Baird I n f r x e d Recording Spectropliotomcter, Model rlB2, fitted with a K B r prism and a Beckinaii Model I K 4 fitted with a CsBr prism. Calibrations were made with t h e 12.38 p band of amnionia. Substantially identical spectra were obtained with both Nujol mulls and freshly prepared K B r and K I pressings of t h e compouuds.
Acknowledgments.-We gratefully acknowledge financial support in the form of the Procter and Gamble Fellowship 195'7-1958 and a Summer KSF Fellowship to M. D >and I. from the U. S. Atomic Energy Commission under Contract KO. AT(30-1)- 1965. ( 2 5 ) American Society for Testing Material:;. Data Cords for t h e Identification of Crystalline Materials. (26) €I. 11. Willard a n d 0.B. Winter, I n d . ortd E n g . C h c m . , Anal. E d . , 5 , 7 (1933). (27) W. F. Hillebrand and G. E. F. Lundell, "Applied Inorganic Analysis," John Wiley and Sons,Inc., N e w York, N . Y.,1953, p. i 4 4 , (,2 8.) F. .T. Welcher. "The Analytical Uses of Ethvlenediamine Tetraacetic Acid," D. Van Nostrand Co., Princeton, N. J., 1958, p. 230. (20) G. H. Gloss, Chemist A ? $ o l y ~ 43, t , 5 0 (1953). (30) I,. A. Azaroff a n d hI. J. Bueryer, " T h e Powder .\Iethnil," LIcCraw-Hill Book Co., New York, N. Y., 1938, Chap. 7.
