P a u l G. Rasmussen University of Michigan Ann Arbor
Some Perspectives on Heteropoly Ion Chemistry
Heteropoly ions are part of the large family of inorganic polynuclear complexes. The heteropoly acids and their salts are a unique group of compounds in whichvanadates, molyhdates, tungstates, or other heavy metal oxyanions condense together along with a "hetero" ion as the pH of their aqueous solutions is lowered. An important characteristic of a heteropoly compound is the ratio of the condensing metal atoms or "addenda" atoms to the "hetero" atoms. While all the integral species from 12:l to 1 : l have been reported ( I ) , the ratios of 12:1, 9: 1, and 6: 1 are by far the most numerous. The predominance of certain ratios suggests that geometry or packing considerations are important in explaining the stability of these ions, and we will see that this is indeed the case. As early as 1826, Berzelius knew that molybdate ions combined in solution with phosphate ion to form a new species but it was not until the eighteen-sixties that Marignac established the stoichiometry of the heteropoly ion 12-molybdophosphate. The determination of the structures of heteropoly ions has evolved slowly over the years, and is still the subject of current research. I n contrast to Werner's work on mononuclear coordination compounds, most of the early structural conjectures for polynuclear ions were incorrect. Following the work of Marignac, various methods for writing systematic formulas were proposed, but they did not predict the correct basicities for the ions, and real knowledge of the spatial arrangements was lacking. The modern period in heteropoly ion chemistry began in 1934 with Keggin's (2) determination of tho structure of Ha[P04W1203e] .5H,O by X-ray diffraction. Figure 1 shows three different ways of representing what is now known as the "Iieggin" structure. The hetero atom is tetrahedrally coordinated to four W3013 clusters, each of which share one oxygen with it. The three octahedra of each cluster share edges and the four clusters share corners. One might get the impression from the diagrams that this structure is rather open, which is not the case. Recently, the "Keggin" structure has been verified for the compound I~[Co(III)04W1203e]~20H20 (S).' The oxygen atom positions were carefully determined and were found to be very nearly in a close-packed arrangement. The tungsten atoms are displaced from the center of the octahedral holes towards the periphery of the anion. This feature has been used to explain the high water solubilities and acid strengths of the "Iieggin" structures as well as the low degree of salvation (4). A 'This is the only known tetrahedral Cog+ compound, and it is high spin with four unpaired electrons.
number of hetero ions including R3+, Si4+,FI+, Ge4+, As5+, Co3+,Co2+,Pe3+, and other first row transition ions form heteropolytungstates of the "Iieggin" type. Other hetero ions such as Ce4+and Th4+are known to form 12:l compounds but these are probably based on a central octahedron because of their size (5). The structures of this latter group have not been rigorously studied. It is noteworthy that the heteropoly nzolyb-
Figure 1. The Keggin Structure. A.Spotia1 diagram. 0 oxygen, or tungsten or molybdenum, 0 hetero atom; 8. Cubo-octahedron formed b y tungsten or molybdenum at vertices; C. Polyhedral diagram, note central tetrahedron
dales of transition row ions form 6: 1 species even though the nonmetallic hetero ions form normal "Keggin" structures. Therc have been recurring reports in the literature of "isomerism" in the 12: 1 series of heteropoly compounds (G), but the precise nature of the structural differences has not been clarified. Although many compounds containing ten or eleven molybdenum or tungsten atoms per hetero atom have been reported, their structures are not well known. It is likely that they have incomplete "Keggin" structures. (An assumption consistent with the known degradation steps of these ions as the pH is raised (7).) The 9 : l series appears in both a monomeric and dimeric form. The dimeric form is typified by (NH& Volume 44, Number 5, May 1967
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[(POI)%~1018061]. 14H20 and the analogous arsenic compound (8). It can be thought of as forming from two 9: 1 fragments which remain after removal of a Mo8OI3cluster from each of two 1 2 : l ions. I n this dimeric, or Dawson structure, the hetero atoms are tetrahed~ally coordinated (9). The monomeric 9 : l series consists a t present of only two ions, [Ni4+08 1 \ 1 0 ~ 0 ~ ~(25) ] ~ - and the analogous Mn4+ compound (11). X-ray diffraction worlc shows that these stmctures contain octahedrally coordinated hetero atoms in an asymmetric structure (12). A preliminary report of the absolute configuration has been given, and the asymmetry is found to persist in solution (13). However, in a more recent investigation, five methods for the resolution of enantiomers failed to resolve this compound (95). Earlier worlc reporting a 9 : l Co4+ species has not been verified ( 1 4 , but other possihilities in this series have not been thoroughly explored. A number of heteropoly compounds having the 6: 1 ratio are known, particularly among the heteropolymolybdates. The fundamental structure of this series is the Anderson-Evans (15), Figure 2 , structure of
Figure 2. T h e Andenon-Evans Structure. A. Spotial diagram. 0 oxygen, t u n g s t e n or molybdenum, o hetero otom; 8. Polyhedral diagram, central octahedron.
[Te08R~Io8018]6-, in which six octahedra share edges in a n annular fashion about the central octahedron. The series includes the trivalent hetero ions Ga3+, Coa+, Ma+, Fe3+, Cr3+, Rh3+ which were formerly thought to he 12:2 dimers as well as the divalent Ni2+ compound. The general formula for this group, [Xn+08M08018H6]'6-");is that of the Anderson-Evans structure except that it contains nonionizable hydrogen (10). It is noteworthy that most of these hetero ions except NiZ+are part of a 12: 1tungstate series which implies that the molyhdate structures are more compact than the corresponding tungstates. The lower ratios have for the most part been postnlated as intermediates (16). An interesting diieric 5-molybdocohaltate(III) has been characterized, however (14). The cobalt atoms appear to be trivalent, diamagnetic, and equivalent, although the exact structural arrangement is unknown. A report of 12-tungsto3-zincate (17) does not seem to be justified on the basis of the data presented. It appears that the octahedra making up heteropoly compounds may share edges or corners but not faces. Equilibria in Solution
While data on solid heteropoly compounds is often incomplete, the situation for solution work can only be described as chaotic. This is in spite of the fact that a large number of experiments, employing many methods, 278 / Journul of Chemical Education
have been carried out. I n a lengthy review article limited to isopolytungstates, Kepert (18) chooses t o classify the field according to the methods used, rather than by the predominant species and their equilihriaan indication of the problems that exist. An instructive (though sobering) example of the need for critical evaluation of experiments pertains to the measurement of diffusion constants in solution. For many years, such constants were related to ionic weights by an equation known as "Riecke's Law" which stated that the product of the diffusion constant and the square root of ionic weight was constant. It was not until 1960 that Baker and Pope (19) reported the results of a simple hut definitive experiment. They measured the diffusion coefficients for the isomorphs [Si04W12088]4(ionic weight 2875) and [Si04Molz036]4-(ionic weight 1820) and found them to be the same within a very small experimental error. Also, a simple theoretical model of diffusion in liquids would predict that size and not mass is the important factor in determining the rate. Another source of confusion in heteropoly research results from the entrance of time as a variable into the experiments. I n contrast to most ionic reactions, equilibrium is established slowly in aqueous solutions of molybdates and tungstates. Thus, many experiments have been carried out unwittingly on solutions of changing composition. Rate studies show that oxygen exchange with tungstate species is rapid at high pH hut decreases markedly as the pH is lowered (20, 21). Recently, matters have been clarified somewhat by Aveston and co-workers (22). They used a combine tion of techniques includmg ultracentrifugation to unravel the important equilibria. They find that the principal species formed upon acidification of a molybdate solution are the heptamer (Mo,OZa6-)and octamer (MosO16'-). The heptamer or paramolybdate is similar in structure to the 1:6 heteromolybdates (25). The octamer has sometimes been confused with tetramer in studies which did not measure the degree of agglomeration independently from the degree of acidification. I n the tungstate system, Aveston found (22) paratungstate A, (HW6OZl5-),in equilibrium with paratungstate Z, (W120n110-),and a t lower pH, metatungstate (W120396-). (The letter designations are those used by Kepert (18).) Lipscomb (24) has pointed out that WI2O4,lo-is an unlikely structure on the basis of Xray symmetry and has suggested H2Wlz042'0-. This species is consistent with Aveston's work since his methods cannot distinguish water of constitution. I n this regard, it is interesting that Aveston's estimation of removable water (22) was closer to the Nalo [HzWlzO~z] .27Hz0 formulation than to the usual Nalo WI2Onll.28H,O. It has been suggested that the slow formation of most heteropoly tungstates is due to the necessity of prior decomposition of paratungstate Z to a hexamer, which also is consistent with Aveston's results. Recently the cuho-octahedral symmetry of the tungsten framework has been verified for [Si04 W12038]4-in solution by X-ray diffraction (35). The metatuugstate on the other hand shows properties similar to the heteropoly ions and is often formed as a by-product in their preparation. Souchay has shown that the metatungstate species is not active in heteropoly ion formation (26). Indeed, one can speculate
that the metatungstate has protons as its "hetero" group (38). In spite of the difficulties posed by heteropoly ions in solution, some of their interesting properties have been investigated. Souchay (7), and recently Pope and Varga (27), have studied the reduction of heteropoly ions by controlled cathode potential and by polarography. They find that from 2 to 6 electrons may be added to a heteropoly ion without the strncture brealing down, although protonation often accompanies the reduction. In the reduced state these ions have a characteristic deep blue color called "ceruleo blue" in the French literature. The classical interpretation of the reduction has been in terms of Mob+ or W5+ species forming within the ions. It seems unlikely, however, that the added electrons are associated with any part,icular metal atom. In fact, the reduction experiments a9 well as the facile electron exchange between 12-tungstocobaltate(I1) and 12-tungstocobaltate(111) (28) suggest the treatment of these ions as conducting spheres, an analogy pursued by Pope and Varga. The magnetic properties of the reduced species are under investigation and should prove enlightening (29). The formation of the heteropoly blue color has had some utility in the colorimetric analysis of easily oxidized materials hut the methods are empirical and require calibration. Solutions of heteropoly ions have successfully been used to precipitate complex cations from solution even when the tendency to form oils is great. However, the large ionic weights of the heteropoly ions detract somewhat from this procedure.
physical and synthetic investigation in spite of their having been discovered over one hundred years ago. Literature Cited
(1) PASCAL, P., Editor, "Nouvau Traite' De Chimie Minerale," D. 903. Vol. XIV., "Hetero~olvaeids"hv L. MALAPRADE. Mnrsnn e.~ t Cin., Paris. - ~1959 ~ ~ ~ ~ , (2) KEGGIN, J. F., Proc. Roy. Soc., 144, 75 (1934). N. F., Ph.D. Thesis, Boston U., (1961). (3) YANNONI, S., Editor, "Proc. 6 I.C.C.C. Detroit," The (4) KIRSHNER, Maemillan Co., New York, N. Y., 1962. G. A,, AND MCCUTCHEON, (5) BAKER,L. C. W., GILLAGHER, T. P., J . Am. Chem. Sac., 75,2493 (1953). P., BuU. C h m . Sac., France, 1951, 365; ( b ) (6) (a) SOUCHAY, STRICKLAND. J. D. H.. J. Am. Chem. Soe.. 74.862 (1952): ?c.) ~asnan;. Rend.. ~ ,- ~ ~~anale; ~ ~ ~ ~ -261. . -3137 i1965). (7) SOUCHAY, P., ~ ua i di ~ & l i e d Chdnistry,'6, 66(1963). A,, AND TRAUBE, A,, Z . Amrg. Chem., 91, (8) (a) ROSENHEIM, ~ SA,, , Ph.D.Thesis, BostonU., 75 (1915); (b) T ~ I G D I NG. (1961), p. 106. B., Ada. Cryst., 6 , 113 (1953). (9) DAWSON, U. C., Ph.D. Disrertation, Boston U, (1960). (10) AGARWALA, C., AND SMUALSON, M. Z., Ana~g.Chem., 24, (11) FRIEDHEIM, fi7 (1900). , - ~ (12) WAUGH, J.'T. L., et al., Ada. Cryst., 7 , 438 (1954). D. P., AbstraUs of A.C.A., Ithaca.N.Y., (1959). (13) SHOEMAKER, G. A., Ph.D. Dissertation (1961) p. 250. (14) TSIGDINOS, (15) EVANS,H. T., J . Am. Chem. Sac., 70, 1291 (1948). G., GEIER,G., AND LITTLER, J., Helv. (16) ~CHWA~EENBACH, Chim. Ada, 45, 2601 (1962). D. H., AND M A I J. ~ A,, J. Chem. Soe. (London), (17) BROWN, 1958, 2599. D. L., ~ ~ I ~ o p ~ l y t u n g ~ tProgress a t e s ~ " in Inorganic (18) KEPERT, Chemistry Vol. IV, 1962 (Editor: COTMN,F. A.). (19) BAKER, L. C. W., AND POPE,M. T.,J . Am. Chem. Soc., 82, 4176 (1960). (20) GEIER,G., AND BRUBAKER, C. H., Inorg. Chem., 5, 321 (1966). V. I., R u m J . horg. Chem., 3, 217 (1958). (21) SPITSYN, J., ANACKER, E. W., AND JOHNSON, J. S., Inorg. (22) AVESTON, New Synthesis Chem.. 3. 735 (1964): . ., AvEsmN.. J... Inoru. Chem.. 3. 981 ( i g ~ j .' (23) LINDGVIST, I., A&". Kemi., 2, 325 (1950) and 349 (1951). In recent years there has been considerable synW. N., I m w . Chem., 4, 133 (1965). (24) LIPSCOMB, thetic activity, particularly in the dodeca-tungstate (25) DKER, L. C. W., 4 N D WEAKLY, T. J. R.,J . h 0 t ' g . Nticl. series of heteropoly ions. Brown and Mair (17, 30) Chem., 28, 447 (1966). have reported 12-tungstates of Cr(III), Fe(III), CuP., Ann. Chim., 18.87 (1943). (26) SOUCHAY, (27) 1 n ( 1 V ) n ( 1 Brief reports on the preparation . . POPE,M. T., AND VARGA,G. M., Inorg. Chem., 5, 1249 (1666). of [TeOnW120a8]4(31) and [TiOaMo120a]4-((58) have C. TI., Inorg. Chem., (28) RnsMr:ssEN, P. G., AND BRUBAKER, appeared in the Russian literature. Baker and Mc3, 977 (1963). Cutcheon reported two unusual dicohalt heteropoly(29) POPE,M. T., AND VARGA, G. M., Private Commnniestion. (30) tungstates in addition to describing [ C O O ~ W ~ ~ O ~ ~ ] ~(a) - Mam, J. A,, J . Chem. Soc., 2364 (1950); (b) BROWN, D. H., AND MAIR,J. A,, J . Chem. Soe., 3946 (1962); ( c ) and [ C O O ~ W , ~ O(33). ~ ~ ] ~ -With the exception of the BROTN,D. H., J . Chem. Sot., 3322 (1962); (d) BROWN, last two (see Simmons (34)), many of the above menD. H., J . Chem. Soc., 4408 (1962). tioned species are poorly characterized. This is due to (31) GAME~JNA, E. Sh., Rusa. J . Ino~g.Chem., 1, 812 (1962). difficult,iesof analysis, low yields of preparative methZ. F., AND SEMENOVSKAYA, E. N., RUBS.J . (32) SHAKHOVA, Imrg. C h a . , 1, 556 (1962). ods, and inadequate investigation. The apparent L. C. W., AND MCCUTCHEON, T. P., J . Am. Chem. (33) BAKER, tetrahedral site symmetry of the hetero ion in many of SOL, 78, 4503 (1956). these compounds suggests the possibility of using V. E., Ph.11. Thesis, Boston U.,(1963). (34) SIMMONS, these frameworks to force this stereochemistry on other P. A,, AND DANFORD, M. D., J . Chem. (35) LEVY,N.A,, AGRON, hetero ions. Other fruitful synthetic paths are indiPhys., 30, 1486 (1959). (36) MATIJEV~C, E., et al., Inorg. Chem., 2, 581 (1963). cated by reports on mixed tungsten molybdenum or V. I., et al., Russ. J . Inorg. Chem., 10, 142 (37) BEZRUKOV, vanadium heteropoly ions, e.g., Matijevic, et al. (36),
and on heteropoly ions of niobium (37). Clearly, heteropoly ions remain a fertile area for both
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(38) POPE,M. T., AND VARGA,G. M., Chem. Comm., 1966, 19, 653.
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