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New Methods and Results in the Study of Polyacids.

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A U G U S T 1966
PAGES 689-750
New Methods and Results in the Study of Polyacids
The present status of research on condensed vanadic, molybdic, and tungstic acids is reviewed. Information about the structure and formation of polyanions can be obtained by
X-ray analysis and from nuclear magnetic resonance, infrared, and Raman spectra, as well
as from complex formation in aqueous solution. Salts of the polyacids can be prepared in
organic solvents by hydrolysis of esters of the metal oxoacids.
1. Introduction
The chemistry of polyacids still presents many unsolved problems, although numerous compounds of
this class have been known and studied for more than
100 years. Whereas preparative and analytical investigations predominated at first, attention later turned
mainly to a systematic study of the state of the polyanions in aqueous solution [ I , 31. The measurement of
diffusion and dialysis coefficients was used as the
principal method for the determination of the pH
ranges in which certain polyanionic species predominate,
and the ranges in which strong aggregation occurs. The
quantitative interpretation of these measurements, i.e.
the determination of the degree of condensation of the
anions, has sometimes led to erroneous results, since
the relationships between diffusion coefficient and
molecular weight are very complex [21. Nevertheless,
these investigations were of great value to further
research in this field. Modern physical methods were
later used to check, and in some cases to correct, the
results, so that a fairly unanimous view on the size of
the dominant particles in the most important polyanion
systems has now been reached. However, many points
of uncertainty still exist in this respect in the polymolybdate system.
2. Methods for the Determination of the Degree
of Condensation of Polyanions
A very accurate and universally applicable method for
the determination of the size[*] of polynuclear complexes was developed by Sillkn 141, who showed that for
the formation of a polynuclear complex in accordance
with Eq. (a)
P A + qB
e ApBq
the values of p and q can be calculated from equilibrium
data. It is possible to recognize several ionic species
existing together, and to determine the equilibrium
constants. To obtain the required experimental data,
the concentration of one component, e.g. CA, is kept
constant, and the concentrations of the other components are varied. Series of experiments are carried
out with different fixed initial concentrations CA, and
the concentration of free A and/or B is found by EMF
Important results have also been obtained by salt
cryoscopy. This method, which was known as early
as 1895 [51, has been greatly improved in recent years
by the use of thermistors. A comparison of the results
of careful salt-cryoscopic investigations with those
obtained by other methods shows that very useful
_ _-
[ I ] Cf. G. Jan& and K . F. Jahr, Kolloid-Beih. 41, 1 (1934).
[2] K . F. Jalir, H . Kudelka, and J . Simon, Z. Naturforsch., in
131 P . Souchay: Polyanions et polycations. Gauthier-Villars,
Paris 1963.
Angew. Chem. internat. Edit. Vol. 5 (1966) No. 8
[ * ] The size of a complex is expressed by the degree of condensation, i.e. the number of metal atoms in the polyanion.
[4] L. G. Silldn, Acta chem. scand. 8, 299, 318 (1954); 10, 186,
8 0 3 (1956); 16, 159 (1962).
[5] R . Lowenherr, Z. physik. Chem. 18, 71 (1895).
3. Some polyanionic Systems
results can be obtained by salt cryoscopy. The recent
criticism [61 of this method therefore seems unjustified.
Ultracentrifugation has also proved a very successful
means of measuring the degree of condensation. This
method can also be applied to electrolytically dissociated compounds, provided an excess of a foreign
electrolyte [71 is added to ensure complete decoupling of
the oppositely charged ions. Like the cryoscopic measurements, determinations using the ultracentrifuge give
only average degrees of condensation [*I, but this
method has the advantage that it is possible181 to
recognize whether the solution studied contains one
type of particle or a mixture of particles of different
A selection from the results obtained for polyvanadate,
polymolybdate, and polytungstate solutions by various
methods is shown in Table 1.
Table 1 . Degrees of condensation in a number of polyanionic systems.
Acidification x 01
the solution [a]
2.4- 2.6
deg. of
Salt cryoscopy
av. deg. of
av. deg. of
3 and 4 19,101
3.4 1151
3.96 1161
4.14-4.33 1171
3.77 1181
4 (191
4 [241
8.1 1151
9.58-9.85 1201
9.2-9.9 [I71
9.7-10 [I91
10 1 1 1 1
10-11 1251
6.1 I211
7 [12,131
7.3 [Zl]
I .80
-6 [261, 7[27]
(a) The Vanadate System
Acidification of a vanadate solution leads first to the
formation of H V 0 4 2 - ions, which exist in equilibrium
with the dimeric
ions, the position of the equilibrium depending on the vanadium concentration. Further
acidification leads via H V 2 0 7 3 - 191 to metavanadate
ions (VO3);by a route that has not yet been fully
elucidated. A concentration-dependent equilibrium 1101
exists at this stage between v 3 0 g 3 - and V 4 0 1 2 4 - . The
trimeric ion is detected only in very dilute solutions.
Addition of further H+ leads to the decavanadate
H V 1 0 0 2 8 ~ - , and H ~ V 1 0 0 2 8 ~ - ,and
ions 1111 V 1 0 0 & ,
finally to the free acid H 6 V 1 0 0 2 8 . The transition from
the metavanadate to the decavanadate also proceeds by
an unknown route; a short-lived red intermediate is
observed in concentrated solutions (311. Free decavanadic
acid H 6 V 1 0 0 2 8 exists in equilibrium with a compound
V 2 0 5 . x H 2 0 of higher molecular weight 1321. On further
acidification, the monomeric cation V02+ is formed by
degradation reactions, the details of which are not yet
Depending on the acidification, the crystalline compounds obtained from polyvanadate solutions are
divanadates M 4 V 2 0 7 , metavanadates ( M V O 3 ) , , which
[9] N . Ingri and F. Brito, Acta chem. scand. 13, 1971 (1959).
[lo] F. Brito, N. Ingri, and L. G. Sill&, Acta chem. scand. 18.
-9 [261, 8 1271
1557 ( 1 964).
[II ] F. J . C. Rossotti and H. Rossotri, Acta chem. scand. 10, 957
20 1211
[I21 Y. Sasaki, I. Lindqvist, and L . G . Sillin, J. inorg. nuclear
Chem. 9, 93 (1959).
lresh soln.7
1221 Ibl
9.4 [22,23]
6 1141
Fresh s o h . 6
aged s o h . di1.6
aged soln.conc.
S o h . of a cryst.
11.6 1221
S o h . of a cryst.
I I .4 (221
12 1271
12 1291
S o h . of amorphous r - m e t a tungstate
!4 [30]
[a] The acidification x of a solution is the ratio of the number of
hydrogen ions to the number of monomeric oxometalate ions.
[h] Slowly established equilibrium between hexamers and dodecamers.
[6] R. S.Tobias, J. inorg. nuclear Chem. 19, 348 (1961).
[7] T. Svedberg and K . 0 .Pedersen: Die Ultrazentrifuge. Steinkopff, Dresden and Leipzig 1940, s. 21.
[*I The ultracentrifuge method gives a weight-average molewhereas salt cryoscopy
cular weight Mw = (&iMiZ)/(&Mi).
gives a number-average Mn = (CniMi)/(sni). The calculation
of the average degree of condensation from Mw and from M,
can lead to differences, which are particularly marked for mixtures o f particles of different sizes, e.g. a mixture of monomeric
and hexameric species.
[8] H. P . Stock, 2. Naturforsch., 206, 933 (1965).
The polyanionic systems listed in Table 1 are described
below in greater detail.
[I31 Y. SasakiandL.G.SillPn,Actachem. scand. 18, 1014 (1964).
[I41 Y. Sasaki, Acta chem. scand. IS, 175 (1961).
[I51 G . Schwarzenbach and G . Parissakis, Helv. chim. Acta 41,
2425 (1958).
[I61 K. F. Jahr and L. Schoepp, Z. Naturforsch. 146, 467 (1959).
[I71 0. Glemser and E. Preisler, Z. anorg. allg. Chem. 303, 303
[IS] K. F. Jahr, H. Schroth, and J . Fuchs, Z. Naturforsch. 186,
1133 (1963).
[ 191 A . W. Naumann and C. J . Hallada, Inorg. Chem. 3, 70 (1 964).
[20] K. F. Jahr and L. Schoepp, Z. Naturforsch. 146, 468 (1959).
[21] G . Wegener, Dissertation, Freie Universitdt Berlin 1964.
[22] H. Ressel, Dissertation, Technische Universitat Berlin 1961.
[23] I. Hapke, Dissertatim, Technische Universitdt Berlin 1955.
[24] H. P. Stock and K . F. Jahr, 2. Naturforsch. 18b, 1134(1963).
[25] 0. Glemser and E. Preiskr, Z. anorg. allg. Chem. 303, 316
[26] J . Aveston, W. Anacker, and J . S . Johnson, Inorg. Chem. 3,
735 (1964).
[27] 0. Glemserand W. Holznagel, Angew. Chem. 72,918 (1960);
0. Glemser, W. Holmagel, and J. S . Ali, Z . Naturforsch. 206,
192 (1965).
[28] 0. Glemser and W . Holtje, Z. Naturforsch. ZOb, 398 (1965).
[29] J . Aveston, Inorg. Chem. 3 , 981 (1964).
[30] 0. Glemser and W . Holtje, Z. Naturforsch. 206, 492 (1965).
[31] G . Jander, K. F. Jahr, and H. Witzmann, Z. anorg. allg.
Chem. 217, 65 (1934).
[32] K. F. Jahr, J . Fuchs, and F. Preuss, Chem. Ber. 96, 556 (1963).
A n g e w . C h e m . internat. Edit.
1 Vol. 5 (1966) i No. 8
have a chain structure, decavanadates M6V10028,
M5HV10028, and M4H2V10028, and “hexavanadates”
( M ~ V ~ O Iwhich
~ ) ~ have
a layer s?ructure[33,341(M =
univalent metal). Nearly all vanadates contain water of
(c) The Tungstate S y s t e m
Acidification of tungstate solutions leads first to the
rapid formation of a hexameric [381 ion, the paratungstate ion A [391.
6 WO42- -1- 7 H ’
(b) The M o l y b d a t e S y s t e m
Diffusion experiments and salt-cryoscopic and spectrophotometric measuremints show that molybdate solutions contain condensed ions after the addition of even
small quantities of acid. The existence of Mo70246may be regarded as certain; this anion is formed in
accordance with
7 M004-7-
( b)
4 HzO
and hence has its maximum stability when the acidification x is 1.14 (8Hf/7Mo042-). The average degree
of condensation increases rapidly at acidifications
higher than 1.50. Figure 1 shows the average degree of
condensation P of acidic molybdate solutions as found
in our salt-cryoscopic measurements 1211.
[ H W ~ O ~ ,aq]j,X
+ (3-X) HzO
(c) [*]
This ion slowly comes to equilibrium, via at least one
intermediate 1401 (the paratungstate ion B [do]), with a
dodecameric ion [22,29,301 (the paratungstate ion Z [**I
[W12036(0H)lo]lO- or [W12040(0H)2]10-). In a still
slower reaction, part of the equilibrium mixture is converted into monotungstate WO42- and “true” metatungstate [W12038(OH)2]6-. Rapid addition of 9 HI, to
6 WO42- (x = 1.50) leads via [HW6021.x aq]5- to another
hexatungstate ion (the metatungstate ion A [41”HW6020.
x aq]3-, which changes very slowly, again via at least
one intermediate, the y-(pseudo-) metatungstate
ion [39,41,41al, into the dodecameric “true” metatungstate ion.
The crystalline products obtained from acidic tungstate
solutions are paratungstates 5 M20.12 WO3mH20 and
metatungstates 3 M20.12 W03mH20. A sodium salt
having the composition Na20.2W03.5 H20 1421 is also
known. The addition of Kf or Ba2+ to y-metatungstate
solutions leads to amorphous precipitates having the
composition K20 or Ba0.4 WO3.nH20 1391.
4. Determination of the Structure
F i g . 1. Average degree o f condensation p of polymolybdate ions as a
function of the acidification x , f r o m salt-cryoscopic measurements 121 1.
EyP (351, who found that the degree of condensation decreased
again at x > 1.14, evidently failed to m ak e adequate allowance
for t h e influence of hydrogen ions o n t h e freezing point
depression. T h e main difficulty in th e elucidation o f t h e
molybdate system is th e fact th at several polyanionic species
exist in every pH range, in rapidly established equilibria.
Crystalline products obtained from acidic molybdate
solutions include paramolybdates 3 M20.7 Mo03.nH20,
“trimolybdates” M20.3Mo03.nH20, and nietamolybdates M20.4Mo03.nH20 (M = univalent metal).
Numerous products ‘361 with still higher molybdenum
contents have been isolated from more strongly acidic
solutions. However, it is not yet certain whether these
are single compounds.
1331 S. Block, Nature (London) 186, 540 (1960).
[34] A. D. Keliners, J. inorg. nuclear Chem. 21, 45 (1961).
[35] J. By@,Ann. Chimic 20, 463 (1945).
[36] Gmelins Handbuch der anorganlschen Chemie, System
Nr. 53, 8th. Edit., Verlag Chemie, Berlin 1935, p. 113.
Angew. Chem. internat. Edit.
1 VoI. 5 (1966) 1 No. 8
The aim of further investigations must be to determine
the structures of the polyions present in solution, and
to discover how they are interconverted. Since the
structure of dissolved particles cannot be determined
directly, except in a few particularly favorable cases,
this aim can only be achieved by a combination of
The elucidation of structures in many fields of chemistry
has been greatly assisted by models developed on the
basis of theoretical considerations. However, attempts
to develop a structural model for polyanions generally
fail simply because the compositions of the ions are not
[37] Cf. D. L. Kepert, Progr. inorg. Chem. 4, 199 (1962).
[38] H . Schulz and G. Jander, 2. anorg. allg. Chem. 142, 141
(1927); G. Jrmder, D . Mojert, and Th. Aden, ibid. 180, 129 (1929);
K . F. fahr and H. Witzmann ibid. 208, 145 (1932).
[39] P . Souchay, Ann. Chimie 18, 61, 169 (1943).
[*] aq = structural water, cf. Section 4.
[40] G. Ewald, DiplomaThesis, Humboldt-Universitiit Berlin 195 1.
[**I The nomenclature used is that proposed by D . L. Kepert [37].
Paratungstate B was formerly called paratungstate A2, and
paratungstate Z was called paratungstate B.
1411 G. Jaizrler and U . Kruerke, Z . anorg. allg. Chem. 265, 244
(I95 I).
[41a] ‘k’-Metatungstate solutions probably contain several species,
mainly hexamers in dilute solutions, and mainly more highly
aggregated particles, regarded by C . Jander and D. Exner as
dodecamers, in concentrated solution [43]. According to more
recent studies [43a], however, the latter are more likely 24-fold
aggregates, since particles of ionic weight 6450 have been found
in a solution of the amorphous potassium ‘1’’-metatungstate.
[42I E. L . Sintons, Inorg. Chem. 3, 1079 (1964).
69 1
accurately known, since the degree of condensation and
the charge are not sufficient to give the exact composition of an ion. Since polyanions may contain structural
water (aq), which forms neutral OH groups with the
oxygen atoms present, the number of oxygen and
hydrogen atoms in the ion cannot be found directly.
For example, it is impossible to decide from investigations in aqueous solution whether paratungstate A has
the composition [W602o(OH)15- or e.g. tW6019(OH)31~(also written as [HW6021.H20]5-). Even in crystalline
compounds, it is often difficult to tell whether or not
structural water is present. Even isobaric or isothermal
degradation and the study of infrared spectra do not
always lead to an accurate answer. Probably the greatest
promise is shown by nuclear magnetic resonance
measurements [441, but until more experience has been
gained, the information obtainable will remain qualitative.
The development of structural models for polyions is
further complicated by the fact that the coordination
number of the metal ions towards oxygen is unknown.
The ionic radii of V5+ (0.59 A), Mo6+ (0.65 A), and
W6f (0.65 A) permit either tetrahedral or octahedral
2:6:2. However, this result cannot be reconciled with the
decavanadate structure found by Evans jr., Swallow, and
Barnestsol for Ca3VloO~s.16H20and K2Zn2V10028.16H20.
Other investigations (cf. Section 8a) indicate that the structure
of decavanadate ions in solution differs from that in crystals.
It is also possible that not all the crystalline decavanadates
contain the same polyanion 1511.
Unfortunately, very few X-ray structure investigations have
been carried out o n salts of polyacids; moreover, some of the
existing investigations are incomplete, since only the heavy
metal positions, but not the positions of the oxygen atoms
could be determined. The great scattering power of heavy
metal atoms (particularly tungsten) for X-rays makes it
extremely difficult to locate the oxygen atoms. Other properties of the salts of isopolyacids, e.g. large unit cells, poor
crystallizing power, and tendency towards twinning, also
make them particularly difficult subjects for X-ray structure
analysis. Fig. 2 shows the structures of isopolyacids that have
been studied by the X-ray method.
5. Importance a n d Results of X-Ray
Structure Investigations
The basis of structure determinations is therefore X-ray
studies on crystalline compounds. However, it must be
established in every case that the structure of the polyanions in solution is the same as, or at least very
similar to, that of the crystalline compounds. This
equivalence cannot be assumed. For example, crystalline
metavanadates contain long chains of linked V 0 4 tetrahedra [451 or V O 5 units 1461 (trigonal bipyramids),
whereas the aqueous solution contains tetravanadate or
trivanadate ions (cf. Figs. 2a and 2b).
The structures in the crystal and in solution can be compared
with the aid of Raman and infrared spectrography and N M R
measurements. Thus Aveston 126,291 found close agreement
between the Raman spectra of crystalline and dissolved
heptamolybdates and paratungstates Z.
The infrared spectra of crystalline and dissolved polytungstates were studied by Spitzyn et al. 147,481. However, no
definite conclusions could be drawn, since agreements were
found as well as differences. It was concluded from N M R
measurements on decavanadate solutions
that three
differently bound vanadium atoms were present in the ratio
[43] G. Jander and F. Exner, Z . physik. Chem. A 190, 195 (1942).
[43a] 0. Glemser, W. Holznagel, W. Hiiltje, and E. Schwarzmann,
Z. Naturforsch. 206, 725 (1965).
[44] Cf. 0. Glemser, Angew. Chem. 73, 785 (1961).
[45] H . T . Evans j r . and S. Block, Amer. Mineralogist 39, 327
(1954); H . T . Evans jr., Z. Kristallogr., Kristallometr., Kristallphysik., Kristallchem. 114, 257 (1960).
[46] C. L. Christ, J. R. Clark, and H . T. Evans jr., Acta crystallogr.
7, 801 (1954).
[47] V. I. Spitzyn and V. J . Kahanow, Z . anorg. allg. Chem. 322,
248 (1963).
[48] A . A. Babushkin, G . V . Yuknevich, Yu. F. Berezkina, and
V. I. Spitzyn, Russ. J. inorg. Chem. 4 , 373 (1959).
[49] 0. W. Howarth and R . E. Richards, J. Chem. SOC.(London)
1965, 864.
2. Anions of the salts: (a) K V 0 3 [451, (b) KVOyH20 1461,
K zZ n2V lo028~16H 20and Ca,Vl0028.16H20 [501, (d) K ~ V 6 0 1 6[331,
(e) (NH4)6M07024.4HzO [521,
( N H ~ ) ~ M o ~ O Z ~ . [52l,
(g) N ~ ~ O W I ~ O ~ ~ ( O H
) ( O-. ~(h)"True"
~ H ~ O metatungstate ion
[HzW1204u16- (isomorphous with H3PW1~040.5
HzO) [541.
Lipscomb 1551 has questioned the correctness of the paratungstate Z structure (Fig. 2 g ) . Lindqvist determined only the
tungsten positions, and the positions of the oxygen atoms
[50] H . T . Evans j r . , A . G . Swnllow, and W:H. Barnes, J. Amcr.
chem. SOC.86,4209 (1964).
[51] A . M . Bystruem and H.T. Evans jr., .4cta chem. scand. 13,
377 (1959).
[52] I. Lindqvist, Ark. Kemi 2, 325, 349 (1950).
[53] I. Lindqvist, Acta crystallogr. 5 , 667 (1952).
[54] J. F. Keggin, Nature (London) 131, 908 (1933); 132, 351
(1933); Proc. Roy. SOC.(London) A 144, 7 5 (1934); J . W. Illingworth and J. F. Keggin, J. chem. SOC.(London) 1935, 575; R. Signer and H . Gross, Helv. chim. Acta 17, 1076 (1934).
[55] W. N . Lipscomb, Inorg. Chem. 4 , 132 (1965).
Angew. Chem. internat. Edit.
1 Vol. 5
1 No. 8
observed X-ray intensities agree equally well with another
tected, but also a short-lived (10-2 sec) polytungstate
structural model (Fig. 3) containing fewer oxygen atoms
ion, which is probably tetrameric, and which the authors
(W12O42 instead of W12046).Thus the formula of the crys- ~ H ~ O as H5W40163-[571. It is interesting in this
talline paratungstates Z would be M ~ O W I ~ O ~ , ~ ( O H ) ~ O formulate
connection to note that Hiillen [581 detected a tetraaccording to Lindqvist, and MloW12040(OH)2*nH20 according to Lipscomb. The results obtained in the isobaric degradatungstate ion as one of the building units in hydrated
tion of potassium paratungstate 5K20-12W03.11 H 2 0 and
lithium tungstate Li2W04.417 H20 by X-ray structure
from the infrared spectra of its degradation products [a71
analysis (see Fig. 4).
agree more closely with Lipscomb’s formula (only one mole
of structural water).
7. Salt Formation i n Organic Solvents b y
Hydrolysis of Esters of Metal Oxoacids
Fig. 3. Structure of the anionofsodiumparatungstateNaloW1204o(OH)z,
according t o Lip’comh [ 5 5 ] .
The data available at present are insufficient to show how
these polyions are formed in solution and how they are
interconverted. It is therefore extremely important to detect
as many as possible of the intermediates, which must exist,
but which are not yet known, since they occur only transiently or only in very low equilibrium concentrations.
6 . Detection of Intermediate Polyanions
Some possibility of detecting short-lived intermediates
is offered by a method described by Meier and Schwarzenbach 1561. The acidification of vanadate and tungstate
solutions was followed potentiometrically in a flow
apparatus permitting very rapid mixing of reaction
solutions (in about 5 msec). Since condensations proceed much more slowly than protonations and deprotonations, these measurements were intended to
detect the protonation stages of anions and polyanions
with a minimum of interference from accompanying
A method for the preparation of salts of polyacids previously not obtainable from aqueous solution was discovered by Jahr, Fuchs, and co-workers [591. Complete
hydrolysis of an ester of a metal oxoacid dissolved
(generally as the monomer) in organic solvents leads
normally to a highly condensed metal oxide hydrate.
The condensation proceeds via the same intermediates
as in the acidification of the metalate solution. The
intermediate polyacids can be intercepted as salts if the
organic solution contains a base, a salt, or a buffer
mixture. This method of precipitating salts of polyacids
from organic solvents, which involves only very small
quantities of water, is very useful not only from the
preparative point of view, but also for the investigation
of structures, since it is possible in this way to obtain
anhydrous compounds. It is then very much easier to
determine whether the polyanions contain structural
water, and if so, how much. The method also offers
new preparative possibilities, since solubility relationships in organic solvents are different from those in
water. Polyanions that have been definitely detected in
aqueous solution, but of which no crystalline salts have
so far been obtained because other polyanions present
in the equilibrium form sparingly soluble salts, can be
obtained as crystalline salts in this way.
Thus the only previously known crystalline paratungstates
were of the 2 type, with dodecameric anions. Hydrolysis of
tetramethyl tungstate(vI), WO(OCH&, in the presence of
piperidine has now led to a pentapiperidinium hexatungstate [59a-611, (C5H1lNH)5HW6021, or ( C ~ H I I N H ) ~ H ~ W ~ O ~ ~ ,
the reactions of which show that it contains the paratungstate
A ion. The corresponding ammonium and pyridinium salts
can also be prepared in this way. We have also obtained salts
of paratungstate ions B, which occur as intermediates in the
conversion of paratungstate A into paratungstate Z, by
hydrolysis of tetramethyl tungstate (VI) in the presence of a
solution of potassium or ammonium acetate in methanol [621.
The X-ray diagrams of ammonium paratungstates A. B,and Z
are shown in Fig. 5 .
I571 G. Schwarzenbach and J . Meier, J. inorg. nuclear Chem. 8,
302 (1958); G. Schwnrzenbach, G . Geiei., and J . Littler, Helv.
chim. Acta 45, 2601 (1962).
[581 A . Hiillen, Naturwissenschaften 5 / , 508 (1964). We thank
Dr. Hiillen, Berlin, for this illustratio.1.
[591 K . F. Jahr and J . Fuchs, Chem. Ber. 96, 2457 (1963).
Fig. 4. Structure of LirWo4.4/7 H ~ O1581.Large spheres: ~ ~small
spheres: Li’ ; tetrahedra: WO$-; group of four octahedra: W 4 0 L h 8 - .
[561 J . Meier and G . Schwarzenbach, Helv. chim. Acta 40, 907
Angew. Chern. internat. E d i t .
1 VoI. 5 (1966)
No. 8
[59a] X-ray studies on this salt are being carried out in our
institute by A . Hiillen and G. Henning.
1601 K . F. Jahr, J . FlXchs, and P. Witre, Angew. Chem. 76, 581
[611 K. F. Jahr, J . Fuchs, P . Wive, and E. P. F/indt, Chem. Ber.
98, 3588 (1965).
[62] J . Fuchs, K . F. Jahr, and E. P . Flindt, unpublished work.
I bl
Fig. 5 . Relative X-ray intensities ( C I J K ~radiation) of (a) ammonium
paratungstate A, (b) ammonium pararungstate B, (c) ammonium
paratungstate Z (Analytical composition: 5 (NH4),0.I2 W O y x solvent.
A and B contain methanol; Z contains water).
Although the compositions are practically identical
[5(NH4)20.12WO3.x solvent; the A and B salts contain
methanol, and Z contains water], the compounds differ
considerably in their chemical properties. Thus a freshly
prepared aqueous solution of ammonium paratungstate Z is
neutral, whereas solutions of paratungstates A and B are
distinctly acidic. Unlike the other two salts, the paratungstate A is very soluble and reacts vigorously with
Hz02 [631, the resulting solution being strongly acidic.
Salt-cryoscopic measurements indicate that the paratungstate
ion B is hexameric, i.e. has the composition [HW6021.X
aqls-. It is not yet known whether the difference between
paratungstates A and B is due to a change in the content of
structural water or to a rearrangement. Interestingly, a
further hydrogen in the paratungstate ion B can be replaced
by metal ions to give [W6021’X aq]6-. The hydrolysis of
tungstate esters in the presence of potassium acetate in
tthanol yielded a salt having the ratio 6 K 2 0 : 12 WO3; the
X-ray diagram of this salt shows that its structure is practically the same as that of the paratungstate B obtained from
methanolic solution, in which the ratio is 5 K 2 0 : 12 W03.
The infrared spectra of 6 K20.12 W03.x H20 are so similar
to those of the Na20.2 wo3.5 HzO obtained from aqueous
solution that we also regard the latter as a salt of the same
hexatungstic acid, and denote it by the formula
“a6w6021.X aq].(l5-x)H20.
Another advantage of the use of organic solvents for the
preparation of salts of polyacids is that compounds
that are immediately hydrolysed in water can be
prepared. For example, Juhr, Fuchs, and Oberhauser 1641 prepared a tetra-c-butylammonium tungstate
[N(C4H9)4]20.6 WO3, which is more strongly acidic
than any of the polytungstates isolated so far from
aqueous solutions, and which is therefore an intermediate stage between metatungstate and the tungsten
oxide hydrates. The salt is insoluble in water, but
[63] K . F. Jalir and E. Lothrr, Ber. dtsch. chem. Ges. 71, 1127
[64] K . F. Jahr, J . Fuchs, and R . Oberhauser, unpublished work.
readily soluble in acetone, from which it can be recrystallized. If a little water is added to the acetone
solution, the polyanion decomposes and a tungsten
oxide hydrate separates out. The degree of condensation
of the anion is not yet known. The salt is obtained
from acetone as very well-formed, highly refractive
crystals [59al.
Interesting results have also been obtained in the
study of the hydrolysis of a molybdate ester. Hydrolysis of the diethyl molybdate - ammonia adduct
3 M o 0 2 ( O C 2 H 5 ) 2 . 2 N H 3 [651 in an ethanolic solution
of ammonia and ammonium chloride led to an ammonium trimolybdate of the composition 2(NH4)2O.
6 M o O 3 . H z O 1661. The structure of the “trimolybdates”
is not yet known. The poor crystallizability of these
compounds and their tendency towards twinning have
so far made an X-ray structure elucidation impossible.
Lindqvist1521 believes that the unit cell of the “trimolybdates” contains heptamolybdate [Mo7024]6- and
octamolybdate ions [MOs026]4- in the ratio 1: 1. This
structure is certainly not correct for the salt prepared
by the hydrolysis of esters, since it must be assumed
from the preparation conditions that the water present
in the compound, the quantity of which can be accurately determined from the water consumed during
hydrolysis, is structural water, i.e. that it is present in
the form of OH groups. Though the degree of condensation of the polyanion is not given unambiguously
by the analytical results (it could also be a multiple
of 6), it is very probable from the results of these and
other experiments(seeSection8) that the salt is a hexamolybdate ( N H 4 M H 2 M o 6 0 2 1 1 or ( N H ~ ) ~ [ M o ~ O I ~ ( O H ) ~ I .
A particularly thorough study has been made of the
hydrolysis of t-butyl orthovanadate, (C4H9)3V04, in the
presence of bases [67-691. All types of polyvanadates that
can be obtained from aqueous solution can also be
prepared in organic solvents. The strength of the base
present during the hydrolysis determines whether the
product obtained is a divanadate, a metavanadate, a
decavanadate, or a hexavanadate, the deciding factor
in aqueous solution being the pH. In addition to the
above salts, a hydrogen divanadate K 3 H V 2 0 7 1681 has
been obtained by the hydrolysis of t-butyl orthovanadate
in the presence of potassium t-butoxide; this confirms
the existence of the H V 2 0 7 3 - ion, discovered by lngri
and Brito 191 in equilibrium studies by Sille‘n’s method.
8. Complex Formation with Polyanions
(a) Complex Formation with Polyvanadate Ions
Salt formation is not the only means by which a polyanionic species can be detected in an equilibrium
mixture. Many polyanions form complexes with cations
1651 A . Nebelung and K. F. Jahr, Z. Naturfnrsch. 15b, 654 (1964).
[66] A . Nebelung, Dissertation, Freie Universitiit Berlin 1965.
[67] J , Fuchs and K. F. Jahr, Chern. Ber. 56, 2460 (1963).
1681 J . Fuchs, K . F. Jahr, and A. Nebelung, Chem. Ber. 58, 3582
[69] J . Fuchs, K . F. Jahr, A . Eberhard, and F. Preuss, Chem. Ber.
98, 3610 (1965).
Angew. Chem. internat. Edit. 1 Vol. 5 (1966)
1 No. 8
in solution. In many cases the polyanion very probably
retains its structure and acts as a polydentate ligand, so
that information about the size of the polyanion can be
obtained from the composition of the complex. Even
the fact that complexes are formed at all sometimes
gives an indication of their structure. During the
titration of a sodium decavanadate solution with a
solution of a neutral salt of a bivalent or trivalent
cation, for example, we observed 1321 that the conductivity of the solution does not increase linearly.
Very strong complex formation takes place, particularly
with trivalent cations. From the titration curve, formation of a 1: 1 complex was deduced. The extent of
complex formation depends on the radius as well as the
charge of the cation, and increases with increasing
ionic radius. A particularly interesting effect is that
resulting from the presence of a neutral salt during the
titration of free decavanadic acid H6V10028 with alkalis.
Figure 6 shows the course of a conductometric titration
of free decavanadic acid with NaOH in the absence and
in the presence of strontium nitrate.
Only a few symmetrical structures are conceivable for a
decavanadate ion having the formula [v10028]6- if the central
cavity is assumed to be large. The only structural units
possible in this case are vo4 tetrahedra, since V06 octahedra
or VOs units give only very compact structures with no central
cavity. The structure proposed [321 for the decavanadate ion
in solution is shown in Fig. 7.
Fig. 7. Proposed structure of VloOz& in solution [321.
(b) Complex Formation with Polymolybdate Ions
Fig. 6. Conductometric titration of 10 rnl o f HaVloOzs (0.01 M with
respect t o VzOs) with 0.05 N N a O H [curve (a)] [321, and a corresponding
titration after addition of 1 nil of 0.1 N Sr (N0 3 )2 solution [curve (b)].
Curve (c) shows a section of (b) with a IS-fold increase in sensitivity.
In the presence of Sr2+, decavanadic acid consumes
but 8 instead of 6 equivalents of OH-. Similar changes
in the titration curve are caused by other metal ions.
When trivalent cations (Al3+, La3+, Ce3+) or Be*+ are
present, 9 OH- are consumed.
Our interpretation of this phenomenon is as follows. The
decavanadate ion probably is approximately spherical with a
cavity in the center. A hydrated cation can enter this cavity,
where it is fixed by complex formation. Owing to the field
effect of the oxygen atoms surrounding the cation, protons
are removed from the enclosed [M(H20),]2+ and can be
neutralized by OH-. This leaves a monomeric hydroxide or
an oxometalate ion, e.g. [Be(OH)3]-, in the interior of the
complex. On the basis of this interpretation, the titration of
H6Vi0028 in the presence of e.g. SrZt may be formulated as
shown in the following equations:
Angew. Chem. internat. Edit. 1 Vol. 5 (1966)
No. 8
If a few drops of methyl red are added to a molybdate
solution of acidification 1.5, the color of the solution
changes from red to qellow after a short time[70,711.
Surprisingly, the solution remains yellow when strong
mineral acid is added. Conductometric titrations show
the formation of a complex containing methyl red and
molybdenum in the ratio 1: 12. The yellow color of the
methyl red in the strongly acidic solution can be understood only if it is assumed that the indicator molecule
is bound in the center of a dodecamolybdate ion, and
that the situation is similar to that in conventional
inclusion compounds of indicators with cyclodextrins [7*,73J, in which the indicator also does not exhibit
its n x m a l color.
The formation of complexes thus indicates that molybdate
solutions of acidification above 1.5 contain dodecameric
anions which, like the decavanadate ion, are approximately
spherical, hollow, and can form inclusion complexes.
Naturally, nothing can be said about the concentration of
polyanions in the solution. It is quite possible that only a
small part of the molybdenum is in the form of dodecameric
particles, and that it is only the formation of the relatively
stable complex that displaces the equilibrium in favor of this
polyanion. The rate of formation of the "inclusion complex"
is greatest in solutions of acidification x = 1.65 to 1.7. The
yellow solution becomes red again on addition of concentrated phosphoric acid, which evidently displaces the
indicator anion to form a more stable complex. The methyl
red complex is also rapidly destroyed by alkalis, owing to
breakdown of the polyanion. It is possible, by very careful
addition of OH-, to achieve the apparently paradoxical
[70] C. Tliieme, Dissertation, Freie Univzrsitst Berlin; see here
for the U V spectra.
[71] K. F. J.7hr, Angew. Chem. 76, 236 (1964); Angew. Chern.
internat. Edit. 3 , 320 (1964).
1721 F. Cranzer : EinschluBverbindungen.Springer, Berlin-Gottingen-Heidelberg 1954, p. 87.
[73] PV. Broser, Z. Naturforsch. 86, 722 (1953).
result that the color of the solution changes from yellow to
red, since the dodecamolybdate ion is destroyed at a pH at
which methyl red is normally red. The yellow indicator anion
is then formed when an excess of OH- is added.
W e have carried out a n intensive study of the complexing power of molybdate solutions of acidification less
than 1.50 towards cations and towards acids that can
form heteropoly compounds [74,751. Many cations can
react with polymolybdate ions in solution t o form
complexes, nearly all of which are very unstable. Al3f
forms a very stable complex having an AI:Mo ratio
of 1 :6. The complexes formed by orthotelluric acid and
periodic acid are still more stable.
In this case, too, a relation is apparent between the
stability of the complex and the ionic radius of the
complex former. The radii of Al3+ (0.55 A), Te6+
(0.56 A), and I7+ (0.50 A) are very similar. These
complexes, which are formed in relatively weakly acidic
solution, require extensive measurements for the determination of their compositions, since the complexing
equilibrium depends not only on the concentration of
ligand ratio, but also to determine the charge on the
hydrogen ion concentration. However, the dependence
of the equilibrium on pH also has the advantage that it
is not only possible t o determine accurately the nucleus:
ligand ratio, but also t o determine the charge o n the
complex a n d to establish the occurrence of protonation
stages. We have elucidated these reactions t o a large
extent [751 by simultaneous thermometric, conductometric, and potentiometric titrations.
The methods used will be illustrated for the complex formation between periodic acid and molybdate ions. When an
HMoO4- solution (x = 1.00) is titrated with periodic acid,
the thermometric and conductometric titration curves contain
discontinuities and the potentiometric curve contains inflections at the molar ratios H ~ I 0 6 / H M o 0 4 -= 1 :7 and 1 : 3.
However, it would be erroneous to conclude that complexes
with the ratios I: Mo = 1:7 and 1 :3 have been formed. In
fact, the discontinuities are due to a 1 :6 complex [IM06024]~which is initially formed by only a part of the molybdenum
[reaction (e)].
One-seventh of the molybdate cannot enter into complex
formation at first, owing to the deficiency in H+.
An interesting phenomenon is observed in this titration.
Owing to the consumption of H+ and the consequent conversion of HMoO4- into MOO& the pH initially increases
rapidly for a time during the titration, although the titrant is
the relatively strong periodic acid.
Since periodic acid itself can supply H+ [*I, the remaining
molybdate is finally converted into the complex ion [reaction
(f)]. If the titration is carried out at different initial acidifi-
cation, the discontinuities and inflections occur at different
values of the ratio H5IO6/MoO42- [*I. The composition of
the complex can be found by mathematical treatment of the
results obtained when molybdate solutions of various
acidifications are titrated with periodic acid.
The complexing processes can in general be formulated as
n MoO42-+ p H - + m H510,j
[ImMonOs](2s-6n-7m)- (5m t p)/2 HzO
where s = 4n-(7m-p)/2
If the acidification x of the molybdate solution is varied and
the atomic ratio I/Mo = Z determined for the initially incomplete complex formation (when all the H+ present have
been consumed) and for the final complex formation (when
all MoO42- is converted into complex), it is found that
Z = mx/p and Z = -x + (m + p)/n, respectively; it is then
possible to establish sets of integral values for m; p; n.
The values found for complex formation with periodic acid
are 6; 7; 1 and multiples of this set, such as 12; 14; 2. These
sets give Equations (h) and (i).
The question of whether the mononuclear complex
[IM06024]5- or a polynuclear complex is formed cannot be
answered on the basis of the titrations; with the ultracentrifuge, however, Stork [761 obtained a value of 1 1 10 for the
ionic weight of the complex ion (Mtheor. for [IM06024]5- is
1086.5), showing that the reaction proceeds according to
Eq. (h). Other experimental points can be easily interpreted
by the formation of the protonation stages [HIM06024]~-,
[H2IM06024]3-, [H31M06024]2-, and [H4IM06024]-.
Complex formation between polymolybdate ions and
orthotelluric acid is more versatile than the reaction
with periodic acid. Salts having the composition
M6TeMo6024mH20 have been known for some time 1771,
complex formation in solution, o n the other hand, was
first studied in 1963 by Gross [741. Conductivity measurements showed that in addition t o the 1:6 complex,
other complexes with different Te: M o ratios occur. By
spectrophotometric studies [781 (molar ratio method)
and by salt-cryoscopic titrations [791 we found the
Te: M o ratios in these compounds to be 1 :7 and 1 : 12.
The charges o n the complexes were determined by
simultaneous thermometric, conductometric, and potentiometric titrations. The ions found in addition t o
[TeMo6024]6- were [HTeMo7028]7- (or [TeMo7027( 0 H ) P ) and [H6TeM0120481~~-(or [TeM012042(oH)6]12-). The ionic weight of the 1:6 complex was
determined with the aid of the ultracentrifuge [801
(Mexp,, 1163; Mtheor, for [TeM06024]~-, 1087.3).
Information about the relative stabilities of the complexes was obtained by salt-cryoscopic titrations.
[*] This example shows clearly th: erroneous conclusions that
can be reached in the study of complex formation unless measurements are performed with a sufficiently wide range of starting
[76] K . F. Jahr, J. Furhs, and C . Wiese, Z . Naturforsch. Z lh , I 1
[%4]K . F. Jahr, J. Furhs, M . Gross, and L. Klingebiel, unpublished
[75] G. Wiese, Dissertation, Freie Universitat Berlin, 1965.
[*I Periodic acid acts as a monobasic acid throughout the pH
range investigated (2.5 to 7.5).
[77] Cf. [36], p. 322.
[78] E. Cegner, Diploma Thesis, Freie Universitat Berlin 1963;
K. F. Jahr and E. Gegner, unpublished work.
1791 H . Schroth, Dissertation, Freie Universitiit Berlin 1964.
[80] H . G. Biibam, Diploma Thesis, Freie Universitat Berlin 1964.
Angew. Cheni. internat. Edit. J Vol. 5 (1966)
/ No. 8
[HM0207.x as]-. [M04013.x aqlz-,
[ M o ~ O Z ~aq14. X [Eq. &)I.
[HMo6020.~ aq13-,
6 MoO42-$
y H20
[HMO60zo.X H2Ol3-+ (y x f 4) H20
The evaluation of the results gives the
[H&1M06024]3- for the aluminum complex.
Fig. 8. Relative freezing point depression AT in the salt-cryoscopic
titration of 2.5 in1 of 0.3 M N a l M o 0 4 solution having a n acidification
1.14 with 0.06 M HnTeOs, mea3u;ed in the Glauber's salt-ice
It can be seen from Figure 8 that the 1 : 6 complex is the
most stable. The freezing-point depressions obtained
almost[*]coincided with those expected for[TeMo6024I6although the measurements, which were carried out in
the Glauber's salt-ice eutectic, had to be made at a
very low concentration ( = 0.01 g-atom of Mo/l) because
of the low solubility of sodium hexamolybdatotellurate.
The eutectic freezes at -1.125"C. The 1:7 and 1:12
complexes are more strongly dissociated than the 1 :6
complex. It is very interesting to observe how the
stability of the 1 :6 complex depends on the acidity of
the molybdate solution (Fig. 9).
Fig. 9. Relative freezing point d e p r e s s i o n l r produccd by [TeMo60:4]6i n the Glauber's salt-ice eutectic, as a function of the acidification x of
the inolybdats solution.
The stability of this complex, which corresponds formally to the acidification x = 1.OO, increases once more
above x = 1.33 (Eq. (j)]. This can be explained by the
assumption that the concentration of the ligands that
react with the orthotelluric acid is lower at x = 1.33 than
in more acidic (x = 1.50) and less acidic (x = 1.14)
solutions (see below).
This assumption is supported by observations made during
the study of complex formation between polymolybdate ions
and Al3+. The curves obtained by thermometric, conductometric, and potentiometric titrations of acidic molybdate
solutions with AIz(SO& solution show not only changes in
slope indicating completion of complex formation, but also
discontinuities and inflections that must be due to other
causes. The results can be explained if it is assumed that the
Al3+ can react only with a certain polyanion, which itself
corresponds formally to x = 1.50. Such a polyanion would
have t o carry one-half of a negative charge per Mo atom, e.g.
["I The expected freezing-point depression was 0 . 2 5 9 ~10-2. The
results of the salt-cryoscopic titration give only the relative
change in the number of particles. To find the absolute degree of
condensation, series of measurements must be carried out over a
wide range of concentrations.
Angew. Chem. internal. Edit.
Vol. 5 (1966) J No. 8
The results obtained in the investigation of complex
formation and hydrolysis of molybdate esters can be
readily explained and related to many other facts if it is
assumed that the acidification of a molybdate solution
leads to consecutive reactions:
The initially formed HMoO4- ions rapidly form a cyclic
polyanion containing 6 Mo atoms. The HM004- ions
may be held together by hydrogen bonds; the formula
of the particle would then be [H6M06024]6-. In contrast
to the tungstate system, in which a polyanion containing
6 W atoms is stable, the hexamolybdate ion is stabilized
by combination with an ion of suitable radius. Thus in a
pure molybdate solution, the heptamolybdate ion
[Mo7024]6- is formed. In the presence of H5IO6 or
Te(OH)6, on the other hand, the heterocomplexes
[IM06024]5- and [TeM06024l6- or [H6TeMo120481~~are formed. We regard [H6TeM01204&- as a complex
in which the Test is situated between two Mo6024
rings, as in a sandwich compound. The reason for the
greater stability of [TeMo6024]6- in relation to
[MoMo6024]6- (heptamolybdate) is probably that the
smaller Test ion fits into the hexamolybdate ring even
better than Mo6+.
This view agrees closely with the results obtained
by Evans[*ll in the X - ray structure analysis of
(NH&TeM06024.7 H20, according to which the Te6+
ion is situated in the center of a ring of six Moo6
octahedra, whereas in [Mo7024]6- the seventh Mo atom
lies outside the plane of the ring, so that this particle
has a "bowl shape" (Fig. 2e). This shape evidently
enables the formation of the complex [HTeMo702817-.
In view of the unsymmetrical structure of the heptamolybdate ion, which according to Aveston [261 probably
has a similar structure in solution to that in the crystal,
the tendency towards complex formation is understandable. Many compounds of composition
MiM"M07024 and M:M111M07024,
which could be regarded as purely salt-like, probably
are complexes. For example, from the color of
(NH4)3Ce111M07024.24 H20,which is orange-yellow
even in solution, although Ce3+ is colorless, Barbieri [*21
assumed it to be a complex.
As was also found by Sasaki[lz,131 with the aid of the
Sillen method, the heptamolybdate ion is stable over a
wide range of acidifications, and exists in solutions
with x > 1.14 as the protonated species [HM07024]5or [H2Mo7024]4-. Complex formation between the
heptamolybdate ion and orthotelluric acid (1 :7 complex) has been detected even in solutions with x = 1.33
both by the simultaneous thermometric, conductometric, and potentiometric titrations and by salt[811 H . T . Evans j r . , J. Amer. chem. SOC.70, 1291 (1948).
[821 G. A . Burbieri, Atti Accad. Lincei (Roma) 17, 1, 540 (1908).
cryoscopic titrations). However, solutions with x = 1.33
also contain other types of particles, as has been shown
by ultracentrifugation experiments 1801. On the other
hand, the existence of (NH4)4H2Mo6021, which is
prepared by hydrolysis of molybdate esters, indicates
the occurrence of [H2M06021]~-.
The question of the existence of a definite polymolybdate
ion corresponding to an acidification x = 1.33 was
disputed for a long time, since no changes in slope were
observed in the curves obtained in potentiometric and
conductometric titrations of molybdate solutions with
acid (except by R. H . Saxena and G . P. Saxerza[831).
A change in slope can, however, be observed in accurate
thermometric titrations; we attribute this change to the
formation of the [H2M06021l4- anion, from which the
“trimolybdates” may be derived. We assume that the
heptamolybdate ion becomes less stable as a result of
protonation, and is partly converted into this hexamolybdate ion (which now has a different structure),
possibly according to
The complexing reactions in solutions with x < 1.50,
and particularly the reaction with Al3+, can be readily
understood if it is assumed that the anion [H3M06021]3is formed in these solutions by protonation of
[H2Mo6021]4-. This protonation also must be associated
with a change in structure, since it would otherwise be
impossible to explain the fact that Al3+ reacts with
[H3Mo6021]3- ([HMo6020.H20]3-), but
not with
[H2M06021]4-; the simultaneous thermometric, conductometric, and potentiometric titrations have shown
that complex formation with A13+ is preceded by
formation of a polymolybdate that corresponds to
x = 1.50 (cf. p. 697). Orthotelluric acid also fails to react
with [H2Mo6021]~-, but probably reacts with
[H6Mo6024]6- and [H3Mo6021]3-. This provides a
simple explanation for the stability minimum of
[TeMo6024]6- at x = 1.33.
I n addition to hexameric polymolybdate ions, the solution
with x = 1.50 must contain also dodecameric anions, since
it is otherwise impossible to explain the complex formation
with methyl red. It is not yet known whether this solution
contains appreciable quantities of octamolybdatt ions.
Many authors believe that the octamolybdate ion [ M 0 ~ 0 2 6 ] ~ is the major component of solutions with x = 1.50. The main
arguments for this view are the facts that this anion1521 is
present in crystalline ammonium metamolybdate, and that
the average degree of condensation of the solution is approximately 8. However, no really conclusive evidence has so far
been produced. On the basis of our views on the interconversions of polymolybdate ions, an octamolybdate would be
quite feasible, since a protonated heptamolybdate ion should
condense with a n HMo4- ion just as with a molecule of
orthotelluric acid. No octamolybdate ion has been detected
by complex formation. However, this ion probably cannot be
expected to form complexes if its structure is similar to that
envisaged by us for the [HTeMo7028I7- ion, i.e. a compact,
approximately spherical particle. Thus we can neither confirm
nor deny the existence of the octamolybdate ion.
No further details on the relationships in more strongly
acidic molybdate solutions (x > 1.50) are available at present.
[83] R. H. Saxeiia and G. P . Saxena, Z . physik. Chzrn. N.F. 29,
181 (1961).
(c) Complex Formation with Polytungstate Ions
Interesting results [84j have also been obtained in the
investigation of complex formation by polytungstates in
solution. The paratungstate ions A and B (which can be
produced by acidification ofW0;-solutions to x = 1.167)
react with bivalent and trivalent cations to form complexes having the atomic ratio M2+ or M3+: W = 1 :6.
The most stable complexes are those formed with cations
of radius M 0.75 ,8,, whereas polymolybdate ions prefer
cations having a radius of about 0.5 A. In solutions
containing paratungstate Z, the same complexes are
formed only after about 24 hours.
Complex formatim with cations (Mz+:W = 1 : 12) has
also been observed in solutions of free metatungstates,
and even in solutions of free metatungstic acid. The
intensity of the complex formation depends on the
charge and on the ionic radius, and decreases roughly
in the order: La3+ > Yb3f > AP+ M Ba2+ > Cd2f M
Sr2+ > Ca2+ M Cu2+ w Zn2+ w Mnzf w C02+ =
Ni2+ > Mg2f > Be2+ Csf.
No complex formation has been observed with Rbi or
the lighter alkali metals. Phosphoric acid does not react
with metatungstates or metatungstic acid. Metatungstate solutions prepared by acidification of WO42solutions to x = 1.50 react with cations only after
boiling for several hours or after prolonged standing
(about 200 days), i.e. only after “true metatungstate
ions” have been formed. On the other hand, freshly
acidified solutions react, though only weakly, with
phosphoric acid. This result provides further confirmation of Junder’s observation 1411 that acidification to
x = 1.50 does not lead immediately to the pseudotungstate ion, but results first in the formation of metatungstate ion A ; recently this observation has been repeatedly questioned. The metatungstate ion and the
pseudometatungstate ion evidently differ from the
paratungstate ions A and B, not only in the degree of
protonation, but also in their structures; otherwise, it
would be impossible to explain the failure of the
first two to form complexes with cations.
Paratungstate solutions (both A and 2) that have been
aged for a long time exhibit the complexing reactions
of the “true” metatungstate ion. This confirms again
that a slow “dismutation” takes place in paratungstate
solutions, with formation of the acidic metatungstate
ion and the basic monotungstate ion[85,861. These two
are probably the only thermodynamically stable tungstate ions.
9. Closing Remarks
Attempts to obtain information about the nature of the
isopolyanions present in a solution from the composition
of complexes of the polyanions with cations which have
been detected in the solution meet with difficulties
similar to those encountered in attempts to apply
[84] P . Schamer, Dissertation, Freie Universitat Berlin 1965.
[ 8 5 ] G.von Knorre, Ber. dtsch. chem. Ges. 18, 2362 (1885).
[86] R. Kubens, Dissertation, Humboldt-Universitat Berlin 1952
Angew. Chem. nternat. Edit. / Vol. 5 (1966) / No. 8
conclusions based on the crystalline state to the dissolved state. A complex of this nature may be formed
in one of two ways:
I . The central cation reacts with monomers or small
isopolyanions ;
2. The central cation reacts with the preformed isopolyanion.
Only in the second case can reliable conclusions regarding the isopolyanion be based on the structure of the
Both reactions evidently are possible in the formation
of hetero-com.plexes. Thus the fact that “true” metatungstate ions do not react with phosphoric acid shows
very clearly that, for example, the dodecatungstatophosphate ion, which is similar to the true metatungstate ion 1541, can be formed in accordance with 1 ; it is
formed on acidification of a solution containing monotungs:ate and phosphate ions. The complexes we have
described, on the other hand, evidently are formed in
accordance with 2, so that there is a direct relation
between these complexes and the isopolyanions originally present in the solution. Some of the many reasons
for this conclusion are:
The formation of different complexes with cations from
paratungstate solutions of the same concentration and
pH but of different ages would not be understandable
if the heterocomplexes were formed from fragments
and not from the different polytungstate ions present in
each case.
True metatungstate ions and decavanadate ions are
broken down only slowly in alkaline solutions. Since
complex formation with these polyanions is instantaneous, it is hardly possible that it could proceed via
The reaction of methyl red with molybdate in the ratio
1 : 12 could not be explained without assuming the prior
existence of dodecamolybdate ions with cavities capable
of accommodating the dye.
We have attempted in this paper to outline, with the
aid of some examples, the present state of research on
polyacids, and to describe a number of new methods
that have contributed to progress in this field. New
insights into the chemistry of the polyacids will undoubtedly be gained in the future by the use of numerous
and versatile chemical and physical methods, and by a
logical combination of all the individual results.
Received: July 30th, 1965
[ A 519 IE]
German version: Angew. Cheni. 78, 725 (1966)
Translated by Express Translation Service, London
[87] K . Lohoff; Dissertation, Freie Universitir Berlin 1966.
Aryl-A’-pyrazolines as Optical Brighteners
Di- and triaryl- A2-pyrazolines and their derivatives can be used as Gptical brighteners for
synthetic fibers andplastics. The heterocycles are readily obtainable by several methods, e.g.
condensation of’ J-halogeno- or J-dialkylaminopropiophenones with phenylhydrazine. A
ncrrnber of rekctionships can be detected between substitution and optical properties.
1. Introduction
The use of organic fluorescent dyes for the optical
brightening of textiles, paper, and plastics and as additives in detergents has shown a rapid increase in recent
years. Without additives these materials are not pure
white; they absorb in the short wavelength region of
the visible spectrum, and consequently appear more or
less yellowish. The use of optical brighteners, which emit
blue-violet fluorescence, leads not merely to the compensation of this so-called “blue deficiency”, but to an
increase in the remission of the material above 100
(relative to magnesium oxide as standard), since the
blue-violet fluorescence gives the impression of increased
whiteness (cf. Fig. I).
Anzew. Chem. internat. Edit. i Vol. 5 (1966) 1 No. 8
As with organic dyestuffs, the compounds used in industry for this purpose can be grouped into a small
number of types[]]. By far the most important optical
hlmglFig. I .
Re-emission R of unbleached cotton cloth without optical
brightener (-)
and with optical brightener (---)
(MgO = 100%).
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stud, method, polyacides, results, new
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