Synthesis of polyoxovanadates from aqueous solutions

Article abstract of DOI:10.1006/jssc.2001.9325

Polyoxovanadates have been synthesized from aqueous solutions of V^v in the presence of inorganic Na⁺ and organic [N(CH₃)₄]⁺ cations. Decavanadate crystals are precipitated at room temperature around pH 7, whereas layered compounds Na[V₃O₈].H₂O and N(CH₃)₄[V₄O₁₀] are formed at the same pH under hydrothermal conditions: In situ ⁵¹V NMR experiments show that upon heating solute decavanadate species are progressively transformed into cyclic metavanadates [V₄O₁₂]^4-. A chemical mechanism is suggested for the formation of these layered vanadium oxides. It involves the ring-opening polymerization of cyclic metavanadates arising from co-ordination expansion favored by protonation or vanadium reduction.

Full text of DOI:10.1006/jssc.2001.9325

Journal of Solid State Chemistry 162, 315}321 (2001)
doi:10.1006/jssc.2001.9325, available online at http://www.idealibrary.com on
Synthesis of Polyoxovanadates from Aqueous Solutions
L. Bouhedja, N. Steunou, J. Maquet, and J. Livageꢀ
Chimie de la Matiere Condensee-UMR 7574, Universite& Pierre et Marie Curie, 4 place Jussieu, 75252 Paris Cedex 05, France
IN HONOR OF PROFESSOR PAUL HAGENMULLER ON THE OCCASION OF HIS 80TH BIRTHDAY
mechanisms leading to the formation of such solids from
Polyoxovanadates have been synthesized from aqueous solu- solutions remain largely unknown. The relationship be-
tions of VV in the presence of inorganic Na؉ and organic
tween the reaction conditions and the nature of molecular
clusters is still far from understood (9, 10). Therefore, this
paper presents a discussion of the relationships between the
nature of molecular precursors in solution and the structure
[N(CH3)4]؉ cations. Decavanadate crystals are precipitated at
room temperature around pH 7, whereas layered compounds
Na[V3O8].H2O and N(CH3)4[V4O10] are formed at the same
pH under hydrothermal conditions. In situ 51V NMR experi-
of crystalline polyoxovanadates.
ments show that upon heating solute decavanadate species are
progressively transformed into cyclic metavanadates [V4O12]4؊.
EXPERIMENTAL
A chemical mechanism is suggested for the formation of
these layered vanadium oxides. It involves the ring-opening
polymerization of cyclic metavanadates arising from co-ordi-
Chemical Syntheses of Compounds I and II
A V O powder (1 g) was dissolved in an aqueous solu-
ꢄ ꢇ
nation expansion favored by protonation or vanadium reduction.
( 2001 Elsevier Science
tion of NaOH (1 M, 5 mL) or TMAOH (10%, 5 mL). The
Key Words: hydrothermal synthesis; polyoxovanadates;
51V NMR spectroscopy.
V O /base molar ratio is close to 1. Dissolution was rather
slow and the suspension was stirred for about 2 days at
ꢄ ꢇ
room temperature. Vanadium pentoxide in excess was then
removed by "ltration and the resulting clear orange solution
(pH+7) was slowly evaporated at room temperature for
1 week giving rise to orange crystals of compounds I (with
NaOH) and II (with TMAOH).
INTRODUCTION
Many new polyoxovanadates have been synthesized dur-
ing the past decade via hydrothermal methods (1}3). They
exhibit a large variety of structures, ranging from chain
Hydrothermal Syntheses of Compounds III and IV
\
metavanadates [VO ] to layered oxides N(CH ) [V O ]
ꢁ L
and compact polyanions [V O ]
ꢀꢃ ꢄꢅ
ꢁ ꢂ ꢂ ꢀꢃ
(4}6). Hollow clus-
ꢆ\
As previously, V O (1 g) was mixed with aqueous solu-
ꢄ ꢇ
ters encapsulating anionic or neutral species such as tions of NaOH (1 M, 5 mL) or TMAOH (10%, 5 mL). These
(TMA) [V O Cl].4H O or (Cs) [H V O I].12H O mixtures were then heated for 48 h at 2003C in a Parr
ꢆ
ꢀꢇ ꢁꢆ
ꢄ
ꢈ
ꢂ ꢀꢅ ꢂꢄ
ꢄ
have also been reported (7). Chemical syntheses via &&chimie Te#on-lined digestion bomb under autogeneous pressure.
douce'' methods favor the formation of open structures such A red pasty material was formed in the presence of NaOH
as layered vanadium oxides that could exhibit improved (compound III) whereas black shiny #at crystals (compound
properties as cathodic materials for advanced lithium batte- IV) precipitated in the presence of TMAOH. In both cases
ries. pH plays a critical role in the formation of vanadium a yellow supernatant solution (pH 7.5) was obtained.
4
oxides from aqueous solutions. The coordination of V de-
creases as pH increases. It turns from [VO ] square pyr-
ꢇ
amids or trigonal bipyramids to [VO ] tetrahedra above
ꢂ
Characterization
pH 7. Therefore, room temperature and hydrothermal
The structure of crystalline compounds was determined
syntheses have been performed in the pH range from by X-ray di!raction. For compounds I to IV, X-ray di!rac-
4
6 to 8 where V coordination turns from "ve to four (1, 8). tion powder patterns were recorded in the 2h range from
The structures and properties of these new polyoxovana- 2 to 503 on a Philips PW 1830 di!ractometer (CuKa
ꢀ
dates have been extensively described but the chemical radiation, j"1.5406 A). For compounds II and IV,
measurements were also collected at room temperature on
single crystals on a CAD4 di!ractometer using MoKa
radiation (j"0.71069 A) and a graphite monochromator.
ꢀTo whom correspondence should be addressed: Fax: (33) 1 44 27 47 69.
E-mail: livage@ccr.jussieu.fr.
315
0022-4596/01 $35.00
( 2001 Elsevier Science
All rights reserved.

316
BOUHEDJA ET AL.
ESR spectra were recorded on an X-band Bruker spec-
trometer (l"9.47 GHz) equipped with a heating device.
ꢇꢀ
V NMR solution spectra were recorded on a Bruker
AC 300 (or MSL 400) spectrometer operating at 78 MHz (or
105.25 MHz), using a 903 pulse width of 11 ls, and a relax-
ation delay of 1 s. Neat VOCl was used as an external
ꢁ
reference for chemical shifts (d"0 ppm). The temperature
dependence of NMR spectra was studied with decavanadate
solutions heated in sealed NMR glass tubes.
Thermogravimetric analyses were carried out under an
ꢁ
oxygen #ux of 5 cm /min at a heating rate of 53C/min using
a Netsch STA 409 thermogravimetric analyzer.
RESULTS AND DISCUSSION
Characterization of Crystalline Compounds
Compound I. Orange crystals corresponding to com-
pound I are grown from aqueous solutions at room temper-
ature. The X-ray di!raction pattern of this compound is
identical to that of the previously reported decavanadate
compound Na [V O ])18 H O (Fig. 1a) (11). The well-
ꢆ
ꢀꢃ ꢄꢅ
ꢄ
known decavanadate anion is a highly condensed polyanion
made of 10 VO octahedra sharing corners and edges. All
ꢆ
vanadium atoms have distorted octahedral geometry and
taking into account the ideal symmetry of the decavanadate
anion D , three types of VO octahedra can be distin-
ꢄF
ꢆ
guished.
Two weight losses around 1003C and 1753C are observed
by thermal analysis (Fig. 2a). They correspond to the depar-
ture of about 19 water molecules per decavanadate, in III.
agreement with the water content reported from the single-
crystal X-ray di!raction study (11). No further weight loss is
FIG. 1. X-ray di!raction patterns of (a) compound I and (b) compound
layered compound (TMA)[V O ] around 3003C (Fig. 3b).
ꢂ ꢀꢃ
observed beyond 2003C and a mixture of NaVO and
ꢁ
A mixture of nonstoichiometric vanadium oxides is then
formed at higher temperature. They are oxidized in air
above 4003C to give orthorhombic V O . The color of
NaV O is obtained upon heating at 5003C as shown by
ꢁ ꢅ
X-ray di!raction.
ꢄ ꢇ
Compound II. Orange crystals are grown from an aque- compound II turns from orange to black above 2003C,
ꢇ>
ous solution of V O in TMAOH. The molecular structure suggesting that some V
are reduced by organic groups
ꢄ ꢇ
of compound II was resolved by single-crystal X-ray di!rac- upon heating. ESR spectra recorded at di!erent temper-
tion showing that it corresponds to a tetramethyl am- atures show that a signal actually appears and grows above
monium decavanadate (TMA) [H V O ].4H O. The
ꢂ
ꢄ ꢀꢃ ꢄꢅ
ꢄ
structure of this compound was found to be identical to the
one recently reported (12). As a consequence, the main
X-ray data are reported in Table 1.
TABLE 1
X-Ray Data for the Structures of Compounds II and IV
The thermal analysis of compound II exhibits four suc-
cessive weight losses around 1003C, 1903C, 3203C, and
3953C (Fig. 2b). The "rst two, associated with endothermic
DTA peaks, should be due to the departure of water mol-
ecules. The last two, above 3003C, correspond to exothermic
processes arising from the combustion of organic groups.
A slight weight increase is observed above 4003C in air.
X-ray di!raction patterns recorded on powders heated at
di!erent temperatures show that decavanadate clusters
(compound II) (Fig. 3a) are transformed into the crystalline
Formula
(TMA) [H V
O
] 4H O
(TMA)[V O
]
'
ꢂ
ꢄ
ꢀꢃ ꢄꢅ
ꢄ
ꢂ
ꢀꢃ
a (A)
b (A)
c (A)
a (3)
b (3)
c (3)
< (Aꢁ)
Z
12.626(3)
12.639(4)
15.733(2)
68.07(2)
83.09(2)
78.71(2)
2281.1(9)
2
17.116(2)
6.6425(9)
11.739(1)
90
90
90
1334.6
4
Space group
P!1
Cmc2
ꢀ

SYNTHESIS OF POLYOXOVANADATES
317
FIG. 2. TGA curves of compounds I+IV under O gas #ow. (a): I; (b): II; (c): III; (d): IV.
ꢄ
2003C. It is made of a single band, without hyper"ne struc- trigonal bipyramids sharing corners and edges (Fig. 5a) (16).
ꢂ>
ꢀ
ture, centered around g"1.97, a value typical of V (3d ) The structure of the hydrated phase NaV O .nH O should
ꢁ ꢅ
ꢄ
>
ions. The intensity of the ESR signal increases as a function be close to that with Na ions and water molecules lying
of temperature, showing that more vanadium is reduced between the vanadium oxide layers.
upon heating (Fig. 4). Above 4003C, the signal decreases
again due to the oxidation of vanadium oxides in air.
Compound IV. Black #at crystals are precipitated via the
hydrothermal treatment of an aqueous solution of V O in
ꢄ ꢇ
Compound III. A red pasty solid is obtained after the TMAOH. The X-ray structure of compound IV was re-
hydrothermal treatment of an aqueous solution of V O in solved by single-crystal X-ray di!raction and found to be
ꢄ ꢇ
NaOH. The X-ray di!raction pattern recorded on the identical to the TMA[V O ] compound already described
ꢂ ꢀꢃ
precipitate dried at room temperature (compound III) is by Whittingham et al. (4). The main X-ray data of com-
identical to that reported for NaV O .nH O (Fig. 1b) (13). pound IV are reported in Table 1. Compound IV is made up
ꢁ ꢅ
ꢄ
A continuous weight loss (&5%) is observed by thermal of double chains of edge-sharing [VO ] tetragonal pyr-
ꢇ
>
analysis upon heating up to 2503C (Fig. 2c). It corresponds amids with TMA
cations distributed between the
\
to the departure of about one water molecule per formula [V O ] layers (Fig. 5b). The neutrality of this compound
ꢂ ꢀꢃ
unit. The crystalline NaV O &&high-temperature'' (NaV O
suggests that one of four vanadium has been reduced to
ꢁ ꢅ
ꢁ ꢅ
ꢂ>
HT) phase is then observed by X-ray di!raction (13}15). Its
V
.
structure has not yet been resolved but it was shown to be
Three weight losses are observed with compound IV,
\
isostructural with LiV O (16). It is made of [V O ] layers followed by a slight increase in weight above 4003C
ꢁ ꢅ ꢁ ꢅ
built up of distorted [VO ] octahedra and distorted [VO ] (Fig. 2d). The TGA curve is similar to that obtained for
ꢆ
ꢇ

318
BOUHEDJA ET AL.
solution in TMAOH is presented in Fig. 6a. At a pH close to
7 and at room temperature, both spectra exhibit three peaks
at d"!427, !502, and !518 ppm due to the decavana-
(ꢆ\L)\
date polyanion [H V O ]
ferent vanadium sites in the ratio 2 : 4 : 4 (17). Two smaller
that contains three dif-
L ꢀꢃ ꢄꢅ
peaks at d"! 581 and !591 ppm can be assigned to the
ꢂ\
ꢇ\
cyclic metavanadates, [V O ] and [V O ] , respec-
ꢂ ꢀꢄ
ꢇ ꢀꢇ
tively (17, 18). Decavanadates are by far the main species.
Integration of NMR peaks shows that they correspond
to 95% (with NaOH) and 93% (with TMAOH) of the
4
whole V .
ꢇꢀ
After hydrothermal synthesis, the V NMR spectra of
the supernatant solutions obtained with NaOH (compound
III) and TMAOH (compound IV) are rather di!erent. Only
two peaks corresponding to metavanadate species
(d"!576 and !585 ppm) are seen in the presence of
NaOH, with very small signals corresponding to traces of
decavanadates. Both decavanadates (d"!423, !498,
and !513 ppm) and metavanadates (d"!582 and !592
ppm) are clearly seen in the presence of TMAOH. Surface
measurements of these NMR peaks show that 44% of the
vanadium is present as decavanadate species and 56% as
metavanadates (30.5 and 25.5 for the tetramer and penta-
mer, respectively).
ꢇꢀ
Temperature Dependence of the < NMR Spectra
of Decavanadate Solutions
These experiments have been performed with the super-
natant orange solution obtained in the presence of
TMAOH (pH+7) from which crystals of compound II
ꢇꢀ
were grown. V NMR spectra are recorded on solutions
FIG. 3. X-ray di!raction patterns of compound II heated at two
di!erent temperatures: (a) 203C, (b) 3003C.
compound II above 3003C. It corresponds to the departure
of organic groups followed by the oxidation of reduced
vanadium oxides into V O .
ꢄ ꢇ
Characterization of the Supernatant Decavanadates
Solutions
ꢇꢀ
The V NMR spectra of the orange solutions obtained
at room temperature in the presence of NaOH and
TMAOH are quite close. The spectrum for the orange recorded at di!erent temperatures.
FIG. 4. ESR spectra of compound II, (TMA) [H V
O ] 4H O,
'
ꢂ
ꢄ
ꢀꢃ ꢄꢅ
ꢄ

SYNTHESIS OF POLYOXOVANADATES
319
the peak corresponding to the cyclic metavanadate
ꢂ\
[V O ]
ꢂ ꢀꢄ
(Fig. 6). The molecular ratio [decavanadate]/
[metavanadate] decreases progressively from 91/9 at room
temperature to 76/24 at 1003C. At a given temperature, it
also decreases as a function of the heating time. Upon
heating at 2003C for example, the ratio decreases from 24/76
after 0.5 h to 3/97 after 3 h. It has to be pointed out that this
decavanadate}metavanadate transformation is partially re-
versible. A ratio of 82/18 is again observed when a solution
heated to 1503C for 12 h is cooled to room temperature.
ꢇꢀ
The overall intensity of V NMR peaks decreases with
the temperature as some black solid phase forms above
1503C while the solution turns green.
DISCUSSION
4
At room temperature, the molecular structure of V
species mainly depends on the pH of the aqueous solution
and several isopolyvanadates have been clearly character-
ꢇꢀ
ized by V NMR spectroscopy (17, 18).
The dissolution of V O at high pH leads to the forma-
ꢄ ꢇ
tion of orthovanadate species as follows:
\
ꢁ\
V O #6OH P2[VO ] #3H O.
[1]
ꢄ ꢇ
ꢂ
ꢄ
The pH of the aqueous solution decreases as dissolution
goes on, leading to the formation of protonated vanadate
(ꢁ\L)\
ions [H VO ]
. Below pH 12, condensation may occur
L
ꢂ
FIG. 5. (a) Projection of the structure of LiV O along [010] showing
ꢁ
ꢅ
the stacking of [V O ]\ inorganic layers between which the Li> cations
ꢁ
ꢅ
are inserted (16). (b) Projection of the structure of N(CH ) [V O ] along
ꢁ ꢂ
ꢂ ꢀꢃ
[100] (4).
heated in sealed tubes at di!erent temperatures from room
temperature to 2003C (Fig. 6).
FIG. 6. ꢇꢀV NMR spectra of decavanadate solutions heated at di!er-
ent temperatures (a) ¹"253C, (b) ¹"803, (c) ¹"1003C, (d) ¹"2003C
during 30 min, (e) 2003C during 3 h (Larmor frequency"78 MHz; number
of scans, NS"400 for (a), (b), (c); Larmor frequency"105.25 MHz;
ꢇ\
The small peak corresponding to the cyclic [V O ]
ꢇ ꢀꢇ
species disappears rapidly around 403C. The relative inten-
sity of the three decavanadate peaks decreases compared to NS"44 for (d) and (e)).

320
BOUHEDJA ET AL.
FIG. 7. Suggested mechanism for the formation of [V O ]\ layers from chain metavanadates.
ꢂ
ꢀꢃ
ꢂ\
when the temperature increases
via the oxolation of two V}OH groups. Pyrovanadates vanadates [V O
ꢂ ꢀꢄ
[V O ] , metavanadates [V O ] , and decavanadates (Fig. 6). At 2003C, under the hydrothermal conditions used
ꢄ ꢉ ꢂ ꢀꢄ
[V O ] are formed as the pH decreases. The "rst two in this paper, only metavanadates are observed by
ꢀꢃ ꢄꢅ
species are formed of corner-sharing tetrahedra whereas
]
ꢂ\
ꢆ\
ꢂ\
ꢇꢀ
V NMR. It may then be assumed that these metavana-
decavanadates are made of 10 densely packed [VO ] oc- dates behave as molecular precursors for the layered oxides,
ꢆ
tahedra sharing corners and edges.
Na[V O ] H O and (TMA)[V O ]. Crystallization from
'
ꢁ ꢅ
ꢄ
ꢂ ꢀꢃ
The "nal pH of the aqueous solutions depends on the metavanadate aqueous solutions is always accompanied by
relative amount of V O and base. In our case, for polymerization of the anion (19) and it may be assumed that
ꢄ ꢇ
V O /base"1, it ranges between 6 and 7, so that decavana- layered polyoxovanadates should be formed via the ring-
ꢄ ꢇ
dates are the main solute species. Therefore, sodium or opening polymerization (ROP) of cyclic metavanadates
ꢂ\
tetramethyl ammonium decavanadates are precipitated [V O ]
ꢂ ꢀꢄ
Actually metavanadates are based on corner-sharing
.
from such solutions at room temperature. There is a
4
straightforward correlation between V molecular species in [VO ] tetrahedra and room-temperature precipitation cur-
ꢂ
the solution and in the solid state.
rently leads to the formation of chain compounds made of
This is no longer the case under hydrothermal conditions. corner-sharing [VO ] tetrahedra as in KVO or edge-shar-
ꢂ
ꢁ
4
The nature of solute V species varies with temperature, ing [VO ] trigonal bipyramids as in KVO H O (20).
'
ꢇ
ꢁ
ꢄ
which becomes a major parameter. Therefore, in situ Hydrothermal conditions may favor the coordination ex-
ꢇꢀ
4
V NMR experiments have to be performed in order to pansion of vanadium V from four to "ve or even six.
4
study the molecular structure of V precursors. There is no Protons are formed when decavanadate species are trans-
obvious structural relationship between the decavanadate formed into metavanadates (Eq. [2]). The &&measured'' pH of
polyanions observed in the supernatant solutions at room the solution decreases upon heating and protonation is
temperature after hydrothermal treatment and the layered known to lead to coordination expansion in this pH range.
polyoxovanadates precipitated from these solutions. In the
The formation of layered vanadium oxides from aqueous
pH range 5}9, both decavanadate and metavanadate spe- solutions of vanadates may then be described as follows.
cies are in equilibrium in aqueous solutions:
ꢂ\ ꢂ\
The ring-opening of cyclic metavanadates leads to the
formation of chains made of corner-sharing tetrahedra. Pro-
>
2[H V O ] #4H O 0 5[V O ] #12H . [2] tonation and reduction (in the case of TMA compounds)
ꢄ ꢀꢃ ꢄꢅ
ꢄ
ꢂ ꢀꢄ
favor coordination expansion via the nucleophilic addition
ꢇꢀ
In situ V NMR experiments show that the equilibrium of [VO ] tetrahedra, giving rise to double chains of
ꢂ
between decavanadate and metavanadate species is pro- edge-sharing [VO ] square pyramids. These double chains
gressively displaced toward the formation of cyclic meta- may be further linked together via corners to form [V O ]-
ꢇ
ꢄ ꢇ

SYNTHESIS OF POLYOXOVANADATES
321
like layers. Such a condensation process may occur via
oxolation between protonated V}OH bonds along the
vanadate chains. Without protonation only chain com-
pounds are formed. A possible mechanism for the formation
2. J. Livage, Coord. Chem. Rev. 178+180, 999 (1998).
3. T. G. Chirayil, P. Y. Zavalij, and M. S. Whittingham, J. Mater. Chem.
7(11), 2193 (1997).
4. P. Y. Zavalij, M. S. Whittingham, E. A. Boylan, V. K. Pecharsky, and
R. A. Jacobson, Z. Kristallogr. 211, 464 (1996).
5. Y. Zhang, C. J. O'Connor, A. Clear"eld, and R. C. Haushalter, Chem.
Mater. 8, 595 (1996).
of layered (TMA)[V O ] from chain metavanadates is
ꢂ ꢀꢃ
4
suggested in Fig. 7. Only V compounds are formed under
6. D. Riou, O. Roubeau, and G. Ferey, Z. Anorg. Allg. Chem. 624, 1021
(1998).
hydrothermal conditions in the presence NaOH whereas
mixed valence polyoxovanadates are formed with TMAOH.
Some vanadium reduction then occurs via the decomposi-
tion of organic compounds upon heating.
7. (a) A. Muller, M. Penk, R. Rohl"ng, E. Krickemeyer, and J. Doring,
Angew. Chem., Int. Ed. Engl. 29(8), 926 (1990). (b) A. Muller, R. Sessoli,
E. Krickemeyer, H. Bogge, J. Meyer, D. Gatteschi, L. Pardi, J. Wes-
tphal, K. Hovemeier, R. Rohl"ng, J. Doring, F. Hellweg, C. Beugholt,
and M. Schmidtmann, Inorg. Chem. 36, 5239 (1997).
8. T. G. Chirayil, E. A. Boylan, M. Mamak, P. Y. Zavalij, and M. S.
Whittingham, Chem. Commun. 33 (1997).
Reduction should also play a major role during the solid-
state transformation of tetramethyl ammonium decavana-
date (TMA) [H V O ] ' 4H O into the layered
ꢂ
ꢄ ꢀꢃ ꢄꢅ
ꢄ
TMA[V O ] oxide. Such a transformation was already
9. T. Duraisamy, A. Ramanan, and J. J. Vittal, Cryst. Eng. 3, 237
(2000).
ꢂ ꢀꢃ
described by Riou et al. in the case of the decavanadate
(NC H ) [H V O ] (6). Such a transformation is not 10. T. Duraisamy, N. Ojha, A. Ramanan, and J. J. Vittal, Chem. Mater.
ꢉ ꢀꢂ ꢂ ꢄ ꢀꢃ ꢄꢅ
11, 2339 (1999).
observed with the sodium decavanadate that directly leads
11. A. Durif, M. T. Averbuch-Pouchot, and J. C. Guitel, Acta Crystallogr.
to the stable oxide phases NaVO and NaV O . The trans-
formation of compact decavanadate clusters into layered
vanadium oxide seems to occur only in the presence of
ꢁ
ꢁ ꢅ
B 36, 680 (1980).
12. S. Nakamura and T. Ozeki, J. Chem. Soc., Dalton ¹rans. 472
(2001).
organic cations. ESR experiments show that the trans- 13. M. E. Spahr, P. Novak, W. Scheifele, O. Haas, and R. Nesper,
J. Electrochem. Soc. 145(2), 421 (1998).
14. Y. Kera, J. Solid State Chem. 51, 205 (1984).
ꢇ>
formation does not occur until some V
cations are re-
ꢂ>
duced to V
.
This reduction, due to the partial
15. M. Pasquali and G. Pistoia, Electrochem. Acta 36, 1549 (1991).
16. A. Wadsley, Acta Crystallogr. 10, 261 (1957).
17. (a) S. E. O'Donnell and M. T. Pope, J. Chem. Soc., Dalton ¹rans. 2290
(1976). (b) O. W. Howarth and M. Jarrold, J. Chem. Soc., Dalton ¹rans.
503 (1978). (c) V. W. Day, W. G. Klemperer, and D. J. Maltbie, J. Am.
Chem. Soc. 109, 2991 (1987).
decomposition of organic species, favors condensation and
coordination expansion. Indeed V ions being larger than
ꢂ>
ꢇ>
V
do not adopt tetrahedral coordination.
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(1981).
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