Electrochemical and spectroscopic studies of the chloro and oxochloro complex formation of Nb(V) and Ta(V) in NaCl-AlCl3 melts

Article abstract of DOI:10.1021/ic050367a

The equilibrium constant for the chloro complex formation of Nb(V) NbCl₆^-?NbCl₅+Cl^- (i) in NaCl-AlCl₃ melts at 175°C was found to be pK_i = 2.86(5). The oxochloro complex formation of Nb(V) and Ta(V) in NaCl-AlCl ₃ melts at 175°C could be explained by the following equilibria: MOCl₄^-?MOCl₃+Cl^- (ii) MOCl ₃?MOCl₂⁺+Cl^- (iii) where M = Nb and Ta. The equilibrium constants determined by potentiometric measurements with chlorine-chloride electrodes were, for M = Nb, pK_ii, = 2.21(4) and pK_iii = 3.95(5) and, for M = Ta, pK_ii = 2.743(15) and pK_iii = 4.521(13). NbCl₆^- has two bands in the UV-vis region, a strong one at 34.7 × 10³ cm^-1 and a weaker one at 41.6 × 10³ cm^-1. The MOCl ₄^- complexes showed in the case of Nb(V) absorption bands at 32.7 and 42.9 × 10³ cm^-1 and in the case of Ta(V) at 38.6 and 48.1 × 10³ cm^-1.

Full text of DOI:10.1021/ic050367a

Inorg. Chem. 2005, 44, 9847−9851
Electrochemical and Spectroscopic Studies of the Chloro and
Oxochloro Complex Formation of Nb(V) and Ta(V) in NaCl−AlCl3 Melts
J. H. von Barner* and N. J. Bjerrum
Department of Chemistry, KemitorVet, Technical UniVersity of Denmark, DK-2800 Lyngby,
Denmark
Received March 10, 2005
The equilibrium constant for the chloro complex formation of Nb(V)
NbCl6 TNbCl5+Cl-
(i)
-
in NaCl−AlCl3 melts at 175
°C was found to be pKi ) 2.86(5). The oxochloro complex formation of Nb(V) and
Ta(V) in NaCl
−AlCl3 melts at 175
°
C could be explained by the following equilibria:
MOCl4 TMOCl3+Cl-
(ii)
-
MOCl3TMOCl2 +Cl-
(iii)
+
where M
) Nb and Ta. The equilibrium constants determined by potentiometric measurements with chlorine−
chloride electrodes were, for M
)
Nb, pKii
)
2.21(4) and pKiii
)
3.95(5) and, for M
)
Ta, pKii
)
2.743(15) and
-
1
pKiii
)
4.521(13). NbCl6 has two bands in the UV
−vis region, a strong one at 34.7
×
103 cm- and a weaker one
-
at 41.6
×
103 cm-1. The MOCl4 complexes showed in the case of Nb(V) absorption bands at 32.7 and 42.9
×
1
1
103 cm- and in the case of Ta(V) at 38.6 and 48.1
×
103 cm-
.
Introduction
ions are present. We have previously reported results on the
chloro complex formation of Nb(V) in KCl-AlCl3 melts2
at 300 °C and of Ta(V) in NaCl-AlCl3 melts3 at 175 °C by
potentiometric and spectrophotometric measurements. It was
obvious2,3 that the niobium and tantalum chloro complexes
would react with oxide ions originating from impurities in
the salts that made up the melts or from contact with water
vapor, resulting in the formation of oxochloro species.
Indeed, oxide impurities often cause serious problems when
depositing niobium and tantalum from halide melts, dis-
turbing the electrochemical reduction and giving impure
deposits of the metals.4 In the solid state, the existence of
the complex ion NbOCl52- has been demonstrated5 by Raman
and IR spectroscopy and by X-ray diffraction studies on
[As(C6H5)4]2NbOCl5‚2CH2Cl2 and it is also known6 to be
present in solid RbNbOCl5 and CsNbOCl5. In solution,
Electrodeposition of niobium and tantalum from molten
halide solvents has been the subject of numerous investiga-
tions. In general, fluoride mixtures seem to be the most
favorable supporting liquids from a deposition point of view.
However, because fluoride melts are very aggressive and may
also have unwanted environmental effects, much research
has been focused on the use of various chloride melts as
solvents for plating processes. However, niobium and
tantalum have in chloride melts a tendency to form insoluble
lower valent cluster compounds, disturbing the plating
process. Despite this, it has, for example, been possible to
deposit Al-Nb alloys from a low-melting chloroaluminate
electrolyte.1 Thus such melts may be interesting candidates
as solvents for deposition of niobium and tantalum.
Knowledge about the species formed by the metals to be
deposited is essential in order to design proper plating
processes. Both NbCl5 and TaCl5 are Lewis acids forming
MCl6- complexes when sufficient amounts of free chloride
(2) von Barner, J. H.; Bjerrum, N. J.; Smith, G. P. Acta Chem. Scand.
1978, A32, 827.
(3) von Barner, J. H.; McCurry, L. E.; Jørgensen, C. A.; Bjerrum, N. J.;
Mamantov, G. Inorg. Chem. 1992, 31, 1034.
(4) Gillesberg, B.; von Barner, J. H.; Bjerrum, N. J.; Lantelme, F. J. Appl.
Electrochem. 1999, 29, 934.
* To whom correspondence should be addressed. E-mail: jvb@kemi.dtu.dk.
(1) Stafford, C. R.; Haarberg, G. M. Plasmas Ions 1999, 1, 35.
(5) Mu¨ller, U.; Lorenz, I. Z. Anorg. Allg. Chem. 1980, 463, 110.
(6) Sabatini, A.; Bertini, I. Inorg. Chem. 1966, 5, 204.
10.1021/ic050367a CCC: $30.25
Published on Web 11/24/2005
© 2005 American Chemical Society
Inorganic Chemistry, Vol. 44, No. 26, 2005 9847

von Barner and Bjerrum
2-
NbOCl5 has been identified in hydrochloric acid,7 in
various nonaqueous solvents,8,9 and in CsCl-NbOCl3 melts
with CsCl in excess.10 NbOCl4 ions formed in the latter
-
melts10 when less CsCl was added, and the presence of this
complex was also indicated in acetonitrile8,9 and dioxane9
solutions. Concerning Ta(V), oxochloro species of types sim-
ilar to the ones for Nb(V) formed in CsCl-TaOCl3 melts.11
Because the equilibrium constants for the chloro complex
formation of Nb(V) in NaCl-AlCl3 melts have not previous-
ly been determined, we decided to investigate this especially
at low temperature (i.e., 175 °C). However, the main focus
of the present work is to clarify which oxochloro complexes,
niobium or tantalum, will form in NaCl-AlCl3 melts.
Experimental Section
AlCl3 was prepared at 700 °C by reaction between molten
aluminum (99.999% from Atomergic Chemetals Corp.) and dry
hydrogen chloride gas. Anhydrous NaCl was made by treating an
analytical-grade material in the molten state with dry HCl gas.
Details about the preparation of these salts, which made up the
solvent, have been given in a previous paper.12
AlOCl was applied as the oxide source. It was synthesized by
the reaction between V2O5 and AlCl3. The latter chemical was added
in an amount approximately 10 times greater than that of V2O5.
After reaction, VOCl3 and the excess of AlCl3 were distilled off at
300 °C. The final AlOCl had a chloride content of 44.97%
(determined by Volhard titration) compared to a calculated content
of 45.0%. NbCl5 was made by the reaction of metallic niobium
(99.99% from Schuchardt) with chlorine gas (99.9% from Fluka)
at 600 °C. The product was purified by 5 times successive
sublimation under chlorine (0.33 atm). TaCl5 (99.9% from Fluka)
was similarly sublimed at 200 °C.
The chlorine-chloride concentration cells for the potentiometric
measurements have been described in detail elsewhere.13 The
apparatus and optical cells applied in the spectrophotometric
experiments were of the same type as that in our previous work.3
The Raman spectra were recorded with the same instrumentation
as was previously used14 and were obtained from melts contained
in Pyrex ampules sealed under argon.
Figure 1. UV-vis gas-phase spectrum of NbCl5 at 300 °C.
From our previous work3 on Ta(V) in NaCl-AlCl3, we
know from UV-vis spectra that oxide addition completely
-
converts TaCl6 to an oxochloro complex, and as we shall
see later, both Raman and UV-vis spectroscopic measure-
ments support this assumption also for niobium.
The measurements at 175 °C were not corrected for
vaporization of niobium or tantalum species into the gas
phase over the melt. In the previous work,3 this situation
with TaCl5 dissolved in NaCl-AlCl3 melts was considered.
For potentiometric measurements performed in a similar pCl-
region as that in the present work, it was concluded that no
correction for vaporization of TaCl5 was needed at 175 °C.
Concerning niobium, we have previously2 measured the gas-
phase spectrum of pure NbCl5 at 300 °C. Unfortunately,
during the calculation of the molar absorptivities, the
measured spectrum was multiplied with a wrong factor. Thus,
the intensities on the spectrum in ref 2, Figure 4A, became
to high by a factor of 1.822. The correct spectrum for gaseous
NbCl5 at 300 °C is given in Figure 1. As can be seen,
maximum absorbance occurs at 35 × 103 cm-1 (286 nm).
At this wavelength, only a slightly visible shoulder appeared
on the vapor-phase spectrum above a NaCl-AlCl3 melt with
NbCl5 added (average chloride coordination number of
approximately 5.5) at 175 °C. This implies for our experi-
ments that less than 1% of the added NbCl5 will be present
in the vapor phase at this temperature. Like in the case for
KCl-AlCl3 melts,2 the vaporization of NbCl5 from acidic
NaCl-AlCl3 melts at 300 °C was not negligible. For the
latter solvent, we estimate, from a spectrophotometric
measurement, the vapor-liquid distribution coefficient for
NbCl5 to be 0.03 at 300 °C.
The program used for testing of the different model equilibria
was similar to the one applied before.13
Definitions and Assumptions
In the calculations, it has been assumed that AlOCl reacts
quantitatively with the pentachlorides of niobium and
tantalum, i.e., according to
MCl5 + AlOCl f MOCl3 + AlCl3
where M ) Nb and Ta.
(1)
(7) Griffith, W. P.; Wickins, T. D. J. Chem. Soc. A 1967, 675.
(8) Furlani, C.; Zinato, E. Z. Anorg. Allg. Chem. 1967, 351, 210.
(9) Miranda, C.; da Silveira, M.; Vernois, J. J. Inorg. Nucl. Chem. 1970,
32, 839.
(10) Rosenkilde, C.; Voyiatzis, G. A.; Jensen, V. R.; Ystenes, M.; Østvold,
T. Inorg. Chem. 1995, 34, 4360.
(11) Rosenkilde, C.; Voyiatzis, G. A.; Østwold, T. Acta Chem. Scand. 1995,
49, 405.
(12) Hjuler, H. A.; Mahan, A.; von Barner, J. H.; Bjerrum, N. J. Inorg.
Chem. 1982, 21, 402.
(13) Zachariassen, K.; Berg, R. W.; Bjerrum, N. J.; von Barner, J. H. J.
Electrochem. Soc. 1987, 134, 1153.
(14) Bjerrum, N. J.; Berg, R. W.; Christensen, E.; Kerridge, D. H.; von
Barner, J. H. Anal. Chem. 1995, 34, 2129.
Our AlCl3 often contained a minor impurity of AlOCl.
This amount of AlOCl can be calculated from a potentio-
metric measurement on the equimolar NaCl-AlCl3 solvent
as described previously.13 Such a measurement was always
performed before the addition of the solutes, and the AlCl3
amounts referred to in this work are the corrected ones. The
calculations of the average coordination numbers and the
equilibrium constants are performed with assumptions for
the solvent and the electrochemical cell similar to those we
used before.3 Because we assume a total reaction between
9848 Inorganic Chemistry, Vol. 44, No. 26, 2005

Chloro and Oxochloro Complex Formation of Nb(V) and Ta(V)
Table 1. pK Values and Variances for Different Models of Chloro
Complex Formation of Nb(V) in NaCl-AlCl3 Melts at 175 °Ca
variance
equilibria
pK1
pK2
× 104
33.06
91.94
44.89
1. NbCl6- T NbCl5 + Cl-
2.86 (0.05)
2. 2NbCl6- T Nb2Cl10 + 2Cl- 4.60 (0.13)
3. NbCl6- T NbCl5 + Cl-
2NbCl5 T Nb2Cl10
2.9 (0.10)
0.08 (2.4)
2.9 (0.2)
3.4 (0.8)
4. 2NbCl6- T Nb2Cl11- + Cl- 1.5 (0.3)
Nb2Cl11- T Nb2Cl10 + Cl-
40.83
37.67
5. 2NbCl6- T Nb2Cl11- + Cl- 2.3 (0.8)
Nb2Cl11- T 2NbCl5 + Cl-
a Number of measurements: 6.
As it appears from Table 1, other models besides NbCl5,
including Nb2Cl10 and Nb2Cl11-, cannot be ruled out on the
basis of an F test at the 90% confidence level. However,
from Raman spectroscopic experiments,16 it does not seem
Figure 2. Average chloride coordination number as a function of pCl-
for a 0.08 M NbCl5 solution in NaCl-AlCl3 melts at 175 °C.
oxide and the added MCl5, we apply the MO3+ entity as the
basis for the average coordination numbers (M ) Nb, Ta).
Thus
-
likely that Nb2Cl11 is formed, whereas Nb2Cl10 may well
be present. Anyhow, including this species does not change
the pK value for eq 5 much. From our measurements, it could
be calculated to be 2.86 ((0.05) at 175 °C. This compares
to a value of 3.89 ((0.04) for the corresponding equilibrium
with tantalum in NaCl-AlCl3 at the same temperature,3
indicating that TaCl6- under these circumstances, as expected
(because of more binding electrons), is a more stable complex
-
njMO3+ ) (CN′ aCl + 3C′
+ 5C′
+ CA′ lOCl - 4[AlCl4 ] -
AlCl3
MCl5
-
7[Al2Cl7 ] - 10[Al3Cl10-] - 6[Al2Cl6] - [Cl-])/C′
(2)
MCl5
where C′ values are the formal concentrations of each of
the added substances, i.e., the initial molar amounts dissolved
in 1 L of the melt. Square brackets symbolize real molar
concentrations. In the present work, the NaCl-AlCl3 solvent
melts have a composition near 1:1. In this case, it has been
shown12 that the cell potential is given by
-
than NbCl6 .
In KCl-AlCl3 melts, we previously found2 a pK value of
4.49 for eq 5 for a 0.1 M solution of NbCl5 but at a higher
temperature (300 °C). To get an idea about the magnitude
of the equilibrium constant at 300 °C in the NaCl-AlCl3
solvent, we performed five measurements at 300 °C, with
average coordination numbers ranging between 5.24 and
5.72. As at 175 °C, the coordination number increased with
decreasing pCl-. If we assume eq 5 to be valid, a pK value
of 1.9 could be estimated at 300 °C. Thus, it seems that
NbCl6- under similar conditions is more stable in KCl-AlCl3
melts than in NaCl-AlCl3 melts because of the generally
higher chloride activity in the former solvent. When we
compare the equilibrium constants in the NaCl-AlCl3 melts
at the two temperatures, it is clear that an increase in
∆E ) (-RT/F) ln([Cl-II]/[Cl- ])
(3)
I
where I and II refer to the measuring and reference
compartments, respectively. Thus, the pCl- of the melt in
the measuring compartment can be calculated from
pCl-I ) -[F/(RT ln 10)]∆E + pCl-
(4)
II
where pCl- is the pCl- of the NaCl-AlCl3 melt saturated
II
with NaCl used as the reference electrode. At 175 °C, the
-
temperature destabilizes NbCl6 ; i.e., eq 5 is shifted to the
value of pCl- has been determined15 to be 1.128.
II
right with an increase in temperature. This is, of course, what
should be expected.
Results and Discussion
We also recorded the Raman spectrum of NbCl5 dissolved
in a NaCl-AlCl3 melt saturated with NaCl at 175 °C. Besides
NbCl5 in NaCl-AlCl3. The results of our potentiometric
measurements with chlorine-chloride electrodes on 0.08 M
NbCl5 solutions at 175 °C can be seen in Figure 2, where
the average chloride coordination numbers are plotted against
pCl-. As was the case for NbCl5 in KCl-AlCl3 melts,2 the
measurements could best be explained by the equilibrium
-
the bands from AlCl4 , a polarized band appeared at 398
cm-1. We assign this band to the symmetrical stretch
-
vibration of NbCl6 , in agreement with the work of Huglen
et al.16
Oxochloro Complexes, Potentiometric Measurements.
The result of the potentiometric measurements can be seen
in Figure 3, where the average coordination numbers (nj) for
NbO3+ are plotted against pCl- for a 0.10 M solution of
NbOCl3 in NaCl-AlCl3 at 175 °C. It can be noticed that
the average coordination number approaches 4 at low pCl-
values (i.e., basic melts). When the free chloride ion activity
NbCl6- T NbCl5 + Cl-
(5)
In the calculations, only the six experimental points with
pCl values greater than 2 are used. Data with smaller pCl
are not useful for model discrimination because all of the
tested models give similar variances in this range.
(15) Fehrmann, R.; von Barner, J. H.; Bjerrum, N. J.; Nielsen, O. F. Inorg.
Chem. 1981, 20, 1712.
(16) Huglen, R.; Mamantov, G.; Smith, G. P.; Begun, G. M. J. Raman
Spectrosc. 1979, 8, 326.
Inorganic Chemistry, Vol. 44, No. 26, 2005 9849

von Barner and Bjerrum
Figure 3. Average chloride coordination number as a function of pCl-
for a 0.1 M NbOCl3 solution in NaCl-AlCl3 melts at 175 ° C.
Figure 4. Average chloride coordination number as a function of pCl-
for a 0.3 M TaOCl3 solution in NaCl-AlCl3 melts at 175 °C.
Table 2. pK Values and Variances for Different Models of Oxochloro
Complex Formation of Nb(V) in NaCl-AlCl3 Melts at 175 °Ca
Table 3. pK Values and Variances for Different Models of Oxochoro
Complex Formation of Ta(V) in NaCl-AlCl3 Melts at 175 °Ca
variance
variance
× 104
equilibria
pK1
pK2
× 104
equilibria
pK1
pK2
1. NbOCl4- T NbOCl2+ + 2Cl-
2. NbOCl4- T NbOCl3 + Cl-
NbOCl3 T NbOCl2+ + Cl-
5.63 (25)
2.207 (39) 3.95 (5)
1521
18.83*
1. TaOCl4- T TaOCl2+ + 2Cl-
2. TaOCl4- T TaOCl3 + Cl-
TaOCl3 T TaOCl2+ + Cl-
7.1 (3)
2.743 (15) 4.521 (13)
1971
2.0*
3. NbOCl4- T NbOCl3 + Cl-
2NbOCl3 T (NbOCl2)22+ + 2Cl-
4. 2NbOCl4- T (NbOCl3)2 + 2Cl-
(NbOCl3)2 T 2NbOCl2+ + 2Cl-
5. 2NbOCl4- T (NbOCl3)2 + 2Cl-
(NbOCl3)2 T (NbOCl2)22+ + 2Cl-
2.16 (16) 6.81 (12) 49.99
3.31 (7) 8.99 (10) 23.07*
3.26 (12) 7.82 (12) 66.24
3. TaOCl4- T TaOCl3 + Cl-
2TaOCl3 T (TaOCl2)22+ + 2Cl-
4. 2TaOCl4- T (TaOCl3)2 + 2Cl-
(TaOCl3)2 T 2TaOCl2+ + 2Cl-
5. 2TaOCl4- T (TaOCl3)2 + 2Cl-
(TaOCl3)2 T (TaOCl2)22+ + 2Cl-
2.68 (4) 8.35 (4)
12.5
28.1
33.7
4.86 (10) 9.53 (9)
4.77 (11) 8.83 (6)
a Number of measurements: 10 (number of different cells: 2). Asterisks
indicate those based on F0.10 (8.8) ) 2.59.
a Number of measurements: 10 (number of different cells: 2). Asterisks
indicate those based on F0.10 (8.8) ) 2.59.
is diminished (i.e., increasing pCl- values), the average
chloride coordination number drops, and in the most acidic
melts, it gets close to 2. This certainly points in the direction
of the formation of several oxochloro complexes. It seems
are the ones that have a probability greater than 90% (based
on an F test). It seems clear that models involving two
equilibria can best explain the experiments. Further, the
+
two models with monomeric NbOCl2 are clearly better
-
obvious to suggest that NbOCl4 is formed at low pCl-
than those with the corresponding dimeric species, i.e.,
(NbOCl2)22+. However, concerning NbOCl3, we cannot
distinguish between the monomeric and dimeric forms of
this species.
because the average coordination number for NbO3+ does
not exceed 4. This is somewhat surprising because it is
known from Raman spectroscopic measurements10 that
2-
The average coordination numbers for TaO3+ as a function
of pCl- based on potentiometric measurements on NaCl-
AlCl3 melts with TaCl5 and AlOCl added can be seen in
Figure 4. Also in this case, the average coordination number
approaches 4 at low pCl-. When the melts are made more
acidic, i.e., with increasing pCl-, the nj values gradually
diminish and reach values of around 2.5 for the most acidic
melts investigated. It therefore seems reasonable to suggest
NbOCl5 is present in a number of alkali chloride melts
2-
with NbOCl3 added. However, in the melts where NbOCl5
was observed, a larger excess of free chloride was present
than in our case. The samples were typically mixtures of
AlkCl-NbOCl3 (Alk ) alkali metal) with XAlkCl greater than
0.66 or diluted solutions of NbOCl3 in binary alkali chloride
mixtures.10
Indeed, NbOCl4- complexes have previously been identi-
fied by vibrational spectroscopy17,18 in the solid state and
were recently also observed in alkali chloride melts where
only limited amounts of chloride were present.10 Thus,
models involving NbOCl4- and various NbOClx(x-3)- species
were tested. For the same reason as was mentioned in the
previous chapter, only the eight most acidic measurements
were used for model discrimination. Both monomeric and
dimeric species were taken into account. In Table 2, the
equilibria that could best explain our potentiometric mea-
surements are shown. The models marked with an asterisk
-
that TaOCl4 is formed in the basic melts. Consequently,
we have, using the eight measurements with the highest pCl,
tested models that involve this complex and one or two more
tantulum(V) oxochloro complexes with Cl/Ta atomic ratios
of less than 4. The result appears in Table 3. As can be seen
from this table, only model 2, which involves two equilibria
-
between the monomeric species TaOCl4 , TaOCl3, and
+
TaOCl2 , is able to explain the measurements on a 90%
confidence level.
Spectroscopic Investigations. The UV-vis spectra of
NbCl5 dissolved in NaCl-AlCl3 melts saturated with NaCl
with and without oxide added have been recorded at 175
°C. To eliminate the influence of oxide impurities, two
(17) Hiller, W.; Stra¨hler, J.; Prinz, H.; Dehnicke, K. Z. Naturforsch. 1984,
39b, 107.
(18) Morozov, I. S.; Lipatova, N. P. Russ. J. Inorg. Chem. 1968, 13, 1101.
9850 Inorganic Chemistry, Vol. 44, No. 26, 2005

Chloro and Oxochloro Complex Formation of Nb(V) and Ta(V)
Figure 6. UV-vis spectra of MOCl4- complexes (M ) Nb, Ta) in basic
Figure 5. UV-vis spectrum of NbCl6- in a basic NaCl-AlCl3 melt (pCl-
) 1.128) at 175 °C. The spectrum is the result of a subtraction of two spectra
with Nb(V) concentrations of 0.042 and 0.028 M, respectively.
NaCl-AlCl3 melts (pCl ) 1.128) at 175 °C. Full line: NbOCl4-. Dotted
-
line: TaOCl4
.
spectrum is much different from the one of the chloro
spectra of melts prepared from the same batch of chemicals
but with different amounts of NbCl5 added were recorded.
The spectrum shown in Figure 5 was obtained by subtraction
of these. It has two bands, with the strongest band appearing
at 34.7 × 103 cm-1 and a minor band at 41.6 × 103 cm-1.
We suggest that this spectrum arises from NbCl6- in NaCl-
AlCl3. It is quite similar to the spectrum of NbCl6- in KCl-
AlCl3 at 300 °C, for which we previously2 observed bands
at 34.4 and 41.3 × 103 cm-1, with the former being by far
the most intensive. Further, the spectrum of TaCl6- in NaCl-
complex formed in the NaCl-AlCl3 melt saturated with
NaCl, i.e., TaCl6 , for which we previously3 found the
-
strongest band at 41.1 × 103 cm-1. For NaCl-AlCl3 melts
saturated with NaCl, we know from our potentiometric
measurements on samples with oxide added that the average
chloride coordination number for MO3+ is 4 in the case of
both M ) Nb and M ) Ta. Thus, the spectra shown in Figure
-
6 should arise from the pure MOCl4 complexes.
Conclusions
-
AlCl3 at 175 °C resembles the one of NbCl6 in shape, but
It has been shown that NbCl6- exists in basic NaCl-AlCl3
melts at 175 °C. In moderately acidic melts, NbCl5 is formed
the bands appear at higher wavenumbers. In this case, the
two main bands were found3 at 41.1 and 49.0 × 103 cm-1,
and again the low-frequency band was the strongest.
Concerning the Raman spectrum, the band at 298 cm-1
disappeared when AlOCl was added to the NbCl5-NaClsat-
AlCl3 melt in such an amount that the molar ratio of O/Nb
-
in equilibrium with NbCl6 , but the existence of other species
such as Nb2Cl10 cannot be completely ruled out. When oxide
is added to NaCl-AlCl3 melts with MCl5 (M ) Nb, Ta)
dissolved, MOCl4- complexes are formed in the basic melts.
When the melts are made progressively more acidic, species
-
became 1. This clearly indicates that NbCl6 reacts with
+
oxide, i.e., supporting our assumption of quantitative forma-
tion of an oxochloro species.
of the types MOCl3 and MOCl2 appear. On the basis of
potentiometric measurements, the equilibrium constants for
the niobium(V) chloro and for the niobium(V) and tantalum-
(V) oxochloro complex equilibria in NaCl-AlCl3 melts have
Also, the UV-vis spectrum changes considerately when
oxide is added. The spectrum of NbCl5 in a NaClsat-AlCl3
melt with oxide added (O/Nb ) 1) can be seen in Figure 6
-
been calculated. The UV-vis spectra of NbCl6, NbOCl4 ,
-
and TaOCl4- have also been obtained. It is found that under
similar conditions Ta(V) forms stronger chloro and oxochloro
complexes than Nb(V). The chloro complexes are more
stable (at the same temperature) in KCl-AlCl3 melts than
in NaCl-AlCl3 melts. Increasing the temperature from 175
to 300 °C (with the same solvent) will destabilize the higher
chloro complexes.
(full line). Like the spectrum of NbCl6 , it has two bands,
but they appear now at 32.7 and 42.9 × 103 cm-1,
respectively. Further, the intensity ratio has changed: the
band with the highest wavenumber is now the most intense.
The spectrum of the corresponding tantalum oxochloro
species formed in NaCl-AlCl3 melts saturated with NaCl
is also shown in Figure 5 (dotted line). When we compare
the spectra of the niobium(V) and tantalum(V) oxochloro
species, it is seen that they resemble each other in shape,
but the two bands on the tantalum spectrum are shifted
toward higher wavenumbers. They appear at 38.6 and 48.1
× 103 cm-1, respectively. As in the case of niobium, the
Acknowledgment. We acknowledge Rolf W. Berg for
recording the Raman spectra and Claus A. Jørgensen for
assistance with the experiments.
IC050367A
Inorganic Chemistry, Vol. 44, No. 26, 2005 9851