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A kinetic study of the hexachloromolybdate(III) ion hydrolysis
INORG.
NUCL.
CHEM.
LETTERS
Vol. 8, pp. 689-694,
1972. Pergamon
Press. Printed
in
Great
Britain.
A KINETIC STUDY OF THE HEXACHLOROMOLYBDATE(III) I0N HYDROLYSIS William Andruchow, Jr. and John DiLiddo Department of Chemistry College of the Holy Cross Worcester, Nassachusetts 01610 (Received 6 March 1972)
A literature examination of molybdenum(III) systems reveals that the reactive behavior of this element in this oxidation state is not clearly understood.
Although there have been
numerous examples of synthetic preparations of molybdenum(III) compounds, notably with simple unidentate ligands and acetylacetonates, there is a definite paucity of quantitative information on the behavior of these systems, particularly in aqueous solution. In addition, the lack of model compounds not only restricts the aqueous solution studies to systems of the type MoX63- where X = F-, C1-, and Br-, but reasonable comparisons with congener compounds of Cr(III) are also limited. Now, Guibe and Souchay(1), Bucknall, et al(2), Hartmann and Schmidt(3), and Bowen and Taube(4) have investigated the behavior of the MoC136- ion and other related species in water.
The conductance
studies of Bucknall, et al and the pH studies of Guibe and Souchay indicate that a rapid hydrolysis of the MoC136- ion takes place at
689
690
A KINETIC STUDY
room temperature, characteristics
Vol. 8, No. 8
and, based on the changes in the spectral
of the aqueous MoCl~- solution, a transformation
takes place even in 6 N NCI(3,5).
In contrast, Bowen and Taube
state in their article that the spectral characteristics NoCl~- ion in 6 M HC1 remainunaltered
of the
over a period of two days.
We wish to point out that the spectral profile of the NoCI~- species can only be determined in HCI solutions whose concentration is greater than 9 ~.
Furlani and Piovesna(5) have reported,
and we have con-
firmed their findings, that the spectra of the hexachloro complex does change in 6 M HCI; and, if care is not taken to run the samples immediately after preparation,
aquation will take place.
Thus, it
appears that what Bowen and Taube measured is an equilibrium mixture of the hexachloro and pentachloroaquomolydate(III) deed stable over a two day period.
ions which is in-
We have found that the NoCl~- ion
in 12 M HC1 exhibits only two bands in the visible region at 5 2 2 n m and $20 n m w i t h
molar absoptivity values of 31.7 and $1.8, respec-
tively, which we feel are perhaps the most reliable values reported to date.
In any event, attempts to quantitatively measure the be-
havior of the NoCl~- ion in aqueous solution have not been made. It has been assumed that the first step in the reaction is the replacement of the chloride ion from the coordination sphere of NoCl~- by a water molecule according to the reaction
MoCI~- + H20
kl-.-~ MoC15(N20)2- + C1-.
Further reaction could take place vi_~a an additional replacement of another chloride ion or through the hydrolysis of the aquo group to give the corresponding hydroxo complex.
Ample empirical evidence
has been given, as mentioned above, to show that a reaction does take
Vol. 8, No. 8
A K I N E T I C STUDY
691
place with water but the exact path has not been clearly elucidated. In order to study this system, the aquation in the first step should be clearly established before any reasonable speculation can be made on subsequent reactions. We wish to report the results of what we believe to be the first kinetic study of the aquation of the MoC163- ion. was studied spectrophotometrically meter.
The reaction
on a Beckman DK2A spectrophoto-
Temperature regulation was achieved by thermostating the
cells in a specially designed cell holder, and the reaction was allowed to take place at T = O°C l 0.05 ° over a pH range of 3.2 $-5.
The temperature O°C was chosen because the reaction proceeds
relatively fast at room temperature; the experimental
the half-life,
limits of the spectrophotometric
found to be in the order of 30-60 seconds.
estimated within
technique used, was
The buffers were prepared
from potassium acidpthalate and either sulfuric acid or sodium hydroxide.
The wavelengths 390 nm and $I0 nm were chosen for the
study because they afforded the greatest molar absoptivity separation between the reactant chromophore, chromophore,
NoCI5(H20)2-.
media had to be considered, Cr(H20)~+_ was examined.
NoC136-, and the product
Since possible reaction with the buffer the interaction of the media with
It was found that no change in the position
of the spectral bands of Cr(H20)36+ occured in the buffer solutions,
thus, there is no reason to believe that Mo(lll) should have any greater tendency to react with the buffer species in preference to the water. The reaction at O°C was found to follow pseudo-first etics consistent with a primary aquation step.
order kin-
The rate constant
(k I)
over the pH range studied was found to be (1.33 + 0.09) x lO-~sec. -I
692
A KINETIC STUDY
with a half-life(t~) of 87 minutes.
Vol. 8, No. 8
We also have found that the
second aquation step, MoCI5(H20)2-
+
H20
k2--~ MoC14(H20) 2
+
C1-
takes place very slowly at this temperature*, and that the pentachloroaquomolydate(III) species has little tendency to recombine with the released chloride ion within the ranges of chloride ion concentrations produced by the primary aquation.
On the other hand,
there is a marked increase in the apparent rate of reaction at pH~
5.5 which suggests that a base hydrolysis is taking place. Although the aquation of the MoCI36- ion is, in general, some-
what faster at this temperature than compounds of its congener, Cr(III), it is worthy to note that the corresponding chromium compound, CrC163-, is very unstable to hydrolysis and will rapidly react with water to form chloroaquo complexes of Cr(III)(6).
Also, the overall
rapidity of the reaction is somewhat surprising in view of expected crystal
field effects.
Not only would a d3-configuration be ex-
pected to exhibit inert kinetics, but the reaction rates of second and third row transition metal compounds are usually slower than their first row congeners(7).
On the other hand, because the d-d
bands do indeed shift, into the ultraviolet region, thermodynamic factors in terms of CFSE certainly favor the formation of the aquo complex.
However, alternative mechanisms may be operative at the
elevated temperatures, and, there is no certainty that the final product would be the hexaquomolydate(III) ion.
Consequently, we
The slowness of this reaction have led us to assume that any contribution to the absorption by the NoCI4(H20) 2 or other related species can be considered to be negligible.
Vol. 8, No. 8
A KINETIC STUDY
693
feel that the spectral profile of the Mo(lll) solution shown by Bowen and Taube(@) is not convincing evidence that the Mo(H20)~ + species is present.
Since it is well known that second and third
row transition metal ions have a greater tendency to undergo dimerization via hydroxolation or oxolation even under acidic conditions(8),
the species present, when one examines the time
allowed for the reaction to take place and the solution media, is more likely to be an oxo- or hydroxo- bridged dimer, of the type
( 2o) o where:
R
=
02-
-
Ryor
Mo( 2O)x
OH-.
The behavior of NoCl~- ion in aqueous solution is not as straight forward as one might imagine, and a considerable amount of work is still needed on this system.
Although the reasons for
the rapidity of the reaction of the NoCl~- ion with water are not clear, it does appear that the bound chloro ligands play an important role in the reactivity of the hexachloro elements.
complexes of group VIB
Temperature dependent rate studies are presently being
carried out which will hopefully lead to additional insight into the reaction intermediate. References I.
L. Guibe and P. Souchay, Compt. rend., 24#, 780 (1957)
2.
W. R. Bucknall, 1927, 512
S. R. Carter, and W. Wardlaw,
J. Chem. S,c.,
694
A KINETIC STUDY
Vol. 8. No. 8
3.
H. Hartmann and H. J. Schmidt, Z. physik. (1957
Chem., ll, 235
4.
A. R. Bowen and H. Taube, J. Am. Chem. Soc., 95, 3287 (1971)
5.
C. Furlani and O. Piovesana, Nol. Phys., 9, 541 (1965)
6.
H. Schlesinger, J. Am. Chem. Soc., 51, 3520 (1929)
7.
F. Basolo and R. G. Pearson, "Mechanisms of Inorganic Reactions", 2nd ed. John Wiley and Sons, Inc., New York, NY (1967)
8.
G. P. Haight, J. Inorg. Nucl. Chem., 2@, 663 (1962)
NUCL.
CHEM.
LETTERS
Vol. 8, pp. 689-694,
1972. Pergamon
Press. Printed
in
Great
Britain.
A KINETIC STUDY OF THE HEXACHLOROMOLYBDATE(III) I0N HYDROLYSIS William Andruchow, Jr. and John DiLiddo Department of Chemistry College of the Holy Cross Worcester, Nassachusetts 01610 (Received 6 March 1972)
A literature examination of molybdenum(III) systems reveals that the reactive behavior of this element in this oxidation state is not clearly understood.
Although there have been
numerous examples of synthetic preparations of molybdenum(III) compounds, notably with simple unidentate ligands and acetylacetonates, there is a definite paucity of quantitative information on the behavior of these systems, particularly in aqueous solution. In addition, the lack of model compounds not only restricts the aqueous solution studies to systems of the type MoX63- where X = F-, C1-, and Br-, but reasonable comparisons with congener compounds of Cr(III) are also limited. Now, Guibe and Souchay(1), Bucknall, et al(2), Hartmann and Schmidt(3), and Bowen and Taube(4) have investigated the behavior of the MoC136- ion and other related species in water.
The conductance
studies of Bucknall, et al and the pH studies of Guibe and Souchay indicate that a rapid hydrolysis of the MoC136- ion takes place at
689
690
A KINETIC STUDY
room temperature, characteristics
Vol. 8, No. 8
and, based on the changes in the spectral
of the aqueous MoCl~- solution, a transformation
takes place even in 6 N NCI(3,5).
In contrast, Bowen and Taube
state in their article that the spectral characteristics NoCl~- ion in 6 M HC1 remainunaltered
of the
over a period of two days.
We wish to point out that the spectral profile of the NoCI~- species can only be determined in HCI solutions whose concentration is greater than 9 ~.
Furlani and Piovesna(5) have reported,
and we have con-
firmed their findings, that the spectra of the hexachloro complex does change in 6 M HCI; and, if care is not taken to run the samples immediately after preparation,
aquation will take place.
Thus, it
appears that what Bowen and Taube measured is an equilibrium mixture of the hexachloro and pentachloroaquomolydate(III) deed stable over a two day period.
ions which is in-
We have found that the NoCl~- ion
in 12 M HC1 exhibits only two bands in the visible region at 5 2 2 n m and $20 n m w i t h
molar absoptivity values of 31.7 and $1.8, respec-
tively, which we feel are perhaps the most reliable values reported to date.
In any event, attempts to quantitatively measure the be-
havior of the NoCl~- ion in aqueous solution have not been made. It has been assumed that the first step in the reaction is the replacement of the chloride ion from the coordination sphere of NoCl~- by a water molecule according to the reaction
MoCI~- + H20
kl-.-~ MoC15(N20)2- + C1-.
Further reaction could take place vi_~a an additional replacement of another chloride ion or through the hydrolysis of the aquo group to give the corresponding hydroxo complex.
Ample empirical evidence
has been given, as mentioned above, to show that a reaction does take
Vol. 8, No. 8
A K I N E T I C STUDY
691
place with water but the exact path has not been clearly elucidated. In order to study this system, the aquation in the first step should be clearly established before any reasonable speculation can be made on subsequent reactions. We wish to report the results of what we believe to be the first kinetic study of the aquation of the MoC163- ion. was studied spectrophotometrically meter.
The reaction
on a Beckman DK2A spectrophoto-
Temperature regulation was achieved by thermostating the
cells in a specially designed cell holder, and the reaction was allowed to take place at T = O°C l 0.05 ° over a pH range of 3.2 $-5.
The temperature O°C was chosen because the reaction proceeds
relatively fast at room temperature; the experimental
the half-life,
limits of the spectrophotometric
found to be in the order of 30-60 seconds.
estimated within
technique used, was
The buffers were prepared
from potassium acidpthalate and either sulfuric acid or sodium hydroxide.
The wavelengths 390 nm and $I0 nm were chosen for the
study because they afforded the greatest molar absoptivity separation between the reactant chromophore, chromophore,
NoCI5(H20)2-.
media had to be considered, Cr(H20)~+_ was examined.
NoC136-, and the product
Since possible reaction with the buffer the interaction of the media with
It was found that no change in the position
of the spectral bands of Cr(H20)36+ occured in the buffer solutions,
thus, there is no reason to believe that Mo(lll) should have any greater tendency to react with the buffer species in preference to the water. The reaction at O°C was found to follow pseudo-first etics consistent with a primary aquation step.
order kin-
The rate constant
(k I)
over the pH range studied was found to be (1.33 + 0.09) x lO-~sec. -I
692
A KINETIC STUDY
with a half-life(t~) of 87 minutes.
Vol. 8, No. 8
We also have found that the
second aquation step, MoCI5(H20)2-
+
H20
k2--~ MoC14(H20) 2
+
C1-
takes place very slowly at this temperature*, and that the pentachloroaquomolydate(III) species has little tendency to recombine with the released chloride ion within the ranges of chloride ion concentrations produced by the primary aquation.
On the other hand,
there is a marked increase in the apparent rate of reaction at pH~
5.5 which suggests that a base hydrolysis is taking place. Although the aquation of the MoCI36- ion is, in general, some-
what faster at this temperature than compounds of its congener, Cr(III), it is worthy to note that the corresponding chromium compound, CrC163-, is very unstable to hydrolysis and will rapidly react with water to form chloroaquo complexes of Cr(III)(6).
Also, the overall
rapidity of the reaction is somewhat surprising in view of expected crystal
field effects.
Not only would a d3-configuration be ex-
pected to exhibit inert kinetics, but the reaction rates of second and third row transition metal compounds are usually slower than their first row congeners(7).
On the other hand, because the d-d
bands do indeed shift, into the ultraviolet region, thermodynamic factors in terms of CFSE certainly favor the formation of the aquo complex.
However, alternative mechanisms may be operative at the
elevated temperatures, and, there is no certainty that the final product would be the hexaquomolydate(III) ion.
Consequently, we
The slowness of this reaction have led us to assume that any contribution to the absorption by the NoCI4(H20) 2 or other related species can be considered to be negligible.
Vol. 8, No. 8
A KINETIC STUDY
693
feel that the spectral profile of the Mo(lll) solution shown by Bowen and Taube(@) is not convincing evidence that the Mo(H20)~ + species is present.
Since it is well known that second and third
row transition metal ions have a greater tendency to undergo dimerization via hydroxolation or oxolation even under acidic conditions(8),
the species present, when one examines the time
allowed for the reaction to take place and the solution media, is more likely to be an oxo- or hydroxo- bridged dimer, of the type
( 2o) o where:
R
=
02-
-
Ryor
Mo( 2O)x
OH-.
The behavior of NoCl~- ion in aqueous solution is not as straight forward as one might imagine, and a considerable amount of work is still needed on this system.
Although the reasons for
the rapidity of the reaction of the NoCl~- ion with water are not clear, it does appear that the bound chloro ligands play an important role in the reactivity of the hexachloro elements.
complexes of group VIB
Temperature dependent rate studies are presently being
carried out which will hopefully lead to additional insight into the reaction intermediate. References I.
L. Guibe and P. Souchay, Compt. rend., 24#, 780 (1957)
2.
W. R. Bucknall, 1927, 512
S. R. Carter, and W. Wardlaw,
J. Chem. S,c.,
694
A KINETIC STUDY
Vol. 8. No. 8
3.
H. Hartmann and H. J. Schmidt, Z. physik. (1957
Chem., ll, 235
4.
A. R. Bowen and H. Taube, J. Am. Chem. Soc., 95, 3287 (1971)
5.
C. Furlani and O. Piovesana, Nol. Phys., 9, 541 (1965)
6.
H. Schlesinger, J. Am. Chem. Soc., 51, 3520 (1929)
7.
F. Basolo and R. G. Pearson, "Mechanisms of Inorganic Reactions", 2nd ed. John Wiley and Sons, Inc., New York, NY (1967)
8.
G. P. Haight, J. Inorg. Nucl. Chem., 2@, 663 (1962)