Characterization, kinetics and mechanism of thermal decomposition of photosubstituted ethylenediamine complexes of molybdate(IV) and tungstate(IV) with chromium(III).

Article abstract of DOI:10.1016/S0040-6031(98)00379-7

Photoinitiated substitution complexes of [M(CN)₈]^4- (where M= Mo(IV) or W(IV)) and ethylenediamine with chromium(III) have been synthesized and characterized. On the basis of elemental analysis, the complexes have been assigned formulas as folows:Cr[Mo(CN)(C₂H₈N₂)(OH) ₆]·2H₂O I for Mo(IV); andCr[W(C₂H₈N₂)(O₂)(OH) ₃]·H₂O II for W(IV)The characteristic IR absorption peaks for different entities present support the assigned formulas. Complex I shows the absorption peaks due to ν(CN) stretching bond, NH asymmetric deformation, NH symmetric deformation and NH₃ rocking mode showing the presence of cyanide and ethylenediamine. The loss of the absorption peak due to ν(CN) stretching in the spectra of complex II confirms the substitution of all the cyanide ligands. The thermal degradation of the complexes has been studied by TG and DSC techniques. Both the complexes have a similar thermal decomposition behavior: involving expulsion of water molecules in the first step followed by the expulsion of other ligands. Kinetics and a thermal decomposition mechanism have been proposed for each complex. Thermodynamic parameters such as activation energy (E_a), pre-exponential factor (A) and entropy of activation (ΔS^#) have been calculated for each step, employing different integral methods of Doyle, Coats and Redfern, and Arrhenius. The reaction enthalpy is obtained from DSC data.

Full text of DOI:10.1016/S0040-6031(98)00379-7

Thermochimica Acta 317 (1998) 183±192
Characterization, kinetics and mechanism of thermal decomposition of
photosubstituted ethylenediamine complexes of molybdate(IV)
and tungstate(IV) with chromium(III).
S.I. Ali*, Kowsar Majid
Department of Chemistry, Jamia Millia Islamia, New Delhi-110 025, India
Received 5 January 1998; received in revised form 20 May 1998; accepted 22 May 1998
Abstract
Photoinitiated substitution complexes of [M(CN)8]4 (where Mˆ Mo(IV) or W(IV)) and ethylenediamine with
chromium(III) have been synthesized and characterized. On the basis of elemental analysis, the complexes have been
assigned formulas as folows:
Cr‰MoꢀCN†ꢀC2H8N2†ꢀOH† Š Á 2H2O I for MoꢀIV†; and
6
Cr‰WꢀC2H8N2†ꢀO2†ꢀOH† Š Á H2O II for WꢀIV†
3
The characteristic IR absorption peaks for different entities present support the assigned formulas. Complex I shows the
absorption peaks due to ꢀ(C>N) stretching bond, NH asymmetric deformation, NH symmetric deformation and NH3 rocking
mode showing the presence of cyanide and ethylenediamine. The loss of the absorption peak due to ꢀ(C>N) stretching in the
spectra of complex II con®rms the substitution of all the cyanide ligands. The thermal degradation of the complexes has been
studied by TG and DSC techniques. Both the complexes have a similar thermal decomposition behavior: involving expulsion
of water molecules in the ®rst step followed by the expulsion of other ligands. Kinetics and a thermal decomposition
mechanism have been proposed for each complex. Thermodynamic parameters such as activation energy (Ea), pre-exponential
factor (A) and entropy of activation (ÁS#) have been calculated for each step, employing different integral methods of Doyle,
Coats and Redfern, and Arrhenius. The reaction enthalpy is obtained from DSC data # 1998 Elsevier Science B.V.
Keywords: Cr(III) complexes; Octacyanomolybdate (IV); Octacyanotungstate (IV); Thermal decomposition; Thermal kinetics
1. Introduction
[2] have reported that Chromium(III) molybdate in the
solid state decomposes to Cr2O3 and MoO3 before
reaching its melting point. The decomposition of
Cr2(MoO4)3 starts at a temperature >8508C and is
accompained by sublimation of MoO3. Thermal
degradation of group VI metal carbonyls have shown
the in¯uence of the metal and ligand on the decom-
position course [3]. In complexes of the type M(CO)5L
(where MˆCr, Mo and W, and Lˆpyridine, triphe-
Different reports on thermal behaviour of chromi-
um(III) molybdate exist in the literature. Plasova and
Kefel [1] have found that Cr2(MoO4)3 melts incon-
gruently at 800±8108C and Getman and Marchenko
*Corresponding author. Tel.: +91 683-1717; fax: +91 682-1232.
0040-6031/98/$19.00 # 1998 Elsevier Science B.V. All rights reserved
P I I S 0 0 4 0 - 6 0 3 1 ( 9 8 ) 0 0 3 7 9 - 7

184
S.I. Ali, K. Majid / Thermochimica Acta 317 (1998) 183±192
nylphosphine), TG and DSC studies have shown
different expulsion behaviour of ligand with different
metals [4].
under UV radiation, until the colour of the solution
changed to deep red. Irradiation was stopped at that
stage and an aqueous solution of Cr(NO3)3 was added
until the solution gave a black precipitate. The whole
solution was subjected to centrifugation, the precipi-
tates left were washed two to three times with water
and then dried over fused CaCl2. The complexes were
analysed for C, H and N and the emperical formula for
the complex I was found to be Cr[Mo(CN)(C2H8-
N2)(OH)6]Á2H2O I for Mo(IV): observed Cˆ9.50;
Hˆ4.5; Nˆ11.20% vs. calculated Cˆ9.68; Hˆ4.87;
Nˆ11.29% andCr[W(C2H8N2)(O2)(OH)3].H2O II for
W(IV): observed Cˆ6.50; Hˆ3.3; Nˆ7.25% vs. cal-
culated Cˆ6.05; Hˆ3.3; Nˆ7.05%.
Ultraviolet irradiation in the ligand ®eld bands of
the octacyanocomplexes of Mo(IV) and W(IV) in
alkaline medium results in the substitution of cyanide
ligand by water [5] which is followed by secondary
thermal steps resulting in the loss of other cyanide
groups with susequent formation of the stable products
with other ligands such as 2,20-bipyridyl, 1,10-phe-
nanthroline, 1,2,3-benzotriazole, ethylenediamime
and triethylenetetramine [6±9]. Ethylenediamine is
reported to form a photoproduct with oxine [10] in
which oxine- and cyano- moieties are removed in
stages at higher temperatures resulting in the forma-
tion of an oxide of polymeric nature as residue.
Thermal behaviour of various cyano compounds have
shown that metal ions and the anion moieties affect the
course of thermally induced reactions [11,12]. Since
mixed ligand complexes of transition metals have
been widely discussed, being biologically signi®cant,
the use of thermal methods of analysis to study the
bonding and structure of coordination compounds are
common [13]. We now report the formation of photo-
4. Physical measurements
Determination of carbon, hydrogen and nitrogen
was carried out by microanalysis. IR spectra of the
complexes were recorded on a Perkin±Elmer 1710
Fourier-transform spectrophotometer in the 400-
1
4000 cm range using a KBr disc. The TG was
conducted on a DuPont 2000 thermal analyser. DSC
was also carried out on a DuPont TA 2000 system with
a DSC attachment. The scanning was done at a rate of
108C/min. An aluminium pan was used as reference
under a dynamic nitrogen atmosphere. The DSC was
calibrated by the fusion of indium (99.99% purity).
The temperature repeatability of the instrument is
Æ0.18C and of ÁH is Æ0.02 mW.
substituted
ethylenediamine
complexes
of
[M(CN)8]4 [where MˆMo(IV) or W(IV)] with chro-
mium(III). The complexes are characterized by IR
spectroscopy and thermal degradation has been stu-
died by TG and DSC. The mechanism of the thermal
dissociation has been proposed and thermodynamic
parameters have been calculated employing different
integral methods.
5. Results and discussion
2. Materials and methods
Photoexcitation of octacyano complexes of Mo(IV)
and W(IV) with ethylenediamine results in the sub-
stitution of cyanide ions by ethylenediamine. Ethyle-
nediamine provides a coordinating entity of amine
group and forms complexes of different stoichiome-
tries with Mo(IV) and W(IV). The IR absorption
bands observed for complex I con®rm the formation
of metal±amine complex showing characteristic peaks
at 3600, 1360, 880 cm 1 assigned as NH asymmetric
deformation [15], NH symmetric deformation and the
NH3 rocking mode [16], respectively. The presence of
cyanide group is shown by the characteristic C>N
stretching band at 2170±2040 cm 1. Since the C>N
Potassium octacyanomolybdate(IV) and octacyano-
tungstate(IV) were prepared by the method of Lei-
poldt et al. [14]. Ethylenediamine, chromium nitrate,
Cr(NO3)3Á6H2O were of AR grade. All the solutions
were prepared in doubly distilled water.
3. Synthesis of complexes
The complexes were synthesized by irradiating
K4M(CN)8 [where MˆMo(IV) or W(IV)] and ethyl-
enediamine solutions of 0.1 M strength in 1 : 2 ratio

S.I. Ali, K. Majid / Thermochimica Acta 317 (1998) 183±192
185
stretching band of the free ion in KCN is observed at
1
2080 cm which is shifted to higher frequencies by
supporting the complete substitution of the cyanide
ligand by ethylenediamine. The stretching vibration of
C±C band is exactly the same at 1050 cm 1. The N±H
asymmetric deformation at 1600 cm 1 and NH3 rock-
ing mode at 840 cm 1 due to ethylenediamine mole-
cule is also supportive of the metal±ammine complex.
The ꢁr (NH3) in case of complex II is at lower
coordination, and as coordination usually weakens the
ligand bond next to the coordinate bond, thus shifting
the ligand stretching vibrations to a lower frequency
region [17], thus justifying the coordination of CN by
appearance of the band at 2100 cm 1. The band at
1050 cm 1 could be the result of the stretching vibra-
tion of either the C±N or C±C vibration. The band at
ca. 1600 cm 1 may be assigned to the NH2 vibrations.
In addition to the C>N stretching bond, the complex
exhibits d(M±C>N) stretching at inner frequency
1
frequency by 40 cm than complex I, which may
be due to the heavier mass of tungsten than molybde-
num, shifting the M±N stretching vibration in lower
frequency region. The presence of the water outside
the coordination sphere is evident from the ꢀ(O±H)
mode at 3450 cm 1, the same as in the complex I.
1
region at 640±660 cm which is in conformity with
1
the reported d(Cr±C>N) [18]. The presence of water as
lattice water trapped by weak forces is shown by the
appearance of the symmetric (O±H) stretching mode
The strong absorption at 1400 cm is assigned as
ꢀ(±N=N±).
at 3450 cm 1 (usually between 3550±3200 cm 1) and
1
of H±O±H bending mode at 1640 cm
(being
6. Thermogravimetry studies
between 1630±1600 cm 1). The absorption band at
1
1500 cm is due to ±N=O or ring vibrations. The
6.1. Cr[Mo(CN)(C2H8N2)(OH)6]Á2H2O I
absoption peaks for the complex II are similar to that
of the complex I with some variation in absorption
peaks justifying the proposed composition. It shows
the loss of absorption peaks due to C>N stretching
The TG and DTG curves of complex I are shown in
Fig. 1. The TG curve indicates the dissociation of the
complex in two signi®cant steps, the DTG shows a
Fig. 1. The TG and DTG curves of complex I.

186
S.I. Ali, K. Majid / Thermochimica Acta 317 (1998) 183±192
sharp peak corresponding to the second thermal
decomposition step and a minor peak for the ®rst
transition.
The ®rst transition initiates from 50 to1708C with
DTG maximum of 110Æ0.18C. The observed mass
loss is 10.15, against the calculated loss of 9.68%,
corresponding to the release of two moles of water.
The temperature range of the transition and DTG
maximum justi®es the removal of water molcules
from the outer coordination sphere. The second tran-
sition is a signi®cant one starting from 1808 to 2808C
with DTG maximum of 2308C, undergoing an
observed mass loss of 50.54 against the calculated
loss of 50.7%, due to the release of the remaining
coordinating moieties of 5 mol of hydroxyl, 1 mol of
ethylenediamnine and 1 mol of cyanide. The observed
mass of the residue is 39.15 against the calculated
39.7%, corresponding to mass of Cr and Mo metals.
The sequential thermal decomposition process of the
complex is shown in Scheme 1. The percentage with,
and without, parentheses correspond to calculated and
observed values, respectively.
Scheme 1.
correspond to the transitions observed in the TG curve.
The exothermic transition observed due to the release
of water molecules starts from 66.58C up to 190.48C
with maximum at 151.08C, giving ÁH value of
272.4 J/g. The endothermic transition observed due
to the release of all other organic ligands and hydroxyl
groups from the inner coordination sphere initiates
from 200.58C up to 297.78C with maximum of
229.28C involving a ÁH value of 174.8 J/g. The
derivative of the second transition involves overlap-
ping of the consecutive peaks, thus justi®ing the
evolution of more than one ligand. The mechanism
involved is shown in Scheme 2.
The DSC of I is shown in Fig. 2. Two transitions,
one exothermic and one endothermic in DSC curve,
Fig. 2. The DSC curve of complex I.

S.I. Ali, K. Majid / Thermochimica Acta 317 (1998) 183±192
187
Scheme 2.
6.2. Cr[W(C2H8N2)(O2)(OH)3]ÁH2O II
against calculated loss of 19.40%, corresponding to
the release of one hydroxyl group and 1 mol of
ethylenediamine molecule. The observed mass of
the residue is 66.9 against calculated 67.4%, which
is due to WO2 and Cr metal. The decomposition
temperature of WO2 in nitrogen atmosphere being
at 15508C justi®es it as residue. The mechanism
showing different stages of decomposition is shown
in Scheme 3. The percentages with, and without,
parentheses correspond to the calculated and observed
percentages, respectively.
Here, the complex formation is somewhat different
from that of complex I. The loss of the IR peaks of
cyanide justi®es the removal of all the cyanide ligands
by different moieties. The TG along with DTG of the
complex is shown in Fig. 3. The decomposition starts
from 408C and ends at 1708C with DTG maximum at
104.38C showing the evolution of uncoordinated
molecules of water and two hydroxyl groups in the
form of the water molecules. The DTG maximum and
temperature range support the expulsion of the moi-
eties from the inner coordination sphere. The observed
mass loss is 12.91 against calculated 13.10%. The
second transition is in continuation with the ®rst,
starting from 1808C and ending at 3108C with DTG
maximum of 217.28C, involving a mass loss of 20.19
The DSC of complex II is shown in Fig. 4. The
transitions observed in the DSC curve correspond to
the transitions observed in the TG curve. Three transi-
tions, two exothermic and one endothermic, are
observed. The ®rst transition starts from 41.38C and
ends at 173.58C with maximum of 133.08C, giving a
Fig. 3. The TG and DTG curves of complex II.

188
S.I. Ali, K. Majid / Thermochimica Acta 317 (1998) 183±192
Scheme 3.
Scheme 4.
ÁH value of 101.3 J/g. This exothermic transitions is
due to the evolution of water molecules and hydroxyl
groups. The second transition is an endothermic
one involving a signi®cant enthalpy value. This
transition shows the evolution of ethylenediamine
in two steps. The ®rst endothermic peak starting
from 184.88C up to 263.98C with maximum of
223.68C involving ÁH value of 125.0 J/g. The second
endothermic transition starts from 277.48C up to
391.58C with DTG maximum of 312.28C involving
ÁH value of 110.9 J/g. These endothermic peaks are
followed by an exothermic transition starting from
420.98C up to 472.98C with maximum of 436.98C
which gives ÁH value of 16.21 J/g. The mechanism
showing transitions with ÁH values are shown in
Scheme 4.(Table 1)
Fig. 4. The DSC curve of complex II.

S.I. Ali, K. Majid / Thermochimica Acta 317 (1998) 183±192
189
Table 1
Thermoanalytical data for the decomposition of the complexes: Cr[Mo(CN)(C2H8N2)(OH)6]Á2H2O I and Cr[W(C2H8N2)(O2)(OH)3]ÁH2O II
Temperature
range/8C
Maximum temperature Observed
in DTG/8C
Calculated
mass loss/%
Remarks: removal of
mass loss/%
Complex I
50±170
100.0(1)
200.0(1)
10.15
50.70
9.68
2 mol of H2O
180±280
50.54
6 mol of OH‡1 mol of
C2H8N2‡1 mol of CN
residue Cr‡Mo; calculated
(39.7); observed 39.15%
Complex II
40±170
104.33(1)
217.17(1)
12.91
20.19
13.10
19.40
1 mol of H2O‡2 mol of OH
1 mol of OH‡1 mol of C2H8N2
residue Cr‡WO2: calculated 67.4;
observed 66.9%
180±310
7. Kinetic parameters
where ꢂTˆt/RH, dw/dt the fraction decomposed at
time `t', R the gas constant, H the heating rate, w the
weight of the sample at time `t', Ea the activation
energy, A the frequency factor, and T the temperature
in Kelvin. Plot of log ꢂT/w vs. 1/T gives a straight line
for a ®rst-order reaction. The best ®t equation obtained
from programme Grapher (ver 1.20) from which
activation energy (Ea) is obtained as he slope of the
Kinetic parameters calculated for different stages of
decomposition of complexes I and II are given in
Tables 2 and 3, respectively.
By using Doyle's [19] equation:
ꢂꢃ
Ea
A
log
ˆ
‡ log
w
4:6T
RH
Table 2
Activation energy, frequency factor and entropy of activation for Cr[Mo(CN)(C2H8N2)(OH)6]Á2H2O I
Stage
Parameters
DTG maximum
temperature/K
Doyle
Coats and
Redfern
Arrhenius
1
I
Ea/(kJ mol
1
)
20.9
10.9
4.9
24.0
0.6
A/(s
)
373.0
573.0
1.6
243.0
19.4
ÁS#/(J K 1 mol
)
)
233.6
1.0
250.9
10.0
0.1
1
1
II
Ea/(kJ mol
1
)
A/(s
)
1.7
5.3
ÁS#/(J K 1 mol
245.9
232.9
266.3
1
Table 3
Activation energy, frequency factor and entropy of activation for the Cr[W(C2H8N2)(O2)(OH)3]ÁH2O II
Stage
Parameters
DTG maximum
temperature/K
Doyle
Coats and
Redfern
Arrhenius
1
I
Ea/(kJ mol
1
)
23.5
18.1
3.9
26.3
1.5
A/(s
)
377.3
490.2
2.0
241.1
21.2
ÁS#/(J K 1 mol
)
)
235.8
15.2
4.4
243.7
22.3
0.8
1
1
II
Ea/(kJ mol
1
)
A/(s
)
1.6
ÁS#/(J K 1 mol
245.4
236.8
250.9
1

190
S.I. Ali, K. Majid / Thermochimica Acta 317 (1998) 183±192
ꢀ
ꢁ
kTs
h
ÁS#
equation using the relation Eaˆ 4.6Âslope and pre-
exponential factor (A) from the intercept of the plot.
The entropy of the activation (ÁS#) is obtained from
the following equation:
A ˆ
exp
(A)
R
where k is the Boltzmann constant, h the Planck
1
Fig. 5. Plot of log ꢂT/w vs. 1/TÂ103 K of complex I.

S.I. Ali, K. Majid / Thermochimica Acta 317 (1998) 183±192
191
1
Fig. 6. Plot of log k vs. 1/TÂ103 K of complex I.
constant and Ts the DTG peak temperature. The Coats
and Redfern [20] equation for nˆ1 is given as follows:
structure than the reactants and the reactions are
slower than normal [22].
ꢂ
ꢃ
ꢂ
ꢃ
From Table 1, it is evident that activation energy is
maximum for the ®rst step and minimum for the
second step. The corresponding value of entropy of
activation is also maximum for step I and minimum
for step II. Since Lozano [23] found that greater
enthalpy and lower activation energy are responsible
for greater binding between pyridine and molybdenum
atom in the complexes, here, the low activation energy
is observed in the second step, and involves release of
ethylenediamine and hydroxyl group, attributing
towards greater stability, hence being removed at
the latter stages in the decomposition process. The
same trend is observed for complex II, where the
thermodynamic parameters show greater coordination
of ethylenediamine and hydroxyl group with the
tungsten metal, being removed in the last step of
the thermal decomposition.
log ꢀ1ꢄ†
AR
2RT
Ea
log
ˆ log
1
T2
HEa
Ea
2:3RT
where ꢄ is the fraction decomposed at time `t', Ea the
activation energy, H the heating rate, R the gas con-
stant and A the pre-exponential factor was also used. A
plot of log ‰ ln ꢀ1 ꢄ†Š=T2 vs. 1/T for nˆ1 was linear
for each stage of decomposition of complex I
(Fig. 5(b)) and complex II (Fig. 6(b)) obtained by
programme Grapher. The activation energy (Ea) and
frequency factor (A) was obtained from the slope and
intercept of the best ®t equation of the plot, respec-
tively. Arrhenius [21] equation was also employed for
the determination of kinetic parameters using equa-
tion dx=dt ˆ kxn, where x is the amount of the sam-
ple undergoining reaction, n the order of the reaction
and k the speci®c rate constant. The best ®t equation of
the plot of log k vs. 1/T gave slope and intercept from
which the values of Ea and A were obtained.
From Tables 2 and 3, the values of Ea and ÁS# of a
particular stage using different methods are compar-
able, although the values obtained by Coats and Red-
fern's lie on the lower side. The negative values of ÁS#
indicate that the activated complexes have a ordered
Acknowledgements
One of the authors Kowsar Majid is grateful to
CSIR, New Delhi for the award of Senior Research
Fellowship.

192
S.I. Ali, K. Majid / Thermochimica Acta 317 (1998) 183±192
[13] W.W. Wendlandt, J.P. Smith, The Thermal Properties of
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