Snythesis, characterization and thermal dissociation of cobalt(II) complexes of the photoproducts of octacyanomolybdate(IV) and -tungstate(IV) with ethylenediamine

Article abstract of DOI:10.1016/S0040-6031(98)00373-6

Photosubstituted mixed ligand complexes of Co(II) with irradiated solutions of Mo(CN)₈^4- and W(CN)₈^4- with ethylenediamine have been synthesized. The complexes have been isolated and characterized by their elemental analysis and IR spectroscopy. The assigned formulae are Co₂[Mo(CN)₂(C₂H₈N₂) ₂(OH)₂]Cl₄.2H₂O I and Co[W(CN)₂(C₂H₈N₂) ₂(OH)₂]Cl₂.2H₂O II for Mo(IV) and W(IV) complexes, respectively. The IR peaks observed for both the complexes show N-H stretching, H-N-H bending and stretching vibrations of the C-N and C-C bonds. Depending on the nature of the absorption bands, complex I is assigned as -trans and complex II as -cis configuration. The thermal decomposition of these complexes has been studied by TG and DSC techniques. The reaction scheme for decomposition of each complex has been proposed. The activation energy (E_a), pre-exponential factor (A) and entropy of activation (ΔS^#) have been calculated by employing the integral methods like Arrhenius, Coats-Redfern and Doyle. Enthalpy (ΔH) values for each transition are obtained from DSC data.

Full text of DOI:10.1016/S0040-6031(98)00373-6

Thermochimica Acta 317 (1998) 141±149
Snythesis, characterization and thermal dissociation of cobalt(II)
complexes of the photoproducts of octacyanomolybdate(IV)
and -tungstate(IV) with ethylenediamine
S.I. Ali*, Kowsar Majid
Department of Chemistry, Jamia Millia Islamia, New Delhi 110 025, India
Received 6 March 1998; accepted 5 May 1998
Abstract
Photosubstituted mixed ligand complexes of Co(II) with irradiated solutions of Mo(CN)8
4
4
and W(CN)8
with
ethylenediamine have been synthesized. The complexes have been isolated and characterized by their elemental analysis and
IR spectroscopy. The assigned formulae are Co2[Mo(CN)2(C2H8N2)2(OH)2]Cl4.2H2O I and Co[W(CN)2(C2H8N2)2(OH)2]-
Cl2.2H2O II for Mo(IV) and W(IV) complexes, respectively. The IR peaks observed for both the complexes show N±H
stretching, H±N±H bending and stretching vibrations of the C±N and C±C bonds. Depending on the nature of the absorption
bands, complex I is assigned as -trans and complex II as -cis con®guration. The thermal decomposition of these complexes
has been studied by TG and DSC techniques. The reaction scheme for decomposition of each complex has been proposed. The
activation energy (Ea), pre-exponential factor (A) and entropy of activation (ÁS#) have been calculated by employing the
integral methods like Arrhenius, Coats±Redfern and Doyle. Enthalpy (ÁH) values for each transition are obtained from DSC
data. # 1998 Elsevier Science B.V.
Keywords: Photosubstituted complex; Octacyanomolybdate(IV); Octacyanotungstate(IV); Ethylenediamine complexes;
Thermal dissociation
1. Introduction
result in varying numbers of ethylenediamine ligands
and the amount of water present [3]. Thermogravi-
metric studies of numerous ethylenediamine com-
plexes of transition metals have been reported [1].
The octacyanomolybdate(IV) and -tungstate(IV) in
alkaline solutions lead primarily to the substitution
of one cyanide ligand by irradiation in the ligand ®eld
band [4], which undergoes thermal substitution of a
hydroxyl group with ethylenediamine [5]. The thermal
behaviour of cyano compounds is complicated due
to the variety of the chemical species (cyanogen,
cyanate, isocyanate, carbonate etc) which is related
to nitrite or cyanide and which can be formed in
Complexes containing ethylenediamine as ligand
have been widely discussed in the literature [1]. The
great complexing capacity of the amine group makes
ethylenediamine most versatile in the synthesis of
inorganic complexes [2]. Thermal studies of tetra-
cyanonickelates having the general formula Ni(en)m-
Ni(CN)4.nH2O (mˆ1, 2 or 3 and n>0) whose
composition is affected by the pH value of the medium
*Corresponding author. Tel.: +91 6831717; fax: +91 6821232.
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 3 - 6

142
S.I. Ali, K. Majid / Thermochimica Acta 317 (1998) 141±149
thermally induced intramolecular reactions with or
without participation of the surrounding atmosphere
[6].
Perkin Elmer 1710 Fourier transform spectrometer in
1
the range of 400±4000 cm using KBr disc techni-
que. TG was conducted on a Du Pont 2000 thermal
analyser. DSC was also carried out on a Du Pont TA
2000 system with a DSC attachment. Nitrogen ¯ow
was maintained at 60 cm3 min 1. Scanning was done
at the rate of 108C min 1. An aluminium pan was used
under a dynamic nitrogen atmosphere. DSC calibra-
tion was effected by indium (99.99% purity). Tem-
perature repeatability of the instrument is Æ0.18C and
of ÁH is Æ0.0 2mW.
In this paper, we report the complex formation of
the photoproducts of octacyanomolybdate(IV) and -
tungstate(IV) and ethylenediamine with cobalt(II)
chloride, their characterization and thermal dissocia-
tion.
2. Experimental
Potassium octacyanomolybdate(IV) and -tungsta-
te(IV) were prepared by the method of Leipoldt et al.
[7]. Ethylenediamine and cobalt chloride CoCl2.6H2O
were of AR grade. All the solutions were prepared in
doubly distilled water.
3. Results and discussion
3.1. IR studies
4
The photoexcitation of M(CN)8
(where Mˆ
2.1. Synthesis of complexes
Mo(IV) or W(IV)) with ethylenediamine at 366 nm
produces a photoproduct in solution in which cyanide
ions are substituted by ethylenediamine molecules [5].
Addition of cobalt (II) chloride to the photoproduct
results in the formation of a complex which is assigned
Co2[Mo(CN)2(C2H8N2)2(OH)2]Cl4.2H2O on the basis
of elemental analysis. IR spectra of the complex show
a very strong peak at 3400 and 1600 cm 1, indicating
the asymmetric and symmetric OH stretching and
bending vibrations, respectively, due to water mole-
Aqueous solutions of K4M(CN)8 (MˆMo or W)
and ethylenediamine were mixed in the ratio of 1:2 of
0.1 M each. The solution mixtures were subjected to
ultraviolet radiation through a mercury lamp for about
15 min until the yellow colour of the solution changed
to deep red. Irradiation was stopped and an aqueous
solution of cobalt chloride was added dropwise until a
reddish black precipitate (Mo) and a light green pre-
cipitate (W) were formed. The precipitate was washed
and separated by centrifugation and dried over fused
CaCl2. The chemical analysis gave ± found: Cˆ11.52;
Hˆ3.18; Nˆ13.45; (%) against calculated ± Cˆ12.00;
Hˆ3.56; Nˆ13.80; (%) The empirical formulae for
the complexes are:
1
cules. The presence of a strong band at 2200 cm
implies the presence of cyano group with the mode M±
CˆN, showing cyanide as a bridging ligand analogous
to a polynuclear complex [Na2Co(CN)5X], in which
the band due to C=N stretching vibration of the
1
cyanide bridge is found at 2202 cm and that of
1
terminal cyanide is at 2130 cm [8]. The four prin-
Co2‰MoꢀCN†2ꢀC2H8N2†2ꢀOH† ŠCl4:2H2O I
2
cipal regions of absorption observed at 1600, 1400,
800 cm 1 can be assigned as asymmetric deformation,
symmetric deformation and NH3 rocking mode, are
supportive of the metal±amine complex. The bands
observed correspond to the ethylenediamime com-
plexes [9]. The main bands observed are between
3300 and 3077 cm 1, which can be due to the NH
stretching region. The band in the region of
and
Co‰WꢀCN†2ꢀC2H8N2†2ꢀOH† ŠCl2:2H2O II
for which the chemical analysis carried out gave ±
found: Cˆ13.5; Hˆ3.5; Nˆ16.0; (%) against calcu-
lated ± Cˆ13.0; Hˆ4.0; Nˆ15.2;(%).
2
1
2.2. Physical measurements
1400 cm is due to vibrations of the ±CH2 group.
1
The band at 1040±1000 cm may be the result of
Determination of carbon, hydrogen, nitrogen and
metal was carried out by micro-analysis methods. IR
spectra of the solid complexes were recorded on a
stretching vibrations of either the C±N or the C±C
bond. The complex formation with W(IV) is assigned
the formula Co[W(CN)2 (C2H8N2)2(OH)2 ] Cl2.2H2O

S.I. Ali, K. Majid / Thermochimica Acta 317 (1998) 141±149
143
on the basis of elemental analysis. The band position is
similar to those in the case of Mo(IV). The presence of
the water molecules in the complex is supported by the
On the basis of the above discussion the possible
structure of the complexes is:
strong peak at 3400 cm 1. The absorption band at
1
2100 cm
shows the presence of cyanide in the
terminal mode. The other principal regions of absorp-
tion are at 1620, 1400 and 840 cm 1, which can be
assigned as asymmetric deformation, symmetric
deformation and NH3 rocking mode. The band at
1125 cm 1 is due to NH2 bending vibration. The peak
1
at 1050 cm is due to vibrations of the CH2 group.
All the peaks correspond to the reported peaks for the
complex of cobalt±ethylenediamine in which ethylene-
diamine is in gauche con®guration [9]. Free ethyl-
enediamine exhibits the bands of bending vibration of
the NH2 group at 1608 and 1597 cm 1 [10]. The bands
due to N±H vibrations are shifted to lower frequencies
in the complexes, which suggest the formation of the
amine complex showing the nitrogen of the amine
groups coordinated to the metal atom [10].
The position of ethylenediamine can result in the
formation of two structures for each complex, cis and
trans. In cobalt ethylenediamine complexes, the most
consistent variation between the spectra of cis- and
trans-isomers have been found in the CH2 rocking
region, i.e. 870±900 cm 1. The complexes with a cis-
con®guration show two bands, while those with a
trans-structure show one. It has also been reported
that the position of the bands in the compounds of the
type [Co(en)2AB]X are not dependent on any one of
the groups [11].
3.2. TA studies
3.2.1. trans-Co2[Mo(CN)2(C2H8N2)(OH)2]Cl4.2H2O
The complexes were subjected to TG and DSC
analysis. The thermograms obtained are shown in
Figs. 1 and 2 respectively. The TG was obtained in
a nitrogen atmosphere. The results are tabulated in
Table 1. The thermogram of I indicates three different
mass loss ranges. The ®rst mass loss is from ambient
temperature 76±1728C, due to the release of OH and
H2O, two moles each in the form of water. The
observed mass loss is 11.60% as against the calculated
value of 11.71%. In the second stage, one mole of the
ethylenediamine along with one mole of cyanide split
off with a mass loss of 12.74% as against the calcu-
lated value of 14.4% in the temperature range of 190±
3498C with the onset temperature of 222.128C and
DTG peak at 225.808C. The third stage occurs in the
range of 356±4718C with DTG maximum at
379.108C, which results in the removal of another
mole of cyanide along with one mole of ethylenedi-
amine. The observed mass loss is 14.54% as against
calculated value of 14.4%. The residue has the
observed mass 59.48% as against the calculated value
of 61.12%. Thermal dissociation is represented in
Scheme 1. Calculated and observed mass loss percen-
tages are given with and without parenthesis, respec-
tively.
For complex I, the spectrum between 870±
1
900 cm shows only a single band, suggesting the
con®guration to be trans one, while in complex II this
band is split, showing the con®guration to be a cis one.
Moreover, Merrit and Wiberly [12] reported the dif-
ference in the spectra of cis and trans complexes of the
type [Co(en)2Cl2]X at 1600 cm 1. In the trans-
bisethylenediamine complexes a single sharp band
occurs in the range of 1605±1575 cm 1, whereas in
the cis form of trisethylenediamine complex, the band
structure is more complex and occurs over a wider
range 1634±1527 cm 1. For the bisethylenediamine
complex (I), a sharp absorption peak is observed at
1600 cm 1, con®rming the structure to be trans, while
in II, the same band is observed at a comparatively
1
wider range of 1620 cm justifying it as a cis±con-
®guration.

144
S.I. Ali, K. Majid / Thermochimica Acta 317 (1998) 141±149
Fig. 1. The TG and DTG curves of complex I.
Fig. 2. The DSC curve of complex I.
3.2.2. cis-Co[W(CN)2(C2H8N2)2(OH)2]Cl2 2H2O
The TG and DTG curve for the II Co [W(CN)2-
(C2H8N2)2 (OH)2] Cl2.2H2O is shown in Fig. 3.
Decomposition of the complex is completed in three
steps. In the ®rst stage of decomposition two moles
of H2O along with one mole of ethylenediamine
are removed in the temperature range of 60±2188C
with the DTG maximum at 938C. The observed mass

S.I. Ali, K. Majid / Thermochimica Acta 317 (1998) 141±149
145
Table 1
Thermoanalytical data for the decomposition of the complexes:Co2[Mo(CN)2(C2H8N2)2(OH)2]Cl4.2H2O
[W(CN)2(C2H8N2)2(OH)2]Cl2.2H2OII
I
and Co
Temp. range
Max. temp. in
Obsd. mass
loss %
Calc. mass
loss %
Remarks
8C
DTG 8C
Removal of
Complex I
76±172
96.14(1)
255.80(1)
379(1)
11.60
12.74
14.54
11.71
14.40
14.40
2 mol H2O‡2 mol OH
222±349
372±471
1 molC2H8N2‡1 mol CN
1 mol CN‡1 molC2H8N2
Residue 2Co‡MoCl4; calc. (59.48)
obsd. 61.12%
Complex II
60±218
93(I)
17.54
11.89
14.26
17.3
2 mol H2O‡1 mol C2H8N2
1 mol C2H8N2
272±379
414±518
327(1)
426(1)
10.89
15.16
2 mol OH‡2 mol CN
Residue Co‡WCl2; calc. (56.62)
obsd. 56.31%
In the ®nal stage two moles of hydroxyl along with
two moles of cyanide are removed in the temperature
range of 414±5188C with DTG maximum at 4168C.
The observed mass loss is 14.26 as against a calculated
loss of 15.16%. The observed mass of the residue is
56.62% as against 56.31%. Scheme 2 shows the
thermal decomposition steps:
Scheme 1.
3.3. Differential scanning calorimetry
The DSC curve for I (Fig. 2) displays two sharp
peaks, a broad exothermic peak starting at 758C
up to 2178C with an enthalpy change of 297.8 J g
due to the expulsion of OH and H2O molecules
along with cyanide and ethylenediamine moiety.
The second peak is endothermic at ca. 3228C up
loss is 17.54% as against calculated mass loss of
17.3%. The second stage occurs at 272±3798C which
corresponds to the removal of another mole of
ethylenediamine resulting in the observed mass loss
of 11.89% as against a calculated loss of 10.89%.
1
Scheme 2.

146
S.I. Ali, K. Majid / Thermochimica Acta 317 (1998) 141±149
Fig. 3. The TG and DTG curve of complex II.
to 4278C with a maximum at 3678C with ÁHˆ
178.9 J g 1. The loss of the remaining ligands occurs
in another endothermic stage, with a low enthalpy
value implying expulsion of the cyanide and hydroxyl
ligands (ÁHˆ31.12 J g 1). The successive stages can
be represented in the Scheme 4.
1
57.88 J g involving the expulsion of the remaining
moiety (Scheme 3).
The DSC curve of II is shown in Fig. 4 which shows
the complex to melt at 76.888C. The expulsion of two
moles of H2O takes place along with the overlapping
of ethylenediamine. The enthalpy change for these
1
3.4. Kinetic parameters
overlapping processes is established to be 371.8 J g
.
The next step involves the dissociation of another
ethylenediamine molecule with enthalpy change of
Thermal decomposition of both complexes I and II
takes place in three steps. The order of the reaction `n,'
Scheme 3.

S.I. Ali, K. Majid / Thermochimica Acta 317 (1998) 141±149
147
Fig. 4. The DSC curve of complex II.
activation energy (Ea) and entropy of activation (ÁS#)
were calculated using different methods:
By Doyle's method [13], the values of Ea and pre-
exponential factor (A) can be calculated from the slope
and intercept of the least square plots of
to be 24.6, 11.4 and 23.5 and in complex II(Fig. 6)
22.9, 9.8 and 13.1 kJ mol 1, respectively. The Arrhe-
nius method [14] was used in calculating the kinetic
parameters on the basis of fundamental equation:
dX
ˆ kXn
(A)
ꢀT
1
vs: using log
ꢀT
Ea
A
dt
log
ˆ
‡ log
W
T
W
4:6T
RH
where X: amount of sample undergoing the reaction, n:
order of the reaction and k: speci®c rate constant.
Entropy of activation ÁS# can be calculated from
Eq. (A). By measuring k at several temperatures the
graph of log k against 1/T gives a straight line whose
slope is Ea/2.3 R and intercept is 2.3 log A. The
activation energy for I for three steps was found to
26.4, 13.0 and 36.9 and for II, 20.9, 11.6 and
where RH: heating rate, ꢀT : t=RH : dw=dt. The
entropy of activation, ÁS#, was calculated from the
equation:
ꢀ
ꢁ
Ts ÁS#
A ˆ k
hexp
R
where k: Boltzmann constant, h: Planck's constant,
ÁS# : entropy of activation and Ts is the DTG peak
temperature. The activation energies calculated for
stages I, II and III in complex I(Fig. 5) were found
1
26.4 kJ mol respectively. TG data were also ana-
lysed by the Coats and Redfern [15] treatment where
the plot of log‰ logꢀ1 ꢁ†==T2Š versus 1/TÂ103
Scheme 4.

148
S.I. Ali, K. Majid / Thermochimica Acta 317 (1998) 141±149
1
Fig. 5. Plot of ꢀA† logꢀT =w; ꢀB† Log‰ logꢀ1 ꢁ†Š=T2; ꢀC† logk vs: 1=T  103K of complex I.
1
Fig. 6. Plot of ꢀA† logꢀT =w; ꢀB† Log‰ logꢀ1 ꢁ†Š=T2; ꢀC† logk vs: 1=T  103K of complex II.

S.I. Ali, K. Majid / Thermochimica Acta 317 (1998) 141±149
149
Table 2
Activation energy (Ea), frequency factor (A) and entropy of activation (ÁS#) for complex I ± Co2[Mo(CN)2(C2H8N2)2(OH)2]Cl4. 2H2O
Stage
I
Parameters
Doyle
Coats and Redfern
Arrhenius
1
Ea (kJ mol
1
)
24.6
15.0
4.5
26.4
1.6
A (s
)
2.3
240.0
11.4
0.4
ÁS# (JK 1 mol
)
)
)
234.2
3.4
242.9
13.0
0.3
1
1
1
1
)
II
Ea (kJ mol
1
A (s
)
6.2
ÁS# (JK 1 mol
258.0
23.5
1.4
234.6
17.8
5.0
258.8
36.9
1.6
1
)
III
Ea (kJ mol
1
A (s
)
ÁS# (JK 1 mol
248.9
238.1
247.5
Table 3
Activation energy (Ea), frequency factor (A) and entropy of activation (ÁS#) for complex II Co[W(CN)2(C2H8N2)2(OH)2]Cl2.2 ±H2O
Stage
I
Parameters
Doyle
Coats and Redfern
Arrhenius
1
Ea (kJ mol
1
)
22.9
13.7
4.7
20.9
1.1
A (s
)
1.9
241.3
9.8
ÁS# (JK 1 mol
)
)
)
233.9
4.4
245.8
11.6
0.2
1
1
1
1
)
II
Ea (kJ mol
1
A (s
)
0.3
6.1
ÁS# (JK 1 mol
262.3
13.0
0.5
231.7
14.8
5.3
264.0
26.4
1.0
1
)
III
Ea (kJ mol
1
A (s
)
ÁS# (JK 1 mol
257.4
238.2
252.1
gives a straight line(Figs. 5 and 6), indicating that the
order of the reaction for the decomposition of all the
complexes is one. The slope of the line is taken as
Ea/2.3R. The entropy of activation is again calcu-
lated from Eq. (A). The activation energy for the three
stages of complex I is 15.0, 3.4 and 17.8 and complex
II is 13.7, 4.4 and 14.8 kJ mol 1, respectively. The
activation energy (Ea), frequency factor (A) and
entropy of activation (ÁS#) for complexes I and II
are listed in Table 2Table 3 respectively. From the
overall kinetic data, it is evident that all the values
obtained by these methods are quite comparable
although those obtained from the Arrhenius method
are more divergent.
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Acknowledgements
One of the authors, Kowsar Majid, is thankful to
CSIR, New Delhi for the award of Senior Research
Fellowship.