TGA and DTA studies on en and tmn complexes of Cu(II) chloride, nitrate, sulphate, acetate and oxalate

Article abstract of DOI:10.1016/S0040-6031(01)00683-9

Dynamic thermogravimetric and differential thermal analysis (TGA and DTA) of ethylenediamine (en) and tetramethylethylenediamine (tmn) complexes of a number of Cu(II) salts (chloride, nitrate, sulphate, acetate and oxalate) have been carried out. The thermal decomposition of these complexes occurs in steps involving dehydration, deamination and deanionation processes. However, majority of the weight loss steps in TGA are found to be composite in nature involving simultaneous loss of the component moieties in varying proportions. The ΔH and E_a values for various steps are deduced and the observed trends are analysed in terms of the nature of the processes underlying the steps and the differences in the nature of co-ordination of the ligands and/or the stereochemistry exhibited by the counter anions about the Cu(II) ion in the complex.

Full text of DOI:10.1016/S0040-6031(01)00683-9

Thermochimica Acta 383 ꢀ2002) 109±118
TGA and DTA studies on en and tmn complexes of CuꢀII)
chloride, nitrate, sulphate, acetate and oxalate
L.S. Prabhumirashi*, J.K. Khoje
Department of Chemistry, Pune University, Pune 411007, India
Received 14 May 2001; accepted 29 June 2001
Abstract
Dynamic thermogravimetric and differential thermal analysis ꢀTGA and DTA) of ethylenediamine ꢀen) and tetramethyle-
thylenediamine ꢀtmn) complexes of a number of CuꢀII) salts ꢀchloride, nitrate, sulphate, acetate and oxalate) have been carried
out. The thermal decomposition of these complexes occurs in steps involving dehydration, deamination and deanionation
processes. However, majority of the weight loss steps in TGA are found to be composite in nature involving simultaneous loss
of the component moieties in varying proportions. The DH and Ea values for various steps are deduced and the observed trends
are analysed in terms of the nature of the processes underlying the steps and the differences in the nature of co-ordination of
the ligands and/or the stereochemistry exhibited by the counter anions about the CuꢀII) ion in the complex. # 2002 Elsevier
Science B.V. All rights reserved.
Keywords: Thermogravimetric analysis ꢀTGA); Differential thermal analysis ꢀDTA); Ethylenediamine ꢀen); Tetramethylethylenediamine
ꢀtmn)
1. Introduction
variety of amine complexes of different CuꢀII) salts
was undertaken. The results on en and tmn complexes
of CuꢀII) nitrate, chloride, sulphate, acetate and oxalate
are found to lead to an understanding of the roles of the
ligand and the counter anion in determining the nature
of thermal decomposition of such complexes and being
reported here in view of general interest. Reports [4±6]
on thermal analysis of some of the systems studied in
the present work dealing with certain limited aspects
are found in literature which are duly referred to at
appropriate stages in the discussion part.
It is well-known that a reaction of CuꢀII) with
bidentate ligands such as ethylenediamine ꢀen) and
tetramethylethylenediamine ꢀtmn) gives well-de®ned
crystalline complexes. Attempts to measure the elec-
trical conductivity of such complexes in solid state at
higher temperatures exhibited a number of disconti-
nuities, indicating structural phase changes on their
heating. In order to understand the nature of the
changes involved; thermogravimetric and differential
thermal analysis ꢀTGA and DTA) of a few CuꢀII) and
NiꢀII) amine complexes had been carried out and the
preliminary results reported in our earlier papers [1,2].
Since then a detailed study [3] of TGA and DTA of a
2. Experimental
2.1. Preparation
* Corresponding author.
The various CuꢀII)-amine complexes were prepared
by treating the aqueous alcoholic solutions of
E-mail addresses: lsp@chem.unipune.ernet.in,
mirashi_lsp@hotmail.com ꢀL.S. Prabhumirashi).
0040-6031/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 0 4 0 - 6 0 3 1 ꢀ 0 1 ) 0 0 6 8 3 - 9

110
L.S. Prabhumirashi, J.K. Khoje / Thermochimica Acta 383 12002) 109±118
CuX2 [X ˆ ClÀ, NO3À, 0.5ꢀSO42À), CH3COOÀ and
0.5ꢀCOOÀ)2] with those of en and tmn by following
the standard procedures [7±19]. Commercially avail-
able AR grade ligands ꢀen and tmn) and the CuꢀII)
salts ꢀnitrate, chloride and sulphate) were used after
distillation and recrystallisation from distilled water,
respectively. The CuꢀII) acetate and oxalate required
were prepared from CuꢀII) carbonate by treatment
with the respective carboxylic acids. The resulting
solid complexes were recrystallised from aqueous
methanol, thoroughly dried and stored in a vacuum
desiccator over P2O5.
3. Results and discussion
3.1. Characterisation
The observed percentage weights of various ele-
ments were ®tted with those calculated on the basis
of different possible metal to ligand stochiometries
ꢀ1:1; 1:2, etc.) together with probable numbers of
water molecules of crystallisation ꢀ0, 0.5, 1, 2, etc.)
for deducing the molecular/empirical formulae of
the complexes. The molecular formulae so obtained
ꢀTables 1 and 2) indicate that all of the present
complexes exhibit 1:2 ꢀmetal:ligand) stoichiometry
except the tmn complex of CuꢀII) chloride which
appears in dimeric form with 1:1 stoichiometry. The
en complexes of CuꢀII) nitrate and sulphate and tmn
complexes of CuꢀII) chloride and acetate are anhy-
drous while the remaining ones appear as hydrated to
different extents. These features as well as the infrared
and electronic spectral data on the complexes are in
good agreement with the reported [24±27] ones on
similar complexes and are indicative of a distorted
octahedral symmetry of the complex cation with the
four N atoms from two ligand molecules forming a
square planar arrangement around the central CuꢀII)
ion.
2.2. Characterisation
The percentage weights of various elements ꢀC, H,
N and O) in all the complexes were ®rst determined on
the basis of elemental microanalysis ꢀC and H), esti-
mation of nitrogen as the total and amine nitrogens,
chlorine as chloride and sulphur and oxygen as sul-
phate by following standard procedures [20]. for
determining their empirical/molecular formulae.
The complexes were further examined by their infra-
red and electronic absorption spectra ꢀin the form of
their nujol mulls and methanolic solutions) to under-
stand the nature of co-ordination of the ligands and the
stereochemistry about the central CuꢀII) ion, respec-
tively.
3.2. Thermograms
2.3. Thermal analysis
The experimentally determined TGA and DTA
curves for all the 10 complexes are presented in
Figs. 1 and 2 and brie¯y described below. ꢀcf. Tables 1
and 2).
The TGA and DTA measurements were carried
out on home-built equipments in air atmosphere at a
constant heating rate of 3 8C/min over temperature
range of 25±700 8C using 60±70 mg and ꢀ100 mg
of the samples for TGA and DTA, respectively in
quartz or ceramic crucibles. The thermobalance was
appropriately standardised by running a thermo-
gram on an empty crucible so as to correct for
the air buoyancy effect as well as by measurements
on recrystallised samples of CaC2O4ÁH2O and
CuSO4Á5H2O which are recommended as the refer-
ence substances [21]. Similarly, the DTA equip-
ment was standardised by ®rst-blank measurements
on ®nely powdered ꢀ200±250 mesh) AR grade
MgO followed by measurements on several recom-
mended samples [22], using the same MgO as the
reference.
The observed percentage weight losses correspond-
ing to various steps in all the TGA curves were
compared with those calculated on the assumption
of possible compositions of the expelled groups
[21,28,29], and those showing the best agreement with
the former are incorporated in Table 1. These TGA
patterns indicate that the thermal decomposition of
these complexes broadly involves three stages; viz
dehydration ꢀin case of hydrated ones), deamination
and deanionation. The dehydration process is gener-
ally found to spread over somewhat large temperature
intervals resulting in broad steps; while the deamina-
tion steps in practically all the cases are quite steep
ranging over an interval of only a few 8C. In all the
cases, the last steps comprising of deanionation with

L.S. Prabhumirashi, J.K. Khoje / Thermochimica Acta 383 12002) 109±118
111
Table 1
TGA data on the en and tmn complexes of various CuꢀII) saltsa
Complex
Step
Temperature
Percentage weight
loss observed
ꢀcalculated)
Probable composition
of expelled group/s
Ea ꢀkJ/mol)b
Mechanism
range ꢀ8C)
CR
HM
Cuꢀen)2ꢀNO3)2
Cuꢀen)2Cl2.H2O
1
1
2
3
4
1
2
1
2
3
1
2
1
2
1
2
3
1
2
3
1
2
1
2
220±550
90±180
90.0 ꢀ74.3)
6.5 ꢀ6.6)
en2 ‡ N2O5
H2O
1438
197
133
164
95
1451
211
158
205
128
182
188
127
146
59
R2
R2
R2
R2
R2
F1
190±275
280±370
375±580
150±330
350±600
75±140
21.5 ꢀ22.0)
21.5 ꢀ22.0))
28.5 ꢀ20.2
67.5 ꢀ67.0)
3.0 ꢀ4.5)
en
en
Cl2 À OÃ
Cuꢀen)2SO4
2en ‡ 0.70SO4
0.30SO4
143
169
117
117
36
R2
R2
R2
D3
R2
R2
D2
R2
F1
Cuꢀen)2ꢀCH3COO)2ÁH2O
14.0 ꢀ14.1)
39.0 ꢀ38.3)
25.5 ꢀ22.7)
65.0 ꢀ64.0)
9.0 ꢀ9.3)
H2O ‡ 0.4en
1.6en ‡ 0.5CH3COO
ꢀ1.5CH3COO)ÀOÃ
1.5H2O ‡ 2en ‡ CO2
CO
140±205
205±450
160±260
270±700
100±120
150±360
60±230
Cuꢀen)2ꢀCOO)2Á1.5H2O
Cuꢀtmn)2ꢀNO3)2Á3H2O
[Cuꢀtmn)Cl2]2
409
28
234
8.3 ꢀ8.5)
2.25H2O
191
114
54
202
140
63
64.0 ꢀ74.5)
26.5 ꢀ26.6)
27.5 ꢀ26.8)
15.5 ꢀ14.9)
28.5 ꢀ28.0)
52.0 ꢀ52.0)
9.5 ꢀ2.8)
0.75H2O ‡ 2tmn ‡ N2O5
1.15tmn
230±425
425±520
50±210
0.85tmn ‡ 0.5Cl2
ꢀ1.5Cl2) À 2OÃ
4H2O ‡ 0.5tmn
1.5tmn ‡ 0.70SO4
0.30SO3
79
98
R2
R2
R2
F1
386
41
406
62
Cuꢀtmn)2SO4Á4H2O
210±290
290±420
100±190
200±400
55±120
232
159
94
268
179
118
906
121
778
D4
D3
R2
R2
D1
Cuꢀtmn)ꢀCH3COO)2
29.5 ꢀ29.3)
46.5 ꢀ44.0)
3.0 ꢀ3.25)
70.0 ꢀ68.0)
0.75tmn
ꢀ0.25tmn ‡ 2CH3COO) À Oà 842
Cuꢀtmn)ꢀCOO)2Á0.5H2O
0.5H2O
107
763
120±370
ꢀtmn ‡ 2CO2) À OÃ
a Negative sign ꢀÀ): gain in weight due to atmospheric oxidation of metallic Cu; CR: Coats and Redfern; HM: Horowitz and Metzager; D1:
one-dimensional diffusion; F1: random nucleation; D2: two-dimensional diffusion, cylindrical symmetry; F2: random nucleation Avarmi
Equation I ꢀn ˆ 2); D3: three-dimensional diffusion, spherical symmetry Jander Equation; R2: phase boundary reaction, contracting area
cylindrical symmetry; D4: three-dimensional diffusion spherical symmetry Ginstling and Brounstein Equation; R3: phase boundary reaction
contracting volume spherical symmetry; A3: random nucleation Avarmi Equation II ꢀn ˆ 3).
b Ea ꢀkJ/mol) refer to per mole of the complex decomposed.
simultaneous oxidation Cu to CuO are found to be
very broad in nature. However, in majority of the
cases, all these steps are not generally isolated but
overlap to varying extents and frequently result in
fractional losses of the constituent moieties. The DTA
curves of all these complexes exhibit a weak and
somewhat broad endothermic peak in the lower tem-
perature region just prior to or accompanying the ®rst
weight loss step in the corresponding TGA curve.
Since the latter corresponds to the loss of crystal water
ꢀin case of the hydrated complexes) or the ligand ꢀin
case of anhydrous complexes); the endothermic nature
of these DTA peaks implies some kind of phase
change associated with an absorption of the required
latent heat. The DTA peaks corresponding to the
dehydration steps in TGA are found endothermic
and broad, while those for deamination, deanionation
and conversion of metallic Cu to CuO processes are all
exothermic in nature. However, there is no distinct
peak corresponding to the last process of oxidation of
metallic Cu to CuO since it occurs almost simulta-
neously with the deanionation process. The TGA steps
corresponding to deamination involving sudden
weight loss are accompanied by relatively sharp peaks
in DTA. In case of the DTA peaks corresponding to the
phase change of a complex prior to any actual dis-
sociation, the TGA curves obviously exhibit no weight
loss steps. In all the cases, the DTA peaks for the Cu to
CuO process are rather weak and spread over a wide
temperature range ꢀꢀ300±600 8C). The TGA plot of

112
L.S. Prabhumirashi, J.K. Khoje / Thermochimica Acta 383 12002) 109±118
Table 2
DTA data on the en and tmn complexes of various CuꢀII) saltsa
Complex
Peak
Temperature range ꢀ8C)
Peak temperature ꢀ8C)
Nature of peak
DH ꢀkJ/mol) Ea ꢀkJ/mol)
Cuꢀen)2ꢀNO3)2
1
2
1
2
1
2
1
2
1
1
2
1
2
3
1
2
1
2
1
2
30±115
65
endo, br
exo, br, s
exo, br, s
exo, br
173
328
149
158
90
10
206
143
20
190±400
100±290
290±580
110±250
250±450
30±150
240 and 370
Cuꢀen)2Cl2ÁH2O
Cuꢀen)2SO4
230
470
190
exo, br
37
375
exo, br, s
endo, br
exo, br
795
249
1550
925
656
1588
115
693
822
235
565
16
83
Cuꢀen)2ꢀCH3COO)2ÁH2O
125
210 and 400
240
22
50
150±550
150±450
30±90
Cuꢀen)2ꢀCOO)2Á1.5H2O
Cuꢀtmn)2ꢀNO3)2Á3H2O
exo, s
84
75
endo, br
exo, br, s
endo, br
exo, br, s
exo, br
30
170±370
30±100
205
65
351
30
[Cuꢀtmn)Cl2]2
100±270
340±580
30±120
160
64
425
44
Cuꢀtmn)2SO4Á4H2O
Cuꢀtmn)ꢀCH3COO)2
Cuꢀtmn)ꢀCOO)2Á0.5H2O
115
290
endo, br, s
exo, br
23
13
150±500
180±190
190±470
30±110
185
endo, s
706
139
26
220
exo, br, s
endo, br
exo, s
529
72
100
220 and 420
120±470
893
83
a Exo: exothermic; endo: endothermic; br: broad; s: sharp.
the en complex of CuꢀII) nitrate exhibits a steep step
accompanied by sudden weight loss. This appears
to be due to a chain type highly explosive reaction
because of simultaneous presence of an oxidising
group ꢀNO3) and a reducing group ꢀen) in the same
complex molecule. Similar explosive effect [30,31] is
reported in the case of thermal decomposition of some
amine complexes of NiꢀII) nitrate. All these observa-
tions are in close agreement with those reported in
literature [32].
except that for the tmn complex of CuꢀII) Chloride
which appears in a dimeric form. The differential
method [23], in which the data are collected from the
medium steep part of the TGA curves, was employed
for better accuracy. The values of the left hand sides
of respective equations were plotted against 1/T for
various assumed values of n; and the most appropriate
value of n was chosen which corresponds to the graph
giving the best straight line ꢀcorrelation coef®cient
r > 0:99) and the respective slope was employed
to obtain the Ea. Similar procedure for evaluation
of kinetic parameters has been applied by a number
of other workers [38].
3.3. Determination of thermodynamic parameters
and probable mechanisms
The data on the fraction decomposed ꢀi.e. the degree
of dissociation a) at various temperatures ꢀT) corre-
sponding to a step in a TGA curve can be used to
deduce the probable mechanism underlying the
respective reaction by using different relations based
on alternative reaction mechanisms [39]. The quantity
Àlogꢀgꢀa)) appearing in various equations is plotted as
a function of 1/T and the relation leading to the best
linear ®t is taken to correspond to the most probable
mechanism [40]. As an illustration, such plots for the
dehydration step of Cuꢀen)2Cl2ÁH2O based on several
relationships are presented in Fig. 3. The mechanisms
A quantitative study of these thermograms was
taken up to deduce the enthalpy change ꢀDH) and
activation energy ꢀEa) related to various decomposi-
tion steps. For this purpose, the observed degrees
of decomposition ꢀa) at various temperatures corre-
sponding to each of the steps in the TGA curves were
®tted to the Coats and Redfern ꢀCR) [33] and
Horowitz and Metzager ꢀHM) [34] equations as
described in standard texts [35,36]. The a values were
calculated on the basis of molecular weights corre-
sponding to monomeric forms of the complexes

L.S. Prabhumirashi, J.K. Khoje / Thermochimica Acta 383 12002) 109±118
113
Fig. 1. Thermograms of en complexes of CuꢀII) salts. TGA ꢀ- - -); DTA ꢀÐ).
for thermal decomposition corresponding to the major
steps in TGA curves for all the complexes so deduced
are incorporated in Table 1 where the symbols A, D, F
and R with subscripts 1,2 and 3 have usual meaning as
explained in standard texts [35,36]. The calculations
involved in all the above-referred ®ttings were carried
out on the basis of computer programs suitably devel-
oped [38] in the Turbo Basic Advanced computer
language.
the equation [23] da=dt ˆ Aꢁ1 À a†n eÀE=RT . The Ea
and DH values corresponding to major peaks in DTA
curves of all the complexes deduced by employing
these relations are presented in Table 2.
The analysis of the observed thermograms and the
DH and Ea associated with various steps leads to some
generalisations as follows.
The thermal decomposition processes underlying
the TGA and the nature of the steps observed in the
DTA of these complexes can be schematically repre-
sented, respectively as
DTA is also a convenient technique for estimating
the Ea and DH involved in thermal decomposition
reactions [37]. The DH for a particular process is given
by the relation [41,42], DH ˆ ꢁk=m†A, where
ÀyL
ÀxH2O
!
À
R
Á
CuL2X2 Á nH2O
CuL2X2deamination
!
dehydration
A ˆ DT dT , k and m are the area of the correspond-
ing peak in a DTA curve; calibration constant and
mass of the sample, respectively. The Ea is given by
ÀzX‡O
CuX2deanionation and oxidation
!
CuO;
ꢀ1)

114
L.S. Prabhumirashi, J.K. Khoje / Thermochimica Acta 383 12002) 109±118
Fig. 2. Thermograms of tmn complexes of CuꢀII) salts. TGA ꢀ- - -); DTA ꢀÐ).
where x, y and z represent the appropriate numbers of
respective species expelled in various decomposition
steps; and
The kind of phase change corresponding to the ®rst
peak in DTA indicated above can be understood in
terms of momentary separation of the crystal water
and/or amine part from the molecular complex by
sublimation followed by immediate condensation
and dissolution of the remainder of the complex
in the former. This mechanism could also explain
why this particular DTA peak, though apparently
endothermic, is rather weak and broad in appearance.
This interpretation is veri®ed from observation of
actual liquefaction of the complexes at the corre-
sponding temperatures when slowly heated in sepa-
rate tubes outside the furnace. The endothermic and
broad nature of the DTA peaks corresponding to the
dehydration steps in TGA curves can also be under-
stood in the same way. Similar observation about
Phase change ! Dehydration ! Deamination
ꢁexothermic†
ꢁendothermic†
ꢁendothermic†
‰OŠ
! Deanionation ! Cu ! CuO
ꢀ2)
ꢁexothermic†
However, for different complexes, the loss of crystal
water, ligand moieties and anionic parts does not occur
in the same relative proportions. The ®tting procedure
also implies the weight loss steps in TGA curves of
various complexes to be composite in nature ꢀi.e.
involve simultaneous expulsion of more than one type
of groups in the complex) and involving loss of various
moieties in a fractional manner.

L.S. Prabhumirashi, J.K. Khoje / Thermochimica Acta 383 12002) 109±118
115
Fig. 3. Illustration for determining the probable reaction mechanism: plots of Àlogꢀgꢀa)) vs. 1/T for the dehydration step of Cuꢀen)2Cl2ÁH2O.
liquefaction has also been reported by some other
workers [32].
activation energies and enthalpy changes are some
kind of averages for the overall steps. Deducing exact
quantitative correlations among the various DHs and
Eas, therefore, does not appear possible. The observed
trends, however, can be qualitatively understood as
follows ꢀThe DHs and Eas referred to in the following
discussion correspond to those deduced from the DTA
and TGA studies, respectively).
The sequence of processes as represented in ꢀi) above
implies that, the crystal water in these complexes is
held less strongly than the ligand moieties which in
turn are bonded less strongly than the anionic moi-
eties. The increasing trend of the decomposition tem-
peratures corresponding to various steps ꢀTable 1) and
that of the DHs and Eas ꢀTable 2) for the respective
processes in the same order, would be consistent with
this conclusion. The DHs and Eas for various decom-
position processes in different complexes would be
expected to exhibit certain general trends dependent
on the strength of bonding of the ligand and anionic
moieties, which in turn would be governed by their
structure and type of co-ordination; provided the
various processes can be observed independent of
one another. However, due to the composite nature
of various steps involving fractional loss of more than
one moeities in varying proportions; the calculated
3.4. Dehydration step
Of the various hydrated complexes; the dehydration
steps of only the en complex of CuꢀII) chloride and tmn
complex of CuꢀII) oxalate are found to be separate,
while those of all others ꢀe.g. tmn complexes of CuꢀII)
sulphate and nitrate) are overlapped by simultaneous
expulsion of ligands and anionic moities to varying
extents resulting in broad steps ꢀDT ꢀ 20±160 8C)
spreading over large temperature intervals ꢀꢀ50±
210 8C). The present Ea value for the dehydration step

116
L.S. Prabhumirashi, J.K. Khoje / Thermochimica Acta 383 12002) 109±118
of the en complex of CuꢀII) chloride is of comparable
order of magnitude as reported by Langfelderova et al.
[5]. The DH and Ea for dehydration step of the en
complex of CuꢀII) oxalate are both larger than those for
the en complex of CuꢀII) acetate. This might be due to a
larger fraction of ligand lost along with H2O in the
former. The activation energy for en complex of CuꢀII)
oxalate appears to be higher than that for en complex of
any other salt, which can be due to the contribution
from the simultaneous loss of ligand and anion parts of
the complex.
be axially compressed, while those of CuꢀII) nitrate or
sulphate are axially elongated. The co-ordination of a
ligand in the former case could therefore be somewhat
weaker, which can also partly explain the observed
trends in the Ea and DH values associated with the loss
of the ligands in these complexes. The Ea values for
the deamination step of the complexes of CuꢀII)
sulphate are generally lower than those for the com-
plexes of CuꢀII) nitrate, which can be attributed to the
2À
stronger co-Àordination of SO4 with CuꢀII) ion than
that of NO3 and hence weaker co-ordination of the
ligands to the CuꢀII) ion in presence of SO4 as the
2À
3.5. Deamination
counter anion. Similarly, the lower activation energy
and temperature corresponding to the loss of tmn from
the complexes of CuꢀII) nitrate, chloride and acetate
than those for the loss of en from the corresponding
complexes are attributable to crowding at N-sites and
packing effects [23] of the coanions in these systems.
The trends in the Ea and DH values for the deamination
step are thus found to be largely determined by the
nature of co-ordination and type of the counter anion
in the CuꢀII) salt. The higher values of DH in the case
of en complexes of CuꢀII) oxalate and acetate than
those for the complexes of CuꢀII) nitrate, chloride and
sulphate could be possibly due to the simultaneous
loss of ligand and anion in the composite steps in the
former. Wendlandt [4] also has reported detailed
studies on TGA and DTA of the bis-en complex of
CuꢀII)SO4. His thermograms appear to differ some-
what from the present ones, which could be due to the
differences in the atmosphere and heating rates
employed. However, there are no other reports on
the relevant DH and Ea values corresponding to ther-
mal analysis so as to present any comparison with our
present values.
The DH for expulsion of en as well as that of tmn are
both higher in case of complexes of CuꢀII) oxalate
than those of acetate indicating somewhat stronger co-
ordination of both of these ligands to CuꢀII) ion in case
of oxalate as the counter ion than that in case of
acetate. This is partly attributable to the chelating
bidentate nature of co-ordination of an oxalate anion
as against unidentate co-ordination of an acetate anion
resulting in higher stability of the complex cation in
the former. The higher values of DH in the former case
could also be partly due to relatively larger fractional
loss of oxalate anion along with the ligands as com-
pared to that of acetate anion. Though the DH values
for deamination step of both the en as well as tmn
complexes of CuꢀII) oxalate are higher than those of
CuꢀII) acetate; the Ea value for the expulsion step of
tmn from its complex with CuꢀII) oxalate is lower than
that from its complex with CuꢀII) acetate. This oppo-
site trend may be partly due to the stronger co-ordina-
tion of the oxalate ion to CuꢀII) due to its chelating
bidentate nature as compared to the unidentate co-
ordination by the acetate ion and the crowding at the
N-sites in tmn resulting in weaker co-ordination by a
tmn moiety to the CuꢀII) in the oxalate salt than that in
the acetate salt. Srivastava et al. [6] also have carried
out thermal analysis studies on a number of amine
complexes of CuꢀII) salts including that on the en
complex of CuꢀII) oxalate. However, they have not
reported any thermodynamic or kinetic parameters.
The DH values for the deamination step of the en
complexes of inorganic saltsÀof CuꢀII) are found to
vary in the order: ClÀ < NO3 < SO4À, which can be
taken to imply the variation of the stability in the same
order. The complexes of CuꢀII) chloride are known to
3.6. Deanionation
The higher values of the Ea and DH for the dec-
arboxylation step of en complex of CuꢀII) oxalate than
those for CuꢀII) acetate are understandable in view of
the oxalate group exhibiting chelating bidentate co-
ordination with CuꢀII) resulting in stronger bonding.
However, the tmn complex of CuꢀII) oxalate exhibits
an opposite trend. This could be due to the crowding at
the N-sites in the latter causing the oxalate anion in
this complex to assume the bridging bidentate co-
ordination to the CuꢀII), which is likely to be weaker in

L.S. Prabhumirashi, J.K. Khoje / Thermochimica Acta 383 12002) 109±118
117
strength as compared to that of the acetate grouping in
5. Summary and conclusions
the tmn complex of CuꢀII) acetate. The temperature
corresponding to the loss of the oxalate group from
Cuꢀtmn)ꢀCOO)2Á0.5H2O is lower than that in Cuꢀen)2-
ꢀCOO)2Á1.5H2O, both of which are lower than that for
CuꢀII) oxalate [43]. The lower decomposition tempera-
tures in case of complexed CuꢀII) oxalate than that for
the uncomplexed one is attributable to weakening of
the co-ordination of an oxalate anion in the former due
to co-ordination by the ligands with the central metal
ion. Similarly, in case of en complexes of CuꢀII) salts of
inorganic anions; the larger Eas for the steps corre-
sponding to the loss of NO3À or SO42À than that for the
step involving the loss of ClÀ could be due to the
contribution from the loss of en in the former cases.
The present TGA and DTA studies, thus enable to
understand the nature of thermal decomposition pro-
cess in CuꢀII) amine complexes. The trends in DH and
Ea values corresponding to various steps for different
complexes can be understood in a semi-quantitative
manner in the light of the structure and co-ordinating
nature of the ligands and the counter anions.
Acknowledgements
A research grant from the University Grants Com-
mission ꢀIndia) to L.S. Prabhumirashi and a JRF to
J.K. Khoje are thankfully acknowledged.
4. Mechanisms
References
Since the mechanism of any reaction depends on a
large number of factors, such as, e.g. temperature,
exothermic or endothermic nature of the process,
stoichiometry, crystal structure, packing, catalysing
or other effect of accompanying species [23], etc. one
cannot expect a unique mechanism to be applicable to
all the decomposition steps or even to similar steps of
different complexes. For the sake of completeness, the
most probable mechanisms corresponding to the
major decomposition steps of all the complexes
deduced as explained earlier are indicated in Table 1
and some common trends observed are noted below.
The dehydration steps of majority of the complexes,
viz. en complexes of CuꢀII) chloride, nitrate, oxalate
and acetate, and the tmn complexes of CuꢀII) sulphate
and oxalate, are found to ®t more closely to the phase
boundary reaction, contracting area cylindrical sym-
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complexes of CuꢀII) chloride, nitrate, sulphate and
oxalate, and tmn complexes of CuꢀII) chloride and
acetate) ®ts closely to the phase boundary reaction
contracting area cylindrical symmetry R2 mechanism.
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