Synthesis, thermal and spectral studies of first-row transition metal complexes with girard P reagent-based ligand

Article abstract of DOI:10.1016/j.saa.2006.11.016

A new series of first-row transition metal complexes with 1-acetylpyridinium chloride-4-benzoyl thiosemicarbazide (H₂GPBzIT) have been prepared and characterized by elemental analysis, spectroscopic and magnetic measurements. The proton-ligand ionization constants were determined potentiometrically using Irving-Rossotti technique. The stability constants of complexes were also calculated and were found in agreement with the seq. uence of stability constants of Irving and Williams. Thermal stability and degradation kinetics have been measured using thermogravimetric analyzer. Kinetic parameters were obtained for each stage of thermal degradation of complexes using Coats-Redfern method.

Full text of DOI:10.1016/j.saa.2006.11.016

Spectrochimica Acta Part A 68 (2007) 211–219
Synthesis, thermal and spectral studies of first-row transition metal
complexes with girard P reagent-based ligand
Usama El-Ayaan∗, I.M. Kenawy, Y.G. Abu El-Reash
Department of Chemistry, Faculty of Science, Mansoura University, Mansoura 35516, Egypt
Received 7 October 2006; received in revised form 9 November 2006; accepted 17 November 2006
Abstract
A new series of first-row transition metal complexes with 1-acetylpyridinium chloride-4-benzoyl thiosemicarbazide (H2GPBzIT) have been pre-
pared and characterized by elemental analysis, spectroscopic and magnetic measurements. The proton-ligand ionization constants were determined
potentiometrically using Irving–Rossotti technique. The stability constants of complexes were also calculated and were found in agreement with
the seq
uence of stability constants of Irving and Williams. Thermal stability and degradation kinetics have been measured using thermogravimetric
analyzer. Kinetic parameters were obtained for each stage of thermal degradation of complexes using Coats–Redfern method.
© 2006 Elsevier B.V. All rights reserved.
Keywords: Synthesis; Thermal degradation kinetics; Spectroscopic; Girard-P derivative
1. Introduction
The present paper deals with the syntheses, spectral
and thermal degradation kinetics of new series of transi-
tion metal complexes of 1-acetylpyridinium chloride-4-benzoyl
thiosemicarbazide (H2GPBzIT). Ligand formation constants
and stability constants of all studied complexes were determined
in solution potentiometrically using Irving–Rossotti technique.
The thermal degradation kinetic parameters such as energy of
activation (Ea) and the pre-exponential factor (A); and ther-
modynamic parameters like entropy (ꢀS), enthalpy (ꢀH) and
activation energy (ꢀG) for each step of degradation have been
evaluated.
Because of their capability to form water-soluble hydrazones,
the girard-P 1-(carboxymethyl)pyridinium chloride hydrazide
and the girard-T (carboxymethyl) trimethylammonium chloride
hydrazide reagents, have long been used in organic chem-
istry to separate carbonyl compounds from other classes of
organic substances [1,2]. Girard-P reagent has already been
used to deprotect imino groups in cephalosporin chemistry [3].
Recently, a very efficient method of preparation of sultamicillin
base was done by deprotection with girard-P reagents [4].
From a series of 33 thioacetals and hydrazones of 2-
(4-formylstyryl)-5-nitro-1-vinylimidazole was prepared and
examined for antitrypanosomal properties. The thioacetals
were inactive as antitrypanosomal agents but three hydra-
zones derived from N-aminoguanidine, pyridylacethohydrazide
chloride (girard reagent P), and dimethylaminoacetohydrazide
(girard reagent D) displayed good activity against Trypanosoma
rhodesiense [5].
2. Experimental
2.1. Instrumentation and materials
All starting materials were purchased from Fluka, Riedel and
Merk and used without further purification. Elemental analyses
(C, H, N) were performed on a Perkin-Elmer 2400 Series II
Analyzer. Electronic spectra were recorded on a UV-UNICAM
2001 spectrophotometer using 10 mm pass length quartz cells
at room temperature. Magnetic susceptibility was measured
with a Sherwood Scientific magnetic susceptibility balance at
297 K. Infrared spectra were recorded on a Perkin-Elmer FT-
IR Spectrometer 2000 as KBr pellets and as Nujol mulls in the
4000–370 cm−1 spectral range. 1H NMR measurements at room
Finally, in view of the fact that these reagents and their Schiff
bases posses ligating atoms they have also been studied as lig-
ands in coordination chemistry [6–8].
∗
Corresponding author. Tel.: +966 553901011.
E-mail addresses: usama@mans.edu.eg, uelayaan@kfu.edu.sa
(U. El-Ayaan).
1386-1425/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2006.11.016

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U. El-Ayaan et al. / Spectrochimica Acta Part A 68 (2007) 211–219
Table 1
Analytical and physical data of H2GPBzIT ligand and its metal complexes
Compound
F. Wt. Color
Found (Calculated) (%)
C
H
N
S
M
–
Cl
–
H2GPBzIT C15H15Cl N4O2S
[Mn(HGPBzIT)2(H2O)2]
350.82 White
51.10 (51.35) 4.25 (4.31) 15.85 (15.97) 9.19 (9.14)
790.60 Pale yellow 45.31 (45.58) 3.96 (4.08) 14.05 (14.17) 8.21 (8.11)
6.81 (6.95)
9.02 (8.97)
C30H32Cl2MnN8O6S2
[Cr(HGPBzIT)Cl2(H2O)]
C15H16Cl3CrN4O3S
490.73 Green 37.3 (36.71) 3.70 (3.29) 11.20 (11.42) 6.88 (6.53) 10.62 (10.59) 21.54 (21.67)
[Co(HGPBzIT)Cl] C15H14Cl2CoN4O2S 444.2
Blue
40.86 (40.56) 3.21 (3.18) 12.80 (12.61) 7.46 (7.22) 13.02 (13.27) 15.84 (15.96)
[Zn(HGPBzIT)Cl](H2O)2
C15H18Cl2N4O4SZn
486.69 Pale yellow 37.22 (37.02) 3.68 (3.73) 11.30 (11.51) 6.69 (6.54) 13.51 (13.43) 14.62 (14.57)
[Ni(GPBzIT)(H2O)] C15H15ClN4NiO3S 425.52 Brown
42.43 (42.34) 3.66 (3.55) 12.94 (13.17) 7.35 (7.53) 13.52 (13.79)
8.14 (8.33)
[Cu2(GPBzIT) Cl2(H2O)2]
15H17Cl3Cu2N4O4S
582.84 Brown
30.86 (30.91) 2.68 (2.94)
9.71 (9.61)
5.63 (5.50) 22.02 (21.80) 18.32 (18.25)
C
[Cd2(GPBzIT) (OAc)2(H2O)2]
C19H23ClCd2N4O8S
727.75 Yellow
30.78 (30.89) 2.98 (3.19)
7.89 (7.70)
4.52 (4.41) 30.62 (30.89)
4.94 (4.87)
temperature were obtained on a Jeol JNM LA 300 WB spectrom-
eter at 400 MHz, using a 5 mm probe head in CDCl3. Chemical
shifts are given in ppm relative to internal TMS (tetramethylsi-
lane). Thermogravimetric measurements were performed on a
DTG-50 Shimazu instrument.
2.3. Synthesis of metal complexes
All complexes were prepared by refluxing H2GPBzIT
(0.35 g, 1.0 mmol) and the hydrated metal salts (1.0 mmol), e.g.
chloride and acetate, in 30 ml ethanol for 2–3 h. The resulting
solid complexes were filtered while hot, washed with ethanol
followed by diethyl ether and dried in vacuo over CaCl2.
2.2. Synthesis of 1-acetylpyridinium chloride-4-benzoyl
thiosemicarbazide (H2GPBzIT)
3. Results and discussion
Preparation of 1-acetylpyridinium chloride-4-benzoyl
thiosemicarbazide (H2GPBzIT) was carried out by refluxing
4.8 g of girard-P with 4 ml benzoyl isothiocyanate in 20 ml
absolute ethanol over a water bath for 4 h. The reaction
mixture was left to cool till white crystals were separated
out. These were filtered off, recrystallized from ethanol and
finally dried in vacuum desiccators over anhydrous calcium
chloride.
The analytical and physical data of (H2GPBzIT) ligand and
its complexes are listed in Table 1.
3.1. IR and NMR spectra of H2GPBzIT ligand and its
complexes
The principle IR bands of H2GPBzIT and its metal complexes
are listed in Table 2.
Yield: 6.50 g (74%) of (H2GPBzIT). Found: C, 51.10; H,
4.25; N, 15.85. Calc. for C15H15ClN4O2S (350.82): C, 51.35;
H, 4.31; N, 15.97%.
IR spectra (KBr) of H2GPBzIT, show a three bands at
3236, 3123 and 3078 cm−1 assigned to νNH(4), νNH(1) and
Table 2
Infrared data (cm−1) of H2GPBzIT ligand and its metal complexes
Compound
IR (cm−1
)
ν(CO)h
ν(CO)b
νCS
νNH(4)
νNH(1)
νNH(2)
ν(C O)
ν(C N)
H2GPBzIT
1708
1730
–
1676
–
1666
1659
–
704
695
711
693
706
3236
–
3131
3131
–
3123
3148
–
–
3084
3078
3052
3053
3052
3062
–
1261
1217
1213
1213
–
1662
1598
1577
1598
[Mn(HGPBzIT)2(H2O)2]
[Cr(HGPBzIT) Cl2(H2O)]
[Co(HGPBzIT)Cl]
–
[Zn(HGPBzIT)Cl](H2O)2
1680
[Ni(GPBzIT)(H2O)]
–
–
–
–
–
–
712
712
713
–
–
–
–
–
–
3064
3052
3052
1193
1208
1632
1600
[Cu2(GPBzIT)Cl2(H2O)2]
1197
1222
1628
1599
[Cd2(GPBzIT)(OAc)2(H2O)2]
b: benzoyl moiety; h: hydrozide moiety.
1193
1208
1655
1590

U. El-Ayaan et al. / Spectrochimica Acta Part A 68 (2007) 211–219
213
Fig. 2. Suggested structure of [Cr(HGPBzIT)Cl2(H2O)] complex.
Fig. 1. Structure of the ligand (H2GPBzIT).
rotation about the thiocarbonyl-nitrogen bond, the weak stabil-
ity of seven membered ring and the lowering of μeff (4.26 BM)
which may be due to antiferromagnetic interaction suggest the
dimeric nature of Co(II) complex.
In the NMR of Zn(II) complex in DMSO-D6, the signal
assigned to NH(4) proton disappear as a result of COb eno-
lization. The absence of H2O signals indicate that H2O is out of
coordination sphere.
In [Ni(GPBzIT)(H2O)], [Cu2(GPBzIT)Cl2(H2O)2] and
[Cd2(GPBzIT)(OAc)2(H2O)2]2 complexes, the ligand behaves
as a binegative tridentate ligand through the enolic oxygen of
both (CO) groups and the nitrogen of NH group. This behaviour
is revealed by the disappearance of ν(CO) for both benzoyl
and hydrazide moieties, νNH(4) and νNH(1) with simultane-
ous appearance of new bands assignable to ν(C O) and ν(C N)
groups. The dimeric nature of Cd(II) complex is confirmed from
the NMR spectrum, where the signals integration of ligand pro-
tons corresponds a twice number of protons in the monomer of
ligand. The sharpening of resonance and the shift of signal (δ
1.82 ppm) to higher field assigned to CH3 protons of CH3COO−
as well as the absorption in the i.r. spectrum at 1560 cm−1
demonstrates the formation of bidentate acetate [12,13]. All
these observations suggested the structure of Cd(II) complex.
The color change of Cu(II) complex to green after addition
of pyridine predicts the presence of chlorine bridge, which is
further confirmed by the lowering of μeff (1.46 BM) which may
be a result of strong antiferromagnetic interaction.
νNH(2) modes, respectively. The two strong bands at 1708 and
1676 cm−1 are attributed to ν(CO) groups of the hydrazide(h)
and benzoyl(b) moieties, respectively. Also, two bands at 704
and 1380 cm−1 are assigned to ν(CS) and a combination of
(νCS + νCN) [9], respectively. The medium intensity band at
975 cm−1 is attributed to the ν(N N) vibration [10]. No bands
exist above 3500 cm−1 or in the 2600–2500 cm−1 [ν(OH) and
ν(SH) vibrations] region. The absence of these bands indicates
the presence of H2GPBzIT in thione and keto form (Fig. 1).
Weak bands at 1900–1800 cm−1 are assigned to the stretching
and bending vibrations (N H O). This result as well as the low-
ering of νNH suggests the presence of intramolecular hydrogen
bonding [11].
The NMR spectrum of H2GPBzIT in DMSO-d6, shows three
signals at δ = 9.28, 8.28 and 8.08 ppm relative to TMS which dis-
appear upon adding D2O and can be assigned to NH(2), NH(4),
NH(1) protons, respectively. The shift of δ NH to down field is
due to hydrogen bonding. The multiplet at (7.35–9.1 ppm) are
assigned to phenyl and pyridyl ring protons. The signal of CH2
protons shifted to down field (6.36 ppm) because the presence
of the quaternary nitrogen (N+) which acts as a strong electron
withdrawing group.
Infrared spectra of [Mn(HGPBzIT)2(H2O)2] complex show
that the ligand behaves as a bidentate, coordinating via CS in
the thione form and the enolized carbonyl oxygen of the benzoyl
moiety with the displacement of a hydrogen atom from the latter
group. This mode of coordination is supported by the following
observations in the complex spectra: (i) the disappearance of
bothν(CO)ofbenzoylmoietyandνNH(4)withthesimultaneous
appearance of new bands assignable to νC O and ν(C N); (ii)
Bands assigned to ν(CO) of hydrazide moiety are shifted to
higher wavenumbers.
In the complexes, [Cr(HGPBzIT)Cl2(H2O)], [Co(HGPBzIT)
Cl] and [Zn(HGPBzIT)Cl](H2O)2, H2GPBzIT behaves as a
mononegative tridentate ligand. In the Cr(III) complex (Fig. 2)
the ligand coordinating via the enolized carbonyl oxygen of the
hydrazide moiety, the CO of the benzoyl moiety in the keto form
and the nitrogen of NH group. In the Zn(II) complex, the ligand
coordinates similarly but through the enolized carbonyl oxygen
of benzoyl moiety. In Co(II) complex, the ligand coordinating
via the enolized CO of hydrozide moiety, the CO of benzoyl moi-
ety in the keto form and the CS in the thione form. The restricted
3.2. Electronic spectra and magnetic moments
Electronic spectra were measured in 10−3 M dimethyl sul-
foxide (DMSO) solution of all studied complexes. The band
positions, magnetic moments and calculated ligand field param-
eters are given in Table 3.
The electronic spectrum of [Cr(HGPBzIT)Cl2(H2O)] shows
two strong absorption bands at 14,350 (ν1) and 21,650 cm−1
(ν2). We could not observe the expected ν3 band on our instru-
ment which might be hidden below ν2. The three spin-allowed
transitions for chromium(III) in an octahedral field are as
4
4
4
4
follows: A2g(F) → T2g(F) (ν1), A2g(F) → T1g(F) (ν2) and
4A2g(F) → T1g(P) (ν3). The ν1 transition is a direct measure-
4
ment of the ligand field parameter 10 Dq. From ν1 and ν2, the
value of B and β can be calculated. In addition, the μeff-value

214
U. El-Ayaan et al. / Spectrochimica Acta Part A 68 (2007) 211–219
Table 3
Magnetic moment, electronic bands and ligand field parameters of the complexes derived from H2GPBzIT
Compounda
Band position (cm−1
)
Dq
B
β
μeff (BM)
[Cr(HGPBzIT)Cl2·H2O]
[Mn(HGPBzIT)2(H2O)2]
[Ni(GPBzIT)(H2O)]
[Co(HGPBzIT)Cl]
[Cu2(GPBzIT)Cl2(H2O)2]
14,350, 21,650, 25,64, 27,933
24,154, 26,954
14,600, 20,964, 25,000
14,662, 16,207, 19,080
12,610, 20,000, 25,900, 27,777
1615
514.13
0.56
3.63
6.70
3.66
4.26
1.46
–
–
–
–
–
–
–
–
–
–
–
–
a
Electronic spectra were measured in 10−3 M dimethyl sulphoxide (DMSO) solution of all studied complexes.
can be taken as additional evidence for the octahedral geometry
[14].
The electronic spectrum of the [Mn(HGPBzIT)2(H2O)2]
complex shows a strong band at 24,154 cm−1 which is assigned
4
to the 6A1g → T2g(D) transition. The other characteristic bands
for d-d transitions are difficult to recognize in this complex and
thus the ligand field parameters could not be calculated.
The electronic spectrum of [Ni(GPBzIT)(H2O)] is consistent
with the tetrahedral geometry showing one broad d–d transition
3
band at 14,600 cm−1 assigned to 3T1(F) → A2(F) (ν3) transi-
tion. Magnetic moment of 3.66 BM is an additional evidence for
the tetrahedral structure.
The electronic spectrum of the blue complex,
[Co(HGPBzIT)Cl] exhibits an intense band at 14,662 cm−1
Fig. 3. Titration curves of metal ions–H2GPBzIT complexes, 0.2 M KCl at
295 K.
4
4
assignable to the A2(F) → T1(P) transition and a shoulder
at 16,207 cm−1 due to spin coupling, indicating tetrahedral
geometry for this complex. The blue color as well as the
magnetic moment of 4.26 BM are a further indication for the
tetrahedral geometry.
The lowering in μeff (1.46 BM) of the binuclear copper
complex, [Cu2(GPBzIT)Cl2(H2O)2] may be attributed to the
covalent nature of the copper-sulphur bond. The electronic spec-
tra show bands at 12,610, 20,000, 25,900 and 27,777 cm−1
region. These band positions are in agreement with those gen-
erally observed for square-planar copper(II) complexes [15].
where Y is the ionizable protons in the ligand, V the volume of the
alkali at desired pH, V0 the initial volume of the titrated mixtures
(25 ml in this study), TL the initial ligand concentration, and N
is the concentration of the alkali.
The proton-ligand formation curves are obtained by plotting
n¯A versus pH as shown in (Fig. 4). Same study was applied for
other two temperatures namely, 308 and 318 K.
From these curves, the maximum n¯A value was found to be
∼1 indicating the presence of one dissociable amide proton in
the ligand.
The proton-ligand stability constant (pKa) of H2GPBzIT was
determined by interpolation at half-n¯A values, i.e. at n¯A = 0.5
from the n¯A-pH curves (Fig. 4). These values are confirmed by
3.3. Potentiometric studies
3.3.1. Determination of the proton-ligand Ionization
constants
The ionization constants of 1-acetylpyridinium chloride-
4-benzoyl thiosemicarbazide (H2GPBzIT) was determined
potentiometrically using Irving–Rossotti technique [16] at dif-
ferent temperatures (295, 308 and 318 K) and at constant ionic
strength of 0.08 M KCl. At constant temperature of 295 K, the
effect of different ionic strength on the proton-ligand ionization
constant was studied using five different KCl concentrations.
The ligand was titrated against carbonate-free sodium hydrox-
ide (0.0105 M) solution. The titration curves for (H2GPBzIT)
and its metal complexes are shown in (Fig. 3) as a representative
example.
The average number of protons associated with the ligand
n¯A, was calculated at different pH-values using Irving–Rossotti
equation [16]:
VN
n¯A = Y −
Fig. 4. Proton-ligand formation curves for H2GPBzIT using half-method at
different ionic strength values.
(V0 − V)TL

U. El-Ayaan et al. / Spectrochimica Acta Part A 68 (2007) 211–219
215
Table 5
The association constants of H2GPBzIT
Method
pKa
0.04 M
0.08 M
0.12 M
0.16 M
0.20 M
Mid-point (half-method)
Least square method
5.2
4.6
5.3
5.6
5.5
–
5.6
5.6
5.6
5.6
(b) The positive values of ꢀH indicate that the dissociation pro-
cesses are endothermic in nature and enhance with the rise
of temperature.
(c) The negative values of ꢀS are owing to the increasing of the
order as a result of salvation process, which can be explained
as the sum of the bound solvent molecules with the dissoci-
ated ligand being more than the solvent molecules originally
accompanying the un-dissociated form.
(d) ꢀG values for the dissociation constants are positive, i.e.
the dissociation processes are non-spontaneous.
(e) The correlations of pKa values versus 1/T are linear in
all cases, suggesting that ꢀCp values for the dissocia-
tion processes are zero over the studied temperature range
(295–318 K).
Fig. 5. Relation between log(n¯A/1 − n¯A) vs. pH for proton-H2GPBzIT, 0.20 M
KCl and at 295 K.
the method of least square by plotting log(n¯A/1 − n¯A) versus
pH as shown in (Fig. 5).
The thermodynamic properties of solutions are not only use-
ful for estimating the feasibility of reaction in solution, but
they also offer one of the better methods for investigating the
theoretical aspects of solution [17].
The standard free energy (ꢀG◦) associated with the acid dis-
sociation can be calculated from the equilibrium constants of
the reaction, as follow:
We also studied the effect of differing the ionic strength on
the proton-ligand stability constant and the result indicate that
the values of pKa of the ligand decrease as the ionic strength
increases due to inert salt effect (Table 5).
ꢀG◦ = −2.303RT pKa
While the standard enthalpy change (ꢀH◦) of dissociation can
be estimated from the use of Van’t Hoff equation:
3.3.2. Determination of stability constants of H2GPBzIT
complexes
d ln K
dT
ΔH◦
RT2
=
First-row transition metal complexes with H2GPBzIT lig-
and have been studied potentiometrically. These include, Cu(II),
Co(II), Ni(II), Zn(II), Fe(III) in addition to Cd(II). The titrations
were performed at different ionic strength values ranging from
0.04 to 0.20 M KCl and at temperature of 295 K.
By plotting pKa versus 1/T, from the slope the enthalpy change
(ꢀH◦) for the dissociation of the ligand can be calculated.
Also, the standard entropy change (ꢀS◦) of dissociation can
be calculated using the following well-known equation:
An inspection of the titration curves shown in (Fig. 3) for
H2GPBzIT reveals that the metal-ligand titration curves are
below and well separated from the ligand titration curves. This
indicates that the complexation processes were occurred with
liberation of hydrogen ions.
Thestabilityconstantareevaluatedfromtheformationcurves
drawn between n and pL, where n is the average number of ligand
attachedpermetalionandpListhefreeligandexponent, nvalues
can be calculated from the following equation:
ꢀG◦ = ꢀH◦ − TꢀS◦
The stoichiometric thermodynamic functions are summarized
in Table 4. From these data the following conclusions can be
obtained:
(a) pKa values decrease with increasing temperature, i.e. the
acidity of the ligand increases with increasing temperature
Table 4.
TL − [L]
n¯ =
TM
Table 4
where TL and TM are the initial concentrations of the ligands and
the metal ion, respectively and [L] is the ligand concentration at
equilibrium. Therefore, the difference between TL and [L] are
the concentration of the bound ligand to the metal ion. The above
equation can rewrite as follow:
Thermodynamic parameters and association constant for H2GPBzIT (0.08 M
KCl) at different temperatures
Temperature (K)
pKa
ꢀG◦
−TꢀS◦
ꢀH◦
(kJ mol−1
)
(kJ mol−1
)
(kJ mol−1
)
295
308
318
5.60
5.18
4.8
31.63
30.55
29.23
4.68
3.6
2.28
(V2 − V1)N0
26.95
n =
(V0 + V1)nATM

216
U. El-Ayaan et al. / Spectrochimica Acta Part A 68 (2007) 211–219
Table 6
decomposition reactions by analysis of curves. Several equa-
tions [20–23] have been proposed as means of analyzing a TG
curve and obtaining values for kinetic parameters. Many others
[24–26] have discussed the advantages of this method over the
conventional isothermal method. The rate of decomposition pro-
cess can be described as the product of two separate functions
of temperature and conversion [24], using:
The stability constants of metal ions–H2GPBzIT complexes at different ionic
strength of KCl and at 295 K
Complex
pKa
0.04 M 0.08 M 0.12 M 0.16 M 0.20 M
[Cu2(GPBzIT)Cl2(H2O)2]
[Co(HGPBzIT)Cl]
[Ni(GPBzIT)(H2O)]
[Cd2(GPBzIT)(OAc)2(H2O)2] 3.86
[Zn(HGPBzIT)Cl](H2O)2
[Fe–GPBzIT]
4.70
3.3
4.14
4.84
3.3
4.00
3.72
–
4.70
2.60
4.00
–
2.60
4.84
4.42
2.60
4.00
–
2.60
4.84
4.42
2.60
4.00
–
2.60
4.84
dα
= k(T)f(α)
(1)
dt
3.02
4.84
4.84
where α is the fraction decomposed at time t, k(T) the temper-
ature dependent function and f(α) is the conversion function
dependent on the mechanism of decomposition. It has been
established that the temperature dependent function k(T) is of
the Arrhenius type and can be considered as the rate constant k:
where V2 is the volume of alkali required to reach the desired
pH in the mixture of complex solution and TM is the initial metal
ion concentration.
k = A e−E/RT
(2)
The pL value can be calculated by the following equation:
where R is the gas constant in (J mol−1 K−1). Substituting Eq.
ꢀ
j=jβjHj
V2 + V0
j=0
(2) into Eq. (1), we get:
pL =
TL − n¯
V0
ꢁ
ꢂ
dα
dt
A
=
f(α)
(3)
andformonobasicacidthefollowingsimplifiedformofequation
can successfully apply [18]:
ϕ e−E/RT
where ϕ is the linear heating rate dT/dt. On integration and
approximation, this equation can be obtained in the following
form:
pL = pKa − pH − log{[HA]init − [OH−]}
The stoichiometric stability constants are evaluated using the
half-method in which the following equation was applied:
ꢁ
ꢂ
−E
AR
ϕE
ln g(α) =
ln
(4)
ꢁ
ꢂ
RT
1
log K = log
[L]
where g(α) is a function of α dependent on the mechanism of the
reaction. Several techniques have been used for the evaluation
of temperature integral. In this work we use the integral method
of Coats and Redfern to evaluate the kinetic parameters and
study the thermal behavior of complexes using the following
equations:
n=0.5
(the pK values are evaluated at n = 0.5)
The experimental stability constant values (Table 6) for com-
plexationoftheinvestigatedligandswasfoundfollowstheorder:
ꢃ
ꢅ
ꢄ
ꢅ
ꢆ
Co2+ < Ni2+ < Cu2+ > Zn2+
1 − (1 − α)1−n
(1 − n)T2
E
AR
ϕE
ꢆ
ln
ln
= −
+ ln
for n = 1
(5)
(6)
RT
In agreement with the will known sequence of stability con-
stants of Irving and Williams [19], clearly this order reflects the
changes in heats of complexation along the series and it arises
from the influence of the polarization ability of the metal ions,
as measured by the ratio of charge to ionic radius.
ꢆ
ꢅ
− ln(1 − α)
E
AR
ϕE
= −
+ ln
for n = 1
T2
RT
where A is the pre-exponential factor.
Moreover, Irving and Williams series is a consequence of
crystal-field stabilization energy which varies in the order of
Co2+ < Ni2+ < Cu2+ > Zn2+, i.e. the splitting caused by the ligand
in the energy level of the d-electrons diminishes the total energy
of the system, thus causes in stabilization.
Also the results show that the trivalent metal ions are more
stable than that of divalent ones, as a result of increasing the
oxidation state.
The correlation coefficient, r, was computed using the least
square method for different values of n, by plotting the left-hand
side of Eq. (5) or (6) versus 1000/T, Figs. 7 and 8. The n value
∼
which gave the best fit (r 1) was chosen as the order parameter
=
for the decomposition stage of interest. From the intercept and
linear slope of such stage, the A and E values were determined.
The other kinetic parameters, ΔH, ΔS and ΔG were computed
using the relationships; ꢀH = E − RT, ꢀS = R[ln(Ah/kT)] and
ꢀG = ꢀH − TꢀS, where k is the Boltzmann’s constant and h
is the Planck’s constant. The kinetic parameters are listed in
Table 7. The following remarks can be pointed out: (i) all com-
plexes decomposition stages show a best fit for (n = 1) indicating
a first order decomposition in all cases. Other n values (e.g. 0,
0.33, and 0.66) did not lead to better correlations. (ii) The value
of ꢀG increases significantly for the subsequently decomposi-
tion stages of a given complex. This is due to increasing the
3.4. Thermal analysis
Thermogravimetric (TG) and differential thermogravimet-
ric (DTG) analysis were carried out for all studied complexes
(Fig. 6).
In recent years there has been increasing interest in determin-
ing the rate-dependent parameters of solid state non-isothermal

U. El-Ayaan et al. / Spectrochimica Acta Part A 68 (2007) 211–219
217
Fig. 6. TG and DTG of (a) [Ni(GPBzIT)(H2O)]; (b) [Mn(HGPBzIT)2(H2O)2]; (c) [Cr(HGPBzIT)Cl2(H2O)]; (d) [Co(HGPBzIT)Cl]; (e) [Zn(HGPBzIT)Cl](H2O)2;
(f) [Cu2(GPBzIT)Cl2(H2O)2]; (g) [Cd2(GPBzIT)(OAc)2(H2O)2] complexes.

218
U. El-Ayaan et al. / Spectrochimica Acta Part A 68 (2007) 211–219
Fig. 7. Coats–Redfern plots of [Co(HGPBzIT)Cl] complex, where Y = ln[−ln(1 − α)/T2].
Fig. 8. Coats–Redfern plots of [Zn(HGPBzIT)Cl](H2O)2 complex, where Y = ln[−ln(1 − α)/T2].
values of TꢀS significantly from one stage to another which
overrides the values of ꢀH. Increasing the values of ꢀG of
a given complex as going from one decomposition step sub-
sequently to another reflects that the rate of removal of the
subsequent ligand will be lower than that of the precedent lig-
and [27,28]. This may be attributed to the structural rigidity of
the remaining complex after the expulsion of one and more lig-
ands, as compared with the precedent complex, which require
more energy, TꢀS, for its rearrangement before undergoing any
compositional change. (iii) The negative values of activation
entropies ꢀS indicate a more ordered activated complex than
the reactants and/or the reactions are slow [29]. (iv) The pos-
itive values of ꢀH mean that the decomposition processes are
endothermic.
Table 7
Temperature of decomposition and the kinetic parameters of studied complexes
Compound
Stage
T (K)
A (S−1
)
E (kJ mol−1
)
ꢀH (kJ mol−1
)
ꢀS (kJ mol−1 K−1
)
ꢀG (kJ mol−1
)
[Ni(GPBzIT)(H2O)]
1st
336
590
662
722
6.38 × 104
2.90 × 108
5.3 × 1010
3.2 × 109
45.1
121.1
165.5
164.6
42.4
116.2
160
−0.154
−0.088
−0.046
−0.07
94
168
190
209
2nd
3rd
4th
158.6
[Mn(HGPBzIT)2(H2O)2]
[Cr(HGPBzIT)Cl2(H2O)]
1st
2nd
3rd
555
740
874
102.5
299.7
51.5
73.9
143.4
46.5
67.8
136.2
−0.211
−0.205
−0.140
−0.152
−0.263
−0.150
−0.217
164
220
259
8.30 × 105
1st
353
500
636
745
8.1 × 104
15.0
20.8
95.2
66
12.0
16.6
89.9
60
66
148
185
222
2nd
3rd
4th
0.189
2 × 105
71.5
[Co(HGPBzIT)Cl]
1st
2nd
3rd
573
726
930
369.5
2.14
4.6 × 105
57.0
45.7
148
52.2
39.7
140
−0.20
168
219
276
−0.246
−0.146
[Zn(HGPBzIT)Cl](H2O)2
1st
2nd
3rd
585
723
887
960.6
397
3.8 × 1011
62
74
239
57
68
232
−0.193
−0.203
−0.032
170
214
261
[Cu2(GPBzIT)Cl2(H2O)2]
1st
2nd
560
880
0.60
25
202.9
20.3
195.6
−0.254
163
259
3.2 × 109
−0.072
[Cd2(GPBzIT)(OAc)2(H2O)2]
1st
2nd
3rd
495
624
913
7 × 105
77.2
98.9
78
73.1
93.8
70.4
−0.137
−0.124
−0.238
141
171
288
4.5 × 106
36.5

U. El-Ayaan et al. / Spectrochimica Acta Part A 68 (2007) 211–219
219
4. Conclusion
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In solution, the stability constant of complexes was found
follows the order: Co2+ < Ni2+ < Cu2+ > Zn2+, in agreement with
the will known sequence of stability constants of Irving and
Williams.
In solid state, the thermal decomposition kinetics of studied
complexes reveals a first order decomposition in all cases. In
addition, the kinetic parameters (E, ꢀH, ꢀS and ꢀG) of all
thermal decomposition stages have been calculated.
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