Synthesis, spectroscopic characterization, pH-metric and thermal behavior on Co(II) complexes formed with 4-(2-pyridyl)-3-thiosemicarbazide derivatives

Article abstract of DOI:10.1016/j.molstruc.2011.08.020

Four new cobalt (II) complexes of some thiosemicarbazides have been synthesized and spectrochemically characterized. The thiosemicarbazides are prepared by the addition of 4-(2-pyridyl)-3-thiosemicarbazide to phenyl isothiocyanate (H₂PPS), benz

Full text of DOI:10.1016/j.molstruc.2011.08.020

Journal of Molecular Structure 1004 (2011) 271–283
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Synthesis, spectroscopic characterization, pH-metric and thermal behavior
on Co(II) complexes formed with 4-(2-pyridyl)-3-thiosemicarbazide derivatives
T.A. Yousef a, O.A. El-Gammal b, S.E. Ghazy b, G.M. Abu El-Reash b,
⇑
a Department of Toxic and Narcotic Drug, Forensic Medicine, Mansoura Laboratory, Medicolegal Organization, Ministry of Justice, Egypt
b Department of Chemistry, Faculty of Science, Mansoura University, Mansoura 35516, Egypt
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 25 June 2011
Received in revised form 7 August 2011
Accepted 8 August 2011
Available online 24 August 2011
Four new cobalt (II) complexes of some thiosemicarbazides have been synthesized and spectrochemically
characterized. The thiosemicarbazides are prepared by the addition of 4-(2-pyridyl)-3-thiosemicarbazide
to phenyl isothiocyanate (H2PPS), benzoyl isothiocyanate (H2PBO), phenyl isocyanate (H2APO) and 2-pyr-
idyl isothiocyanate (H2PPY). The complexes are characterized by elemental analysis, spectral (IR, 1H NMR
and UV–Vis), thermal and magnetic measurements. The studies revealed that structures of complexes are
of two types octahedral and tetrahedral. The octahedral complexes are of H2PPS, which acts as di-anionic
tetradentate SSNN and H2APO acts as mono-anionic tridentate NON. The tetrahedral complexes are of
H2PBO, which acts as di-anionic tridentate NSO and H2PPY acts as mono-anionic bidentate NS. From
the modelling studies, the bond length, bond angle, HOMO, LUMO and dipole moment had been calcu-
lated to confirm the geometry of the ligands and their investigated complexes. From TG and DTA studies
kinetic parameters are determined using Coats–Redfern and Horowitz–Metzger methods. From pH met-
Keywords:
Thiosemicarbazide
PM3
Co complexes
Coats–Redfern
Horowitz–Metzger
pH-metry
ric studies at 298, 303 and 308 K and
l (0.1, 0.15 and 0.2) in 50% dioxane–water mixture the protonation
constants of the ligands, the stepwise stability constants of the complexes and their thermodynamic
parameters are calculated.
Crown Copyright Ó 2011 Published by Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
Heterocyclic thiosemicarbazones have aroused considerable
interest in chemistry due to their remarkable coordination proper-
ties and in biology owing to a wide spectrum of potential biologi-
cal activities [1–4]. The biological activity of thiosemicarbazone
complexes is related to its chelating ability toward the transition
metal ions, whether the bonding through nitrogen, and sulfur
atoms [4] or oxygen, nitrogen and sulfur atoms [5–7]. The activity
of heterocyclic thiosemicarbazones is affected by the presence of
N(4) substitution [8,9]. We report here the preparation of Co(II)
complexes of N1-phenyl-N2-(pyridin-2-yl)hydrazine-1,2-bis(car-
bothioamide) (H2PPS), N-phenyl-2-(2-(pyridin-2-ylcarbamothioyl)
hydrazinyl)-2-thioxoacetamide (H2PBO), N-phenyl-2-(pyridin-2-
ylcarbamothioyl)hydrazinecarboxamide (H2APO) and 1-(aminoN-
(pyridin-2-yl)methanethio)-4-(pyridin-2-yl)thiosemicarbazide
(H2PPY) with study the substituent effect on the thiosemicarbazi-
dic moiety and its coordination behavior. Also, the thermal degra-
dation kinetic parameters such as energy of activation (Ea) and the
pre-exponential factor (A) and thermodynamic parameters like
2.1. Instrumentation and materials
All the chemicals were purchased from Aldrich and Fluka and
used without further purification. Elemental analyses (C and H)
were performed with a Perkin–Elmer 2400 series II analyzer. IR
spectra (4000–400 cmꢀ1) for KBr discs was recorded on a Mattson
5000 FTIR spectrophotometer. Electronic spectra were recorded on
an Unicam UV–Vis spectrophotometer UV2. Magnetic susceptibili-
ties were measured with a Sherwood scientific magnetic suscepti-
bility balance at 298 K. 1H NMR measurements in d6-DMSO at
room temperature was carried out on a Varian Gemini WM-
200 MHz spectrometer at the Microanalytical Unit, Cairo Univer-
sity. Thermogravimetric measurements (TGA, DTA, 20–800 °C)
were recorded on a DTG-50 Shimadzu thermogravimetric analyzer
at a heating rate of 10 °C/min and nitrogen flow rate of 20 ml/min.
2.2. Synthesis of the ligands
entropy (DS), enthalpy (DH) and activation energy (DG) for each
step of degradation have been evaluated.
4-(2-Pyridyl)-3-thiosemicarbazide was synthesized as reported
earlier [10]. Derivatives 1–4 were prepared by boiling ethanolic
solution of 4-(2-pyridyl)-3-thiosemicarbazide (1.6 g, 100 mmol)
with an equimolar amount of phenyl isothiocyanate, benzoyl iso-
thiocyanate, phenyl isocyanate and 2-pyridyl isothiocyanate Fig. 1.
⇑
Corresponding author. Tel.: +20 100373155; fax: +20 502219214.
E-mail address: gaelreash@mans.edu.eg (G.M. Abu El-Reash).
0022-2860/$ - see front matter Crown Copyright Ó 2011 Published by Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2011.08.020

272
T.A. Yousef et al. / Journal of Molecular Structure 1004 (2011) 271–283
2.3. Synthesis of metal complexes
mational analysis has been performed using PM3 [12] forcefield
as implemented in hyperchem 8 [13]. The low lying obtained from
MM+ was then optimized at PM3 using the Polak–Ribiere algo-
rithm in RHF–SCF, set to terminate at an RMS gradient of
All complexes were prepared by refluxing H2PPS, H2PBO, H2APO
or H2PPY (1.0 mmol) and CoCl2ꢁ6H2O (1.0 mmol) in 30 ml ethanol
for 2–3 h. The solid complexes were filtered off, washed with eth-
anol followed by diethyl ether and dried in a vacuum over CaCl2.
0.01 kcal molꢀ1
.
3. Results and discussion
2.4. Procedure for the pH-metric titration
The physical and analytical data of each ligand and their Cobalt
complexes are listed in Table 1.
The pH-metric readings were measured to 0.01 unit with Or-
ion’s research model 601A/digital Ionalyzer standardized before
and checked after each titration with buffer solutions produced
by Fischer (New Jersey, USA). The following mixtures were pre-
pared and titrated potentiometrically at 298, 303 and 308 K against
9.2 ꢂ 10ꢀ3 M NaOH in 50% (v/v) dioxane–water at a constant ionic
strength KCl (0.1, 0.15 and 0.2 M). The solution mixtures (i–iii)
were prepared as follows:
3.1. Infrared and 1H NMR spectra of ligands
The most important IR bands of ligands are recorded in Table 2.
The spectra exhibit three bands between 3234 and 3100 cmꢀ1 due
to m(NH) groups. The m/d modes of (CN) group of pyridyl ring are
found at ꢄ1560 and 620–635 cmꢀ1
.
An inspection on the Table 2 describing the IR spectra of H2PPS,
H2PBO, and H2PPY indicates that the appearance of strong bands
(i) 1.25 ml (1.1 ꢂ 10ꢀ2 M) HCl + 1.25 ml (0.1, 0.15 and 0.2 M)
KCl + 10 ml doubly-distilled H2O.
assigned to
m(C@N) (azomethine), m(CAS) and m(SH) as well as
(ii) 1.25 ml (1.1 ꢂ 10ꢀ2 M) HCl + 1.25 ml (0.1, 0.15 and 0.2 M)
KCl + 2.5 ml (5 ꢂ 10ꢀ3 M) H2PPS, H2PBO, H2APO and H2PPY +
10 ml bi-distilled H2O.
(C@S) modes suggested that these ligands exist in thione–thiol
form. As these ligands contain two C@S groups, (C@S)1 and
(C@S)2, we proposed that (C@S)1 is in thione form and (C@S)2 in
thiol form. This assumption is confirmed by the absence of bands
(iii) 1.25 ml (1.1 ꢂ 10ꢀ2 M) HCl + 1.25 ml (0.1, 0.15 and 0.2 M)
KCl + 2.5 ml (5 ꢂ 10ꢀ3 M) H2PPS, H2PBO, H2APO and
H2PPY + 0.5 ml (5 ꢂ 10ꢀ3 M) Co2+ in bi-distilled water +
9.5 ml H2O.
due to m(C@N) (azomethine), m(CAS) and m(SH) vibrational modes
in the IR spectrum of the start (4-pyridyl thiosemicarbazide) [14]
and the missing of SH signal in the 1H NMR spectrum of H2APO.
In the IR spectrum of H2PPY, the band due to
difficult to recognize because it is overlapped with
m
(C@N) mode was
(C@C) of pyr-
m(CAS) at higher wave-
m
The ultimate volume (25 ml) was adjusted by adding dioxane in
each case and after adding of each increment of the titrant, the
solution was stirred for about two minutes and the pH-reading is
then recorded. For converting the pH-meter reading (B) in 50%
(v/v) dioxane–water and 0.1, 0.15 and 0.2 M KCl to [H+] values,
the equation of Van Uitert and Hass [11] was applied,
idyl ring. Furthermore, the appearance of
number, 683 cmꢀ1 than that of other ligands may be due to the
presence of pyridyl groups at the extremes of H2PPY structure,
which act as electron withdrawing groups. The IR spectrum of
H2APO exhibits a sharp band at 3521 cmꢀ1 due to
m
(OH) in addition
to the
tautomerism.
m
(CO) band at 1675 cmꢀ1 suggested the keto–enol
ꢀ log½Hþꢃ ¼ B þ log UH
where logUH is the correction factor for the solvent composition
and ionic strength for which B is read.
The 1H NMR spectra of H2PPS, H2PBO and H2PPY (Figs. 2–4)
derivatives in DMSO-d6 show two signals at approximately
d@11.10 and 15.4 ppm relative to TMS that disappear upon adding
0
2.5. Molecular modeling
D2O. These signals are attributed to the amide (NHa,a ) and thiol
(SH) protons. The signal at d = 8.29 ppm is due to the (NHb), while
the multiplets at 7.00–7.86 ppm are characteristic of the pyridine
ring protons [15]. The appearance of signal at d 15.4 ppm in the
An attempt to gain better insight on the molecular structure of
the ligand and its complexes, geometric optimization and confor-
4
4
1
1
S
H
H
3
2
5
6
S
H
H
5
6
3
2
a
C
N
N
C
N
N
b
b
a
c
a'
c
a'
N
N
C
S
C
O
N
N
C
S
N
N
1
1
2
2
H
H
H
H
N1-phenyl-N2-(pyridin-2-yl)hydrazine-1,2-bis(carbothioamide)
N-phenyl-2-(2-(pyridin-2-ylcarbamothioyl)hydrazinyl)-2-thioxoacetamide
(H2PPS)
(H2PBO)
4
4
S
C
H
H
N
S1
C
H
H
5
6
3
3
2
5
6
a
N
b
2
a
b
c
N
N
a'
c
N
N
C
O
a'
N
N
C
S
N
N
1
1
H
H
2
N
H
H
N-phenyl-2-(pyridin-2-ylcarbamothioyl)hydrazinecarboxamide
1-(aminoN-(pyridin-2-yl)methanethio)-4-(pyridin-2-yl)thiosemicarbazide
(H2APO)
(H2PPY)
Fig. 1. Structure of H2PPS, H2PBO, H2APO and H2PPY.

T.A. Yousef et al. / Journal of Molecular Structure 1004 (2011) 271–283
273
Table 1
Analytical and physical data of H2PPS, H2PBO, H2APO, H2PPY and their Co(II) complexes.
Compound
Empirical formula
H2PPS
Color
M.p. (°C)
Found (Calcd.)%
Yield (%)
(F.Wt)
M
–
Cl
–
C
H
Yellow
180
51.33
(51.46)
37.91
(37.68)
50.81
(50.74)
37.75
(38.0)
54.51
(54.35)
35.59
(35.76)
47.47
(47.31)
33.85
4.19
(4.32)
4.35
(4.14)
3.88
(3.95)
3.59
(3.87)
4.43
(4.56)
4.03
(4.15)
4.55
(3.97)
3.25
93
89
90
88
90
90
93
85
C
13H13N5S2
[Co(PPS)(H2O)2](H2O)
13H17CoN5O3S2
H2PBO
14H13N5OS2
[Co(PBO)(H2O)](H2O)2
14H17CoN5O4S2
H2APO
13H13N5OS
(303.41)
(414.36)
(331.42)
(442.39)
(287.27)
(436.69)
(304.35)
(433.83)
Dark
Brown
Yellow
>300
270
14.1
(13.9)
–
–
–
–
–
C
C
Dark
Green
White
>300
182
13.5
(13.3)
–
C
C
[Co(HAPO)Cl(H2O)2](H2O)
C13H18CoN5O4SCl
H2PPY
Dark
Green
Yellow
>300
310
13.29
(13.6)
–
8.47
(8.15)
–
C
12H12N6S2
[Co(HPPY)Cl(H2O)](H2O)
12H16CoN6O2S2Cl
Green
>300
14.1
(13.6)
8.6
(8.2)
C
(33.19)
(3.48)
Table 2
Assignments of the IR spectral bands of H2PPS, H2PBO, H2PBO, H2PPY and their Co(II) complexes.
0
Compound
m
(NH)a,a
m
(NH)b,c
m(C@N)py
m
(C@N)⁄
m(C@S)
m(C@N) d(C@N)py
m
(NAN)
m(C@O)
m(CAO)
m(OH)
m(SH)
m(CAS)
H2PPS
[Co(PPS)(H2O)2](H2O)
H2PBO
[Co(PBO)(H2O)](H2O)2
H2APO
[Co(HAPO)Cl(H2O)2](H2O)
H2PPY
3160,3099 3220
1562
–
–
1540
–
–
–
1563
–
1629
860
–
861
–
865
–
861
–
1646
1616
1641
1621
–
1626
–
1620
624
643
630
620
634
670
625
595
971
1006
990
1008
1008
1043
1000
1006
–
–
1672
–
–
–
–
3337
–
–
–
–
2390
–
2380
–
2380
660
635
660
634
–
620
683
630
3133
3118
3129
3158
–
3258
3228
–
3239
3262
3234
–
1562
1562
1542
1527
1562
1565
1164
1206
1243
1216
–
–
1675
3363
–
–
–
–
–
–
3174
3126
[Co(HPPY)Cl(H2O)](H2O)
–
spectrum of H2PPY due to SH proton and a signal at d 12.57 ppm
due to OH group in the spectrum of H2APO (Fig. 5) confirms the
thiol–enol tautomerism in solution.
on the hydrazine nitrogen atoms. Additional evidence for
coordination of the imine nitrogen is the presence of a band
at 449 cmꢀ1 due to
m(CoAN) vibration falling in the 435–
450 cmꢀ1 range for the divalent metal centers [16].
3.2. Infrared spectra of complexes
(iii) The pyridine in-plane deformation mode at 625–634 cmꢀ1
in the spectra of the ligands shifts to 620–621 cmꢀ1 in the
IR spectra of the above complexes, suggesting coordination
of the heteroaromatic nitrogen [17–20].
A careful comparison of the IR spectra of ligands and their Co(II)
complexes in Table 2 gave a good insight on the mode of bonding
and the nature of metal–ligand bond. The IR spectra of the polymer
[Co(PPS)(H2O)2](H2O) complex showed that H2PPS behaves as
binegative tetradentate via two thiol (CAS) and two azomethine
nitrogen groups Fig. 6.
(iv) The disappearance of m(CAOH) band in the cobalt complexes
of [Co(PBO)(H2O)](H2O)2 and [Co(HAPO)Cl(H2O)2](H2O) sug-
gests its coordination with displacement of proton. The shift
of
m(CAO) to higher wavenumber confirms the suggested
H2PBO acts as binegative tridentate through the (C@N)⁄, (C@S)
that adjacent to Nc substitutent (i.e. phenyl group) in thiol form
and enolized (CAO) groups Fig. 7.
behavior for these ligands. In addition the bands at 540
and 553 cmꢀ1, respectively due to
coordination.
m(MAO) reveals this
H2APO acts as mononegative tridentate coordinating via
(C@N)py, enolized (CAO) and (C@N)⁄ groups Fig. 8.
3.3. Electronic spectra and magnetic moments
Finally, H2PPY behaves as mononegative bidentate through
(C@N)py and one of (C@S) groups in thiol form Fig. 9.
This is supported by the following:
The electronic spectral features of ligands and their complexes
in DMF and Nujol mull are summarized in Table 3.
The spectrum of [Co(PPS)(H2O)2](H2O) exhibited two strong
bands at 18,050 and 19,084 cmꢀ1 while of [Co(HAPO)Cl(H2O)2]
(H2O) at 14,925 and 14,577 cmꢀ1. These bands corresponding to
(i) Disappearance of bands due to m(NH)b,c and (C@S) modes
with appearance of new bands at 1540, 1563 and 630,
635 cmꢀ1 attributable to (C@NAN@C) and (CAS) vibrations
in the IR spectra of [Co(PPS)(H2O)3] and [Co(PBO)(H2O)]
(H2O)2 complexes, respectively.
4
4T1g(F) ? 4T1g(P) and T1g(F) ? 4A2g(F) transitions [21,22] arising
from the high-spin d7 configuration Co(II) in an octahedral
geometry.
(ii) The clear shift of the band due to
m
(C@N) of free ligands
Finally, the green complexes, [Co(HPPY)Cl(H2O)](H2O) and
[Co(HPBO)(H2O)2] exhibit an intense bands at 14,553 and
25,381 cmꢀ1 for the first complex and at 14,662 and 23,697 cmꢀ1
H2PPS and H2PBO at 1641 and 1646 cmꢀ1 to a lower wave-
number (1616 and 1621 cmꢀ1) in their complexes is consis-
tent with coordination of the azomethine nitrogen to the
central Co(II) atoms, which is further supported by the
increase in frequency of the hydrazinic NAN bond as a conse-
quence of the reduction between the lone pairs of electrons
4
for the second assignable to the 4A2ðFÞ ! T1ðPÞ and
ꢀ1
4
4A2ðFÞ ! T2ðFÞ transitions. The shoulders at 16,722 cm and at
16,393 cmꢀ1 due to spin coupling [21] respectively, confirming a
tetrahedral geometry for these complexes.

274
T.A. Yousef et al. / Journal of Molecular Structure 1004 (2011) 271–283
Fig. 2. 1H NMR spectrum of H2PPS.
Fig. 3. 1H NMR spectrum of H2PBO.
3.4. Molecular modeling
Analysis of the data in Tables 1S–14S (Supplementary materi-
als) calculated for the bond lengths and angles for the bond. One
can conclude the following remarks:
The molecular structure along with atom numbering of Co(II)
complexes are shown in Figs. 2–4.

T.A. Yousef et al. / Journal of Molecular Structure 1004 (2011) 271–283
275
Fig. 4. 1H NMR spectrum of H2APO.
Fig. 5. 1H NMR spectrum of H2PPY.
1. The bond angles of the thiosemicarbazide moiety are altered
somewhat upon coordination; the largest change affects
N(10)AN(11)AC(12) and C(12)AN(14)AC(15) angles which
are reduced from 124° and 121.4° on ligand to 100.9° and
112.2° in case of [Co(HPBO)(H2O)2] complex, C(5)AC(4)AN(7)
and N(7)AC(8)AS(9) from 126.6° and 128.8° on ligand to 117°

276
T.A. Yousef et al. / Journal of Molecular Structure 1004 (2011) 271–283
N
NH
N
S
S
C
N
S
Co
S
N
N
Co
C
NH
Molecular modeling of H2PPS
[Co(PPS)(H2O)2](H2O) complex
Fig. 6. Molecular modeling of H2PPS and [Co(PPS)(H2O)2](H2O) complex.
Molecular modeling of H2PBO
Molecular modeling [Co(PBO)(H2O)](H2O)2 complex
Fig. 7. Molecular modeling of H2APO and molecular modeling of [Co(HAPO)Cl(H2O)2](H2O) complex.
Molecular modeling of H2APO
Molecular modeling of [Co(HAPO)Cl(H2O)2](H2O) complex
Fig. 8. Molecular modeling of H2APO and molecular modeling of [Co(HAPO)Cl(H2O)2](H2O) complex.
and 112.6° in case of [Co(HAPO)Cl(H2O)2](H2O) complex and
N(11)AC(12)AS(13) from 123.6° on ligand to 130.4° in case
of [Co(HPPY)Cl(H2O)](H2O) as a consequence of bonding.
higher HOMO energy implies that the molecule is a good electron
donor. LUMO energy presents the ability of a molecule receiving
electron as in Table 15S [22].
2. All bond angles in [Co(PBO)(H2O)](H2O)2 and [Co(HPPY)Cl
(H2O)](H2O) complexes are quite near to a tetrahedral geometry
predicting sp3 hybridization while for [Co(PPS)(H2O)2](H2O)
and [Co(HAPO)Cl(H2O)2](H2O), which have octahedral geome-
try predicting sp3d2 hybridization.
3. All the active groups taking part in coordination have bonds
longer than that already exist in the ligand (like C@S, NAH,
(C@N)py and C@O) especially C(8)AN(10) from 1.42–1.45 Å to
1.31–1.33 Å
3.5. Thermogravimetric studies
The stages of decomposition, temperature range, decomposition
product as well as the found and calculated weight loss percent-
ages of the complexes are given in Table 4. In most complexes,
the first decomposition step representing the elimination of water
of hydration.
3.6. Kinetic data
The lower HOMO energy values of H2PBO and H2PPY show that
molecule donating electron ability is the weaker. On contrary, the
Non-isothermal calculations were used extensively to evaluate
the thermodynamic and kinetic parameters for the different thermal

T.A. Yousef et al. / Journal of Molecular Structure 1004 (2011) 271–283
277
Molecular modeling of H2PPY
Molecular modeling of [Co(HPPY)Cl(H2O)](H2O) complex
Fig. 9. Molecular modeling of H2PPY and molecular modeling of [Co(HPPY)Cl(H2O)](H2O) complex.
Table 3
Magnetic moment, electronic bands and ligand field parameters of the Co(II) complexes of H2PPS, H2PBO, H2PBO and H2PPY.
Compound
H2PPS
Solvent
Band position (cmꢀ1
)
Dq
B
b
leff (B.M.)
DMF
Nujol
DMF
35,211 31,847 29,069
–
–
–
–
–
–
–
0.84
–
[Co(PPS)(H2O)2](H2O)
35,211 31,055 25,773
18,050 14,925
24,390 22,321 19,762
17,857 16,556
35,461 32,680 29,070
–
782.7
815.3
3.18
Nujol
774.3
806.5
0.83
H2PBO
DMF
Nujol
DMF
–
–
815
–
–
849
–
–
0.87
–
[Co(PBO)(H2O)](H2O)2
35,211 29,940 24,510
18,797
2.9
Nujol
23,697 20,161 18,305
16,393 14,662
35,211 32,051
–
35,211 32,467 22,624
19,084 16,835 14,577
26,178 23,364 18,797
14,837
793.8
826.8
0.85
H2APO
DMF
Nujol
DMF
–
–
827
–
–
862
–
–
0.89
–
[Co(HAPO)Cl(H2O)2](H2O)
3.1
Nujol
815
849
0.87
H2PPY
DMF
Nujol
DMF
35,211 30,120 18,050
–
35,211 31,847 29,585
25,126 19,157 16,667
14,706
–
–
–
–
–
–
0.74
–
[Co(HPPY)Cl(H2O)](H2O)
317.5
721.5
3.91
Nujol
25,381 19,380 16,722
14,535
313.8
713.1
0.73
equation. The differential conversion function, g(
various functional forms but its most commonly form for solid-state
a
) may present
decomposition steps of the Co(II) complexes were determined using
the Coats–Redfern [23] and Horowitz–Metzger [24].
The rate of decomposition of a solid depends upon the temper-
ature and the amount of material. The expression for the thermal
decomposition of a homogeneous system has the following general
form:
reactions is g( )n, where n is the reaction order, assumed
a) = (1 ꢀ
a
to remain constant during the reaction [25,26].
The rate constant is normally expressed by the Arrhenius
equation:
ꢀ
ꢁ
Ea
RT
da
K ¼ A exp
ꢀ
ð2Þ
¼ KðTÞgð
aÞ
ð1Þ
dt
where Ea is the activation energy, A is the Arrhenius pre-exponential
factor which indicates how fast the reaction occurs and R is the gas
constant in (J molꢀ1 K). Substituting in Eq. (1), we get:
where t is the time, T is the absolute temperature and
gree of transformation defined as:
a
is the de-
ꢀ
ꢁ
wo ꢀ wt
da
Ea
a
¼
¼ A exp
ꢀ
gð
aÞ
ð3Þ
wo ꢀ w
1
dt
RT
In which wo, wt and w are the weights of the sample before the
1
degradation, at temperature t and after total conversion, respec-
tively. K(T) is the rate coefficient that usually follows the Arrhenius
When the reaction is carried out under a linear temperature
program (T = To + bt, where b = dT/dt is the heating rate and To the

278
T.A. Yousef et al. / Journal of Molecular Structure 1004 (2011) 271–283
Table 4
Decomposition steps with the temperature range and weight loss for ligands and their cobalt complexes.
Compound
H2PPS
Temp. range (°C)
Removed species
Wt. loss
Found%
Calcd%
141–228
229–371
513–691
691–800
APy + NH
APy + NH + CS
AN2H2
30.84
45.76
9.58
30.68
45.18
9.8
CS (residue)
13.8
14.5
[Co(PPS)(H2O)2](H2O)
32–115
AH2O
A2H2O
AC5H4 + N4 + C2 + H
ANH + C6H5
Co + 2S (residue)
4.34
9.35
36.29
22.37
30.2
4.35
8.7
35.02
22.23
29.7
176–300
300–392
392–582
582–800
H2PBO
0–210
210–370
370–700
APy + NH
C3H3S2O
C6H5
28.58
48.16
23.28
28.09
48.64
23.27
[Co(PBO)(H2O)](H2O)2
44–115
A2H2O
8.27
8.14
290–483
483–716
716–800
AH2O + Py + N3H2 + C2 + S
AN + C7H5
CoO + S (residue)
44.15
22.77
24.81
44.36
23.31
24.19
H2APO
126–217
217–344
344–700
AC5H5N2 + N2H2
AC6H5 + CO + NH
CS (residue)
42.15
41.11
16.74
42.85
41.82
15.34
[Co(HAPO)Cl(H2O)2](H2O)
10–78
AH2O
A2H2O + Cl
APy + NH + C + HN2
AC6H5 + NH + C
CoO + S (residue)
4.42
16.9
30.48
23.9
24.3
4.13
78–274
274–387
387–574
574–700
16.37
30.71
23.84
24.5
H2PPY
188–246
246–400
400–700
AN2H2
A2Py + N2H2
2CS (residue)
10.57
60.96
28.78
9.86
61.18
28.96
[Co(HPPY)Cl(H2O)](H2O)
30–157
AH2O
AH2O
ACl + NH + N
APy
ANH + 2C + NH + Py
Co + 2S (residue)
4.28
5.35
15.5
17.64
30.1
4.15
4.15
14.86
17.99
30.46
28.28
157–296
296–376
376–471
471–595
595–700
27.13
the straight line equal (Ea/R) and the intercept the pre-exponential
factor, A can be determined. The data obtained are represented in
Table 5.
starting temperature). A large number of decomposition processes
can be represented as first order reaction [27]. Particularly, the
degradation of the investigated series of metal complexes was sug-
gested to be first order in sample weight reaction. Therefore we
will assume n = 1 for the remainder of the present text. Under this
assumption the integration of Eq. (3) leads to:
3.6.2. Horowitz–Metzger method
The Horowitz–Metzger [24] relation was used to evaluate the
degradation kinetics is
ꢀ
ꢁ
Z
T
A
b
E
RT
lnð1 ꢀ
aÞ ¼ ꢀ
Exp
dT
ð4Þ
Eah
RT2S
ln½ꢀlnð1 ꢀ
aÞꢃ ¼
for n ¼ 1
ð6Þ
To
On the basis of Eq. (4), it is possible to analyze experimental
data by the integral method, in order to determine the degradation
kinetic parameters A, Ea. The temperature integral in the right-
hand side of Eq. (4) has no exact analytical solutions and several
kinds of approximations are generally used. Two methods that dif-
fer on the way of resolving Eq. (4) are compared using the TGA data
of the studied complexes. These methods are:
"
#
!
1ꢀn
1 ꢀ ð1 ꢀ
a
1 ꢀ n
Þ
A RT2S
Ea
RTS
Eah
ln
¼ ln
ꢀ
þ
for n–1
ð7Þ
RT2S
b
E
where h = T ꢀ Ts, Ts is the DTG peak temperature, T the temperature
corresponding to weight loss Wt. In this method a straight line
Ea
should be observed between ln[ln(1 ꢀ
a
)] and h with a slope of
.
RT2S
Fig. 11 show the Horowitz–Metzger plots for the metal com-
3.6.1. Coats–Redfern method
plexes under study. The obtained data are recorded in Table 6.
The Coats–Redfern [23] method is as follows:
ꢂ
ꢃ
ꢀ
ꢁ
3.6.3. Thermodynamic parameters
The other thermodynamic parameters of activation can be cal-
culated by Eyring equation [28,29]:
gð
a
Þ
AR
bE
Ea
RT
ln
¼ ln
ꢀ
ð5Þ
) = ꢀln(1 ꢀ
T2
where g(
a
) = 1 ꢀ (1 ꢀ
a
)1 ꢀ n/1 ꢀ n for n – 1 and g(
a
a)
D
D
D
Hꢅ ¼ Ea ꢀ RT
ð8Þ
for n = 1, R is the universal gas constant. The correlation coefficient,
ꢀ
ꢁ
r, was computed using the least square’s method for different val-
hA
KBT
ues of n (n = 0.33, 0.5, 0.66 and n = 1) by plotting ln½ Þꢃ versus 1/
gð
a
Sꢅ ¼ R ln
ꢀ 1
ð9Þ
2
T for the investigated metal complexes are shown inT Fig. 10. The
n-value which gave the best fit (r ꢄ 1) was chosen as the order
parameter for the decomposition stage of interest. The slope of
Gꢅ ¼
D
H ꢀ T
D
S
ð10Þ

T.A. Yousef et al. / Journal of Molecular Structure 1004 (2011) 271–283
279
(A)-13.0
-13.5
(C)
-10
lnX n=1
lnX n=1
lnX n=0.66
lnX n=0.33
lnX n=0
lnX n=0.66
lnX n=0.33
lnX n=0
lnX n = 0.5
lnX n = 0.5
-14.0
-12
-14.5
-15.0
0.0030
0.0032
1000/T
0.0034
0.00125 0.00130 0.00135 0.00140
1000/T
lnX n=1
(B)
(D)
lnX n=1
lnX n=0.66
lnX n=0.33
lnX n=0
-11.0
-11.2
-11.4
-11.6
-11.8
-12.0
-12.2
lnX n=0.66
lnX n=0.33
-11
lnX n=0
lnX n = 0.5
lnX n = 0.5
-12
-13
0.0024 0.0025 0.0026 0.0027 0.0028
0.0026
0.0028
0.0030
1000/T
1000/T
Fig. 10. Coats–Redfern plots of (A) [Co(PPS)(H2O)2](H2O), (B) [Co(PBO)(H2O)](H2O)2, (C) [Co(HAPO)Cl(H2O)2](H2O) and (D) [Co(HPPY)Cl(H2O)](H2O).
where
activation (kJ/mol K) and
(kJ/mol), h the Planck constant and KB the Boltzmann constant. The
kinetic parameters evaluated by Coats–Redfern and Horowitz–
Metzger methods are listed in Tables 5 and 6, respectively. From
the results obtained, the following remarks can be pointed out:
D
H⁄ is the enthalpy of activation (kJ/mol),
G⁄ the Gibbs free enthalpy of activation
DS the entropy of
quent species will be lower than that of the precedent one
[33–35]. This may be attributed to the structural rigidity of
the remaining complex after the expulsion of one or more
ligands, as compared with the precedent complex, which
D
requires more energy, TD
S⁄, for its rearrangement before
undergoing any decomposition change.
(i) All decomposition steps show best fit for n = 1.
(ii) The negative value of the entropy of activation,
3.7. Potentiometric metric studies
D
S⁄ of some
decomposition steps in case of the ion exchanger and its
metal complexes with all investigated metal ions indicates
that the activated fragments have more ordered structure
than the undecomposed ones and the later are slower than
the normal [30,31].
The potentiometric measurements were carried out according
the procedure developed by Clavin and Bjerrum [36,37].
3.7.1. Determination of the proton–ligand ionization constants
The ionization constants of H2PPS, H2PBO, H2APO and H2PPY
were determined pH-metrically using Irving–Rossotti technique
(iii) The positive sign of activation enthalpy change,
D
H⁄ indi-
cates that the decomposition stages are endothermic
processes.
at (298, 303 and 308 K) and at constant ionic strength of (l = 0.1,
0.15 and 0.2 M KCl). The free acid (0.011 M) and its mixture with
the ligand (0.0005 M) were titrated against carbonate-free sodium
hydroxide (0.0092 M) solution. The titration curves for ligands are
shown in Figs. 1S–4S respectively.
(iv) The high values of the energy of activation, Ea of the com-
plexes reveals the high stability of such chelates due to their
covalent bond character [32].
D
G⁄ for the investigated complexes
ꢀ
(v) The positive sign of
The average number of protons associated with the ligand, nA
reveals that the free energy of the final residue is higher than
that of the initial compound, and hence all the decomposition
steps are nonspontaneous processes. Moreover, the values of
was calculated at different pH-values using Irving–Rossotti equa-
tion [38].
ꢀ
The proton–ligand ionization curves, obtained by plotting nA
D
G⁄ increase significantly for the subsequent decomposition
versus pH at different temperature are shown in the Figs. 5S–8S.
ꢀ
steps of a given compound. This results from increasing the
S⁄ clearly from one step to another which override the
values of
H⁄ reflecting that the rate of removal of the subse-
From these curves, the maximum nA value is found to be ꢆ2 indi-
TD
cating that the investigated ligands have two dissociable protons of
the enolized hydrogen ion of NHb and NHc respectively.
D

280
T.A. Yousef et al. / Journal of Molecular Structure 1004 (2011) 271–283
Table 5
Kinetic parameters of ligands and their cobalt complexes evaluated by Coats–Redfern equation.
Compound
H2PPS
Peak
Mid temp (K)
Ea (kJ/mol)
A (Sꢀ1
)
D
H⁄ (kJ/mol)
D
S⁄ (kJ/mol K)
D
G⁄ (kJ/mol)
1st
2nd
3rd
447
572
864
94.10
118.72
129.03
1.2406Eꢀ5
6.2067Eꢀ6
7.5270Eꢀ6
90.38
113.96
121.85
ꢀ0.3422
ꢀ0.3500
ꢀ0.3518
243.24
314.20
425.76
[Co(PPS)(H2O)2](H2O)
1st
346
511
620
761
33.65
73.77
279.8
190.0
1.1907Eꢀ5
1.6481Eꢀ6
5.3642Eꢀ6
3.8043Eꢀ6
30.77
69.52
274.73
183.70
ꢀ0.3404
ꢀ0.3601
ꢀ0.3519
ꢀ0.3564
148.61
253.70
492.87
454.83
2nd
3rd
4th
H2PBO
1st
2nd
379
565
42.51
50.04
1.0950Eꢀ6
1.3166Eꢀ6
39.36
45.34
ꢀ0.3610
ꢀ0.3628
176.17
250.39
[Co(PBO)(H2O)](H2O)2
1st
2nd
3rd
351
638
841
30.17
97.38
147.06
3.15944
2.30197E6
1.87037E6
27.24
92.07
140.06
ꢀ0.2367
ꢀ0.1485
ꢀ0.1334
110.56
186.96
252.40
H2APO
1st
2nd
447
550
193.17
71.05
3.5313Eꢀ5
2.4248Eꢀ5
189.45
66.48
ꢀ0.3335
ꢀ0.3383
338.43
252.63
[Co(HAPO)Cl(H2O)2](H2O)
1st
2nd
3rd
318
452
604
25.16
48.19
198.17
1.5483Eꢀ5
2.2100Eꢀ6
4.6809Eꢀ6
22.52
44.43
193.14
ꢀ0.3375
ꢀ0.3566
ꢀ0.3528
129.74
205.54
406.41
H2PPY
1st
2nd
490
598
224.02
122.65
6.1209Eꢀ6
5.5964Eꢀ7
219.94
117.68
ꢀ0.3488
ꢀ0.3703
390.95
339.22
[Co(HPPY)Cl(H2O)](H2O)
1st
366
503
609
698
807
16.83
62.73
216.26
104.33
267.26
1.5898Eꢀ6
1.7970Eꢀ6
3.1089Eꢀ6
1.6301Eꢀ6
9.6117Eꢀ7
13.79
58.55
211.19
98.53
ꢀ0.3576
ꢀ0.3592
ꢀ0.3562
ꢀ0.3627
ꢀ0.3683
144.86
239.08
428.28
351.64
557.86
2nd
3rd
4rd
5rd
260.55
1
0
1
(A)
(C)
lnXn=1
lnXn=1
lnXn=0.66
lnXn=0.33
lnXn=0
lnXn=0.66
lnXn=0.33
lnXn=0
lnXn = 0.5
lnXn = 0.5
0
-1
-1
-20
0
20
-20
0
T- Ts
T- Ts
1
lnXn=1
(B)
(D)
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
-0.1
-0.2
-0.3
lnXn=1
lnXn=0.66
lnXn=0.33
lnXn=0
lnXn=0.66
lnXn=0.33
lnXn=0
lnXn = 0.5
lnXn = 0.5
0
-1
0
20
-20
0
T- Ts
T- Ts
Fig. 11. Horowitz–Metzger plots of (A) [Co(PPS)(H2O)2](H2O), (B) [Co(PBO)(H2O)](H2O)2, (C) [Co(HAPO)Cl(H2O)2](H2O) and (D) [Co(HPPY)Cl(H2O)](H2O).

T.A. Yousef et al. / Journal of Molecular Structure 1004 (2011) 271–283
281
Table 6
Kinetic parameters of ligands and their cobalt complexes evaluated by Horowitz–Metzger equation.
Compound
H2PPS
Peak
Mid temp (K)
Ea (kJ/mol)
A (Sꢀ1
)
D
H⁄ (kJ/mol)
D
S⁄ (kJ/mol K)
D
G⁄ (kJ/mol)
1st
2nd
3rd
447
572
864
94.10
118.72
129.03
1.2406Eꢀ5
6.2067Eꢀ6
7.5270Eꢀ6
90.38
113.96
121.85
ꢀ0.3422
ꢀ0.3500
ꢀ0.3518
243.24
314.20
425.76
[Co(PPS)(H2O)2](H2O)
1st
346
511
620
761
33.52
73.51
279.62
190.47
1.124Eꢀ5
1.528Eꢀ6
5.330Eꢀ6
3.785Eꢀ6
30.64
69.26
274.47
184.15
ꢀ0.3409
ꢀ0.3607
ꢀ0.3519
ꢀ0.3565
148.64
253.77
492.64
455.30
2nd
3rd
4th
H2PBO
1st
2nd
379
565
42.51
50.04
1.0950Eꢀ6
1.3166Eꢀ6
39.36
45.34
ꢀ0.3610
ꢀ0.3628
176.17
250.39
[Co(PBO)(H2O)](H2O)2
1st
2nd
3rd
351
638
841
31.10
97.71
143.00
5.977
1.5811E5
4.2788E5
28.17
92.40
136.00
ꢀ0.2314
ꢀ0.1517
ꢀ0.1457
109.63
189.28
258.66
H2APO
1st
2nd
447
550
193.17
71.05
3.5313Eꢀ5
2.4248Eꢀ5
189.45
66.48
ꢀ0.3335
ꢀ0.3383
338.43
252.63
[Co(HAPO)Cl(H2O)2](H2O)
1st
2nd
3rd
318
452
604
25.48
48.64
198.27
1.4979Eꢀ5
2.2297Eꢀ6
4.5854Eꢀ6
22.84
44.88
193.25
ꢀ0.3377
ꢀ0.3565
ꢀ0.3529
130.15
205.95
406.62
H2PPY
1st
2nd
490
598
224.02
122.65
6.1209Eꢀ6
5.5964Eꢀ7
219.94
117.68
ꢀ0.3488
ꢀ0.3703
390.95
339.22
[Co(HPPY)Cl(H2O)](H2O)
1st
366
503
609
698
807
16.62
62.58
216.37
104.69
267.41
1.5925Eꢀ6
1.6449Eꢀ6
2.8629Eꢀ6
1.4482Eꢀ6
9.6035Eꢀ7
13.58
58.41
211.30
98.89
ꢀ0.3576
ꢀ0.3599
ꢀ0.3569
ꢀ0.3637
ꢀ0.3683
144.64
239.31
428.81
352.69
558.02
2nd
3rd
4rd
5rd
260.70
Table 7
The dissociation constants of H2PPS in 50% (V/V) dioxane–water and KCl at different temperatures and it’s thermodynamic parameters.
l
mol/L Dissociation constant
Free energy change (
298 K 303 K
D
G) kJ molꢀ1
Enthalpy change (D D
H) kJ molꢀ1 Entropy change ( S) J molꢀ1 Kꢀ1
298 K
pK1
303 K
308 K
308 K
pK2
pK1
pK2
pK1
pK2
0.1
0.15
0.2
13.45
13.44 10.2
12.90
8.25 12.89 8.15
11.98 8.56 11.8
7.80 10.88 7.26
8.72 6.76 82.7 24.8 83.3 25.0 83.9 25.2 43.37
7.88 26.1 21.7 26.3 39.9 27.5 40.2 15.06
9.25 4.35 63.2 60.9 63.7 61.4 64.2 61.9 33.50
13.66
21.29
31.63
ꢀ131.9
ꢀ37.12
ꢀ99.57
ꢀ37.55
ꢀ61.28
ꢀ98.30
Table 8
The dissociation constants of H2PBO in 50% (V/V) dioxane–water and KCl at different temperatures and it’s thermodynamic parameters.
lmol/L Dissociation constant
Free energy change (
D
G) kJ molꢀ1
Enthalpy change (
D
H) kJ molꢀ1 Entropy change (
D
S) J molꢀ1 Kꢀ1
298 K
303 K
308 K
298 K 303 K
308 K
pK1
pK2
pK1
pK2
pK1
pK2
0.1
0.15
0.2
12.89 8.99 10.51 8.15 10.02 7.60 48.9 22.8 49.3 23.0 51.9 23.2 26.35
12.98 9.02 11.6 8.55 11.57 7.36 22.0 27.7 22.2 27.9 22.3 28.2 12.94
12.49 7.52 10.46 7.41 9.80 6.48 45.7 16.8 46.0 16.9 46.4 17.1 24.69
12.77
15.22
9.53
ꢀ75.58
ꢀ30.46
ꢀ70.41
ꢀ33.89
ꢀ42.00
ꢀ24.44
Table 9
The dissociation constants of H2APO in 50% (V/V) dioxane–water and KCl at different temperatures and it’s thermodynamic parameters.
l
mol/L Dissociation constant
Free energy change (
298 K 303 K
D
G) kJ molꢀ1
Enthalpy change (
D
H) kJ molꢀ1 Entropy change (
D
S) J molꢀ1 Kꢀ1
298 K
303 K
308 K
308 K
pK1
pK2
pK1
pK2
pK1
pK2
0.1
0.15
0.2
14.00 8.48 13.54 8.04 10.98 6.03 51.2 42.4 27.7 42.7 51.9 43.0 27.67
22.46
34.73
18.36
ꢀ78.75
ꢀ100.9
ꢀ108.3
ꢀ66.85
ꢀ106.2
ꢀ53.04
13.65 9.05 10.81 8.33
13.36 8.64 9.60 7.41
9.93 5.30 64.2 66.4 64.7 66.9 65.2 67.4 34.14
9.41 6.64 68.5 34.2 69.1 34.4 69.6 34.7 36.25
The first proton–ligand stability constant (pK1) for all ligands
3.7.1.1. Effect of substituent. A glance at Tables 7–10 reveals the or-
der of the ionization constants of the thiosemicarbazides as,
H2APO > H2PPS > H2PPY > H2PBO. This can be explained on the ba-
sis of the formation of an intramolecular hydrogen bond [39]. In all
cases such thiosemicarbazides the geometry of their molecules is
ꢀ
ꢀ
was determined by interpolation at half-nA values, i.e. at nA ¼ 0:5
ꢀ
from the nA—pH curves. The second proton-ligand stability con-
stant (pK2) for all ligands was determined using mid point method,
ꢀ
where 2pH (nA ¼ 1) = pK1 + pK2.

282
T.A. Yousef et al. / Journal of Molecular Structure 1004 (2011) 271–283
such to allow the C@S, C@O, (C@N) pyridine and NH groups to be
oriented in close proximity to each other. Consequently, it is diffi-
cult to lose H+ so pK is high. ON substituting carbonyl group by thi-
one group, pK value decreased as oxygen is more electronegative
than sulfur atom. Also, the presence of pyridyl group in H2PPY
(i.e. electron withdrawing group) will decrease the electron density
by its high negative inductive effect, withdrawing electrons more
effectively than phenyl group so the stronger is the hydrogen bond
and consequently the smaller the pK value.
13S–16S. The corresponding standard free energy change,
the standard entropy change, S°, for such dissociations are calcu-
lated using the following well-known relationship:
DG° and
D
D
Gꢇ ¼ ꢀ2:303RT log KH ¼ 2:303RTpKH
D
Gꢇ ¼
D
Hꢇ ꢀ T
D
Sꢇ
The calculated thermodynamic functions are recorded in Tables 7–
10. Inspection of these values reveals that:
3.7.1.2. Effect of ionic strength. By the addition of the inert back-
ground salt (to maintain the ionic strength constant), the dissocia-
tion constants are modified. That is because the activity coefficient
values vary with the change of the nature and the concentration of
such salt Figs. 9S–12S depicts the dissociation constant of ligands
as a function of the square root of the ionic strength, which was ad-
justed with KCl. An inspection of Tables 7–10 and Figs. 9S–12S re-
veals that there is a regular increase in the value of pK with
increasing ionic strength of the medium, in agreement with that
reported [40].
(1) pKa values decrease with increasing temperature from 298
to 308 K, i.e., the acidity of ligands increase with increasing
temperature. On the other hand, the stepwise stability con-
stants decrease with increasing temperature indication that
the complexation processes are unfavorable with increasing
temperature.
(2) The positive values of
DG° indicate that the dissociation pro-
cesses are non-spontaneous process while the chelation has a
spontaneous character as revealed by the negative
values.
DG°
(3) The negative values of
DS° for dissociation and complex for-
mation due to the increased order as a result of salvation
process, i.e. the sum of total of the bound solvent molecules
with the dissociated ligand is more than the originally
accompanying the undissociated form [41].
3.7.1.3. Effect of temperature on the ionization constants, the stepwise
formation constant and the thermodynamic parameters. The stan-
dard enthalpy changes,
DH°, for the dissociation of studied ligands
were deduced from the slope of the plot of the pKH versus 1/T Figs.
Table 10
The dissociation constants of H2PPY in 50% (V/V) dioxane–water and KCl at different temperatures and it’s thermodynamic parameters.
l
mol/L Dissociation constant
Free energy change (
298 K 303 K
D
G) kJ molꢀ1
Enthalpy change (D D
H) kJ molꢀ1 Entropy change ( S) J molꢀ1 Kꢀ1
298 K
303 K
308 K
308 K
pK1
pK2
pK1
pK2
pK1
pK2
0.1
0.15
0.2
13.04 9.00 12.33 8.47 9.93 6.65 53.1 40.4 53.5 40.7 53.9 41.0 28.51
14.25 8.65 11.47 7.77 8.85 4.22 94.8 78.6 95.6 79.3 96.3 79.9 49.54
21.54
40.62
23.48
ꢀ82.54
ꢀ152.0
ꢀ80.94
ꢀ63.23
ꢀ127.6
ꢀ69.99
11.85 8.75
9.92 7.55 8.84 6.19 51.8 44.3 52.2 44.7 52.6 45.0 27.63
Table 11
The stability constants of Co(II) complexes in 50% (V/V) dioxane–water and KCl at different temperatures.
Compound
lmol/L
Dissociation constant
298 K
303 K
308 K
LogK1
LogK2
LogK1
LogK2
LogK1
LogK2
[Co(PPS)(H2O)2](H2O)
[Co(PBO)(H2O)](H2O)2
[Co(HAPO)Cl(H2O)2](H2O)
[Co(HPPY)Cl(H2O)](H2O)
0.1
0.15
0.2
13.13
19.14
11.01
9.80
10.02
8.14
12.33
–
12.20
9.35
6.20
7.45
10.5
–
10.12
3.25
7.61
5.93
0.1
0.15
0.2
17.08
19.08
–
6.58
9.36
13.37
15.58
–
–
5.31
9.07
9.02
11.11
15.95
–
3.81
11.00
12.48
0.1
0.15
0.2
18.87
11.58
–
8.38
6.94
10.19
14.51
–
–
7.88
12.26
7.86
13.67
–
–
5.67
7.29
7.89
0.1
0.15
0.2
19.52
7.13
12.23
10.19
–
9.19
15.78
10.29
13.75
5.51
–
7.88
10.24
10.24
–
4.46
6.34
6.95
Table 12
Thermodynamic parameters of Co(II) complexes in 50%(V/V) dioxanel–water and 0.1 M KCl at different temperature.
Compound
Free energy change (
298 K
D
G) kJ molꢀ1
Enthalpy change (
D
H) kJ molꢀ1
Entropy change (
D
S) kJ molꢀ1 Kꢀ1
303 K
308 K
Co(II) + H2PPS
Co(II) + H2PBO
Co(II) + H2APO
Co(II) + H2PPY
ꢀ75.4
ꢀ56.7
ꢀ35.9
ꢀ48.1
ꢀ58.3
ꢀ68.9
ꢀ81.8
ꢀ93.9
ꢀ86.8
ꢀ38.4
ꢀ38.4
ꢀ40.9
ꢀ42.3
ꢀ62.4
ꢀ62.4
ꢀ80.4
ꢀ61.3
ꢀ20.1
ꢀ20.1
ꢀ33.7
ꢀ26.4
ꢀ461.4
ꢀ1048.3
ꢀ913.7
ꢀ1629.4
ꢀ1149.4
ꢀ484.42
ꢀ476.0
ꢀ1.3
ꢀ3.2
ꢀ2.7
ꢀ5.1
ꢀ3.7
ꢀ1.5
ꢀ1.4
ꢀ3.2
ꢀ97.7
ꢀ107.5
ꢀ112.2
ꢀ1006.6

T.A. Yousef et al. / Journal of Molecular Structure 1004 (2011) 271–283
283
(4) The positive values of
DH° indicate that the ionization of
Appendix A. Supplementary data
ligands in aqueous solution are endothermic indicating
that the ionization processes are favorable at higher tem-
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.molstruc.2011.08.020.
peratures. The negative values of
DH° of the chelation
processes indicating that the processes are exothermic
revealing that the complexation reactions are favorable at
lower temperature.
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ꢀ
Figs. 17S–20S represent the formation curves (nversus pL) of the
studied metal ion–ligand mixtures. The metal–ligand stoichiome-
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ꢀ
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D
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D
Sꢇ
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4. Conclusion
A new series of thiosemicarbazides ligands and their Co(II) com-
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structure of all studies complexes. The stability of complexes was
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