Acid-catalyzed aquation of ni(II)-hydrazone complexes: Kinetics and solvent effect

Article abstract of DOI:10.1002/kin.20861

Complexes of the type [Ni(L)(H₂O)]Cl₂·nH ₂O, where L = 2-pyridyl-3-isatinbishydrazone ligands, have been synthesized and characterized on the bases of elemental analysis, molar conductance, IR, electronic spectra, and ther

Full text of DOI:10.1002/kin.20861

Acid-Catalyzed Aquation of
Ni(II)-Hydrazone
Complexes: Kinetics and
Solvent Effect
AHMAD DESOKY M. MOHAMAD, MOSTAFA K. RABIA, NABAWIA M. ISMAIL,
ALI ABDO MAHMOUD
Chemistry Department, Faculty of Science, Sohag University, 82534, Sohag, Egypt
Received 3 February 2014; revised 26 April 2014; accepted 1 May 2014
DOI 10.1002/kin.20861
Published online 27 June 2014 in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: Complexes of the type [Ni(L)(H2O)]Cl2·nH2O, where
L
=
2-pyridyl-3-
isatinbishydrazone ligands, have been synthesized and characterized on the bases of elemental
analysis, molar conductance, IR, electronic spectra, and thermal analysis (TGA and DTA). Acid-
catalyzed aquation of the Ni(II) isatin-bishydrazone complexes was followed spectrophotomet-
rically in various water–methanol and water–acetone mixtures at temperature 298 K. Kinetic
behavior of the acid aquation is a linear rate law, indicating that the acid-catalyzed aquation of
these complexes in water–methanol and water–acetone mixtures follows a rate law with kobs
=
k2[H+]. The effect of the mole fraction of the ganic solvent, i.e., methanol and acetone, on the
acid aquation has been analyzed; the decrease in the rate constant values with increasing of the
methanol or acetone ratios is attributable to the effect of the co-ꢀCorganic solvent on the initial
states of the acid aquation by the destabilization of the H+ ion. 2014 Wiley Periodicals, Inc.
Int J Chem Kinet 46: 451–461, 2014
ing used as biological models in understanding of the
structure of biomolecules and biological process [2].
Schiff base ligands containing various donor atoms
(such as N, O, S) show broad biological activity and
are of special interest because of the variety of ways
in which they are bonded to metal ions. It is known
that the existence of metal ions bonded to biologically
active compounds may enhance their activities [3]. In
the past few years, a wide range of metal compounds,
which contain the isatin moiety, have received a lot of
attention in the fields of inorganic chemistry, biochem-
istry, and environmental chemistry. Isatin, an endoge-
nous indole, and its derivatives have exhibited a wide
range of biological activities [4–8]. A significant in-
creasing interest in the designing of metal compounds
INTRODUCTION
The preparation of a new ligand was perhaps the most
important step in the development of metal complexes,
which exhibit unique properties and novel reactivity.
Schiff bases are an important class of ligands with wide
applications in biology, clinics, analysis, and industry
in addition to their important roles in catalysis and or-
ganic synthesis [1]. A considerable number of Schiff
base complexes have potential biological interest, be-
Correspondence to: Ali Abdo Mahmoud; e-mail: alyabdo8081@
yahoo.com; aliabdo@science.sohag.edu.eg.
Supporting Information is available in the online issue at
www.wileyonlinelibrary.com.
C
ꢀ
2014 Wiley Periodicals, Inc.

452
MOHAMAD ET AL.
R
H2N
NH2
O
O
N
NH
NH2
+
NH
N
O
O
Isatin
2-Pyridyl
Isatin monohydrazone
Ligand
Abbrev.
cpish
Metal
Complex
Empirical formla
Nicpish [Ni(cpish)(H2O)]Cl2 nH2O
Niapish [Ni(apish)(H2O)]Cl2 nH2O
Reflux 4–5 h HCl
R
H
Abbrev.
.
.
.
NiII
R
CH3 apish
ph
bpish
Nibpish [Ni(bpish)(H2O)]Cl2 nH2O
N
N
N
N
H
O
R
N
.
N
NiCl2 6H2O
NH
.
Cl 2H O
2
2
(1:1)
Ni
O
N
H O
2
Scheme 1 Schematic diagram of the isatin-bishydrazone ligands and their Ni(II) complexes.
as drugs and diagnostic agents is directly observed
in the area of scientific inquiry appropriately termed
medicinal inorganic chemistry [9]. Metal complexes
with Schiff bases of isatin derivatives exhibited remark-
able biological activity [10–12]. Although much atten-
tion has been directed to study the metal complexes
of the Schiff base ligands derived from isatin [13,14],
no investigations have appeared in the literature to de-
scribe the metal complexes of the Schiff base derived
from isatin monohydrazone and 2-pyridyl. Thus, our
interest is in the synthesis and structural characteri-
zation of new Schiff bases [15] We synthesized and
characterized new isatin-bishydrazone compounds de-
rived from isatin monohydrazone and 2-pyridyl, which
show notable biological activity, also reported in other
work [16,17], and new Ni(II) complexes with the pre-
viously synthesized bioactive isatin-bishydrazone lig-
ands, with studies on their biological activity.
aqueous solutions is sensitive to the nature and molar
fraction of organic cosolvent. The rate constants in bi-
nary aqueous mixtures depend on the composition of
the solvent [18,19]. Less information is available in the
literature on the acid-catalyzed aquation of Ni(II) com-
plexes in various reaction media and their mechanistic
pathways.
Thus, in view of the significant importance of these
complexes, we are interested in the investigation and
report the acid hydrolysis of these complexes, in addi-
tion investigate the effects of methanol and acetone as
organic cosolvents on the kinetics of the acid hydroly-
sis of these complexes to gain more information about
the acid hydrolysis of these new bioactive complexes.
Methanol and acetone have been chosen as a cosolvent
due to the availability of previously measured chemical
potentials, which aided in the interpretation of the rate
constant data.
The rate coefficient for the reaction between or-
ganic and inorganic compounds with hydrogen ions in
The structures of the prepared Ni(II) complexes in
this investigation are shown in Scheme 1.
International Journal of Chemical Kinetics DOI 10.1002/kin.20861

ACID-CATALYZED AQUATION OF NI (II)-HYDRAZONE COMPLEXES
453
EXPERIMENTAL
Chemicals and Apparatus
Chemicals. All chemicals were used as received with-
out further purification. Isatin, 2-acetyl pyridine, 2-
benzoyl pyridine, and pyridine-2-carboxaldhyde were
obtained from Sigma-Aldrich. Hydrazine hydrate and
hydrated nickel chloride (NiCl2·6H2O) were obtained
from BDH. All other reagents and solvents (methanol,
ethanol, and DMF) were purchased from commercial
sources and were of analytical grade. Hydrochloric
acid was used as received from Merck (37 wt% in
water (99.99%)), and water used was doubly distilled.
The stock solutions of HCl and NaCl were freshly pre-
pared by dissolving the calculated amounts of AnalaR
samples in redistilled water.
Apparatus and Methods. IR spectra of the current
Isatin-bishydrazone ligands and their corresponding
complexes were monitored using a Shimadzu FTIR
model 8101 (within the region of 4000–400 cm−1 us-
ing dry KBr disks). Conductivity measurements were
carried out using a Jenway conductivity meter model
4320 at 298 K, which was controlled by an ultrathermo-
stat (Crioterm model 190) to control the temperature
at 25 0.1°C). The elemental analyzer Perkin–Elmer
model 240c was used for microanalysis. The thermal
gravimetric analysis (TGA) of the recent Ni(II) com-
plexes was performed by a Shimadzu thermal analyzer
model 50H using air as the carrier gas. The complexes
were heated at the rate of 10°C per min from 25 to
600°C. The UV–vis spectra and the kinetic measure-
ments of the current complexes were performed using
10-mm matched silica cells in the thermostated cell
holder of a Jasco UV–vis model V-530 spectropho-
tometer. The thermostated cell holder was supplied
with an ultrathermostate water circulator (Crioterm
model 190) to control the temperature at 25 0.1°C
and other temperatures.
Synthesis of the Isatin-Bishydrazone
Ligands and Their Corresponding Ni(II)
Complexes
Synthesis of the Isatin-Bishydrazone Ligands.
Isatin-bishydrazone ligands, namely [(pyridine-2-
carboxaldhyde)-3-isatin]-bishydrazone (cpish), [(2-
acetyl pyridine)-3-isatin]-bishydrazone (apish), and
[(2-benzoyl pyridine)-3-isatin]-bishydrazone (bpish),
were prepared in two steps: The first step was
the synthesis of isatin-monohydrazone, followed by
condensation with the 2-pyridyl, giving the isatin-
bishydrazone ligands.
International Journal of Chemical Kinetics DOI 10.1002/kin.20861

454
MOHAMAD ET AL.
Table II Infrared Absorption Frequencies (cm−1) and Electronic Spectra of the Investigated Isatin-Hydrazone Ligands
and Their Metal Complexes
Electronic Spectra
ν
ν
ν
ν
ν
ν
ν
λmax
εmax
No. Compound (OH)1
(N–H)2 (C=O)3 (HC=N)4 (C=N)5 (M–O) (M–N) (nm) (dm3 mol−1 cm−1
)
Assignment
*
1
Cpish
3421.2 3276.5
1721.7
1613.7
1460.3
. . .
. . .
237
324
284
399
627
676
254
274
325
244
320
625
680
252
271
327
244
352
399
636
682
1744.05
865.49
696.97
636.84
320.12
293.02
538.30
580.97
339.05
1578.13
886.64
723.22
256.57
1096.47
882.56
653.87
1828.62
817.06
722.84
268.80
295.95
π–π
n–π
*
2
Ni-cpish
3430.8 3287.1
1685.0
1611.7
1409.0
656.8
510.2
Intraligand band
LMCT band
d–d band
d–d band
*
3
4
Apish
3393.2 3180.0
3357.5 3159.8
1721.7
1672.5
1618.5
1592.4
1458.4
1463.2
. . .
. . .
π–π
π–π
n–π
*
*
Ni-apish
637.6
504.4
Intraligand band
LMCT band
d–d band
d–d band
*
5
6
Bpish
3424.0 3181.0
3359.4 3160.7
1721.7
1671.5
1607.9
1594.4
1452.6
1463.2
. . .
. . .
π–π
π–π
n–π
*
*
Ni-bpish
636.6
503.5
Intraligand band
LMCT band
LMCT band
d–d band
d–d band
1Vibrations of the water molecules, 2Vibrations of the Indol ring (N–H), 3Vibrations of the lactonyl group (C=O), 4Vibrations of the
azomethine group (C=N), 5Vibrations of group from the β-hydrazone of isatin (C=N), LMCT = ligand metal charge transfer.
Synthesis of Isatin-Monohydrazone. Isatin (1.47 g,
10 mmol) was dissolved in methanol (40 mL) and
was added to a solution of hydrazine hydrate (0.05 g,
10 mmol) dissolved in hot methanol (5 mL). The re-
sulting mixture was refluxed for 3 h in a water bath. On
cooling, the yellow compound that formed was filtered,
washed with cold methanol, dried, and recrystallized
from methanol [20].
sulting solid product was filtered, washed with a water–
methanol solvent, dried, and finally recrystallized from
the water–methanol mixed solvent. The yield and melt-
ing point of each product were determined [16].
Acid-Catalyzed Aquation of Ni(II)
Complexes
Kinetics of the acid-catalyzed aquation of the Ni(II)–
isatin-bishydrazone complexes was measured by fol-
lowing the time dependence of absorbance using 10-
mm silica cells in a thermostated cell compartment of
a Jasco’s model V-530 spectrophotometer. The tem-
perature (25 0.1°C) was adjusted by a Crioterm 190
ultrathermostate. The required volume of the isother-
mal reacting stock solutions was taken by a syringe
and mixed in 10-mm cells in the thermostated cell
jacket compartment. Chemical reactions were moni-
tored in solutions held at constant ionic strength us-
ing appropriate amounts of sodium chloride over at
least three half-live times. The reaction was carried out
under pseudo–first-order conditions with respect to a
substrate by mixing multifold greater concentrations of
HCl than that of the compound. The substances were
added to the cell in the following order: (1) substrate,
(2) methanol fraction, (3) salt, (4) water, and finally
HCl; the total volume of the cell was 3 mL. Before
Synthesis of Isatin-Bishydrazone Ligands. 1.0 mmol
of 2-pyridyl (2-acetyl pyridine or 2-benzoyl pyridine
or pyridine-2-carboxaldhyde) was added dropwise to
a hot methanolic solution of isatin-monohydrazone
(1.0 mmol), the resulting mixture was refluxed for
1 h with constant stirring, and then two to three drops
of glacial acetic acid was added with continued re-
fluxing for 4 h under constant magnetic stirring. On
cooling, the formed ligand was filtered, washed with
cold methanol, dried, and finally recrystallized from
methanol [21].
Synthesis of Ni(II) Complexes. A solution of the metal
salt in a minimum amount of water (NiCl2·6H2O;
1.0 mmol) was added dropwise to a hot methanolic so-
lution of the ligand (cpish or apish or bpish; 1.0 mmol).
The resulting mixture was refluxed at 70°C for 10 h un-
der constant stirring. By evaporation overnight, the re-
International Journal of Chemical Kinetics DOI 10.1002/kin.20861

ACID-CATALYZED AQUATION OF NI (II)-HYDRAZONE COMPLEXES
455
adding HCl into the cell, the cell was allowed to take
the required temperature of the jacket compartment
of the spectrophotometer. The run started simultane-
ously after the addition of HCl. It was confirmed that
there was no interference from other reagents at the
selected wavelength absorption maxima for the inves-
tigated compounds.
istic bands due to (–NH) and lactonyl carbon ν(C=O)
at ࣈ3180–3200 and ࣈ1722 cm−1 [24], respectively.
In addition, the strong band at ࣈ1460 cm−1 and a
characteristic high intensity band at ࣈ1621 cm−1 in
the IR spectra of the isatin-bishydrazone ligands are
assigned to ν(C=N) and ν(HC=N), respectively. In
comparison with the spectra of the isatin-bishydrazone
ligands, all the Ni(II) complexes exhibited the band
of ν(HC=N) in the region of ࣈ1590 cm−1, showing
the shift of the band to lower wave numbers, indicating
that the azomethine nitrogen is coordinated to the metal
ion [25,26]. The band of ν(C=O) in the region of 1670–
1680 cm−1 in the metal complexes showing the shift to
lower wave numbers confirms that the carbonyl oxy-
gen is coordinated to the metal ion [27]. The unaltered
position of a band due to ν(NH) and ν(C=N) in all the
metal complexes indicates that these groups are not in-
volved in coordination. The new bands in the region of
500–510 and 630–650 cm−1 in the spectra of the com-
plexes are assigned to stretching frequencies of (M–
N) and (M–O) bonds, respectively [28]. Thus, the IR
spectral results provide strong evidences for the com-
plexation of Isatin-bishydrazone ligands with the metal
ion in the tridentate mode via isatin-carbonyl (C=O),
azomethine-N (C=N), and pyridine-N (C = N).
RESULTS AND DISCUSSION
Identification of the Prepared Compounds
Microanalysis and Molar Conductance Measure-
ments. The results of the microanalysis of the prepared
isatin-bishydrazone ligands and their Ni(II) complexes
in addition to the molar conductance measurements
(cf. Table I) suggested that the subject ligands act as
neutral tridentate and form complexes in the 1:1 molar
ratio (metal to ligand), with 1:2 electrolytic nature of
all the complexes [22,23].
Spectroscopic Studies
IR Spectra and Mode of Bonding. The significant
infrared bands of the subject ligands and their metal
complexes are given in Table II. The observed bands
may be classified into those originating from the lig-
ands and those arising from the bonds formed between
metal ions and the coordinating sites. The IR spectra
of the isatin-bishydrazone ligands exhibited character-
Electronic Spectra. The absorption maxima bands are
listed in Table II and the spectra are given in Fig. 1. The
UV–vis spectra of the isatin-bishydrazone ligand show
[Ni(bpish)(H O)]Cl .2H O Complex
2
2
2
bpish-Ligand
5
4
3
2
1
0
2
1
0
600
700
Wavelength (nm)
800
200
300
400
500
600
700
800
Wavelength (nm)
Figure 1 UV–vis spectra of the Ni(II) isatin-hydrazone complexes.
International Journal of Chemical Kinetics DOI 10.1002/kin.20861

456
MOHAMAD ET AL.
Table III Thermoanalytical Data (TG, DTG): Ni(II) complexes
Mass Loss
Observed
(Calculated) (%)
DTAmax Thermal
Metallic
Residue
Complex
Step
TGrange (°C)
(°C)
Effect
Assignment
Ni-cpish complex
I
II
III
I
II
III
I
53.11–106.55
106.55–426.69
426.69–600.53
54.07–219.16
219.16–389.07
389.07– 497.8
56.07–210.55
210.55–404.91
404.91–580.93
78.38
209.42
506.16
78.71
196.61
460.33
87.68
Endo
Endo
Endo
Endo
Endo
Endo
Endo
Endo
Endo
8.98 (8.30)
5.13 (4.14)
58.10 (57.68)
8.55 (8.03)
5.22 (4.02)
75.11 (72.86)
7.35 (7.06)
4.05 (3.53)
65.78 (63.99)
2H2O (hydrated)
1H2O (coordinated)
Organic moiety
2H2O (hydrated)
1H2O (coordinated)
Organic moiety
2H2O (hydrated)
1H2O (coordinated)
Organic moiety
NiCl2
NiCl2
NiCl2
Ni-apish complex
Ni-bpish complex
II
III
265.57
519.39
Table IV Observed First-Order Rate Constants, kobs (s−1), Derived Second-Order Rate Constants k2 (103 dm3 mol−1
ࣔ
s
−1), and Transfer Free Energies of Activation, δmꢀG (KJ mol−1) for Acid-Catalyzed Aquation of Ni-cpish, Ni-apish,
and Ni-bpish complexes at 4.50 × 10−4 M in Different Water-Methanol Binary Mixtures and INaCl = 0.16 mol dm−3 at
T = 298 K
[H+] (mol dm−3
)
0.14
0.1
104 kobs (s−1
11.4
0.06
0.02
ࣔ
MeOH (vol%)
)
k2 103 (dm3 mol−1 s−1
)
δmꢀG (KJ mol−1
)
Ni-cpish complex
0
20
40
60
0
20
40
60
0
26.6
13.1
3.93
19
3.79
1.88
18.965
9.389
2.81
0.841
5.9
–
9.39
2.81
5.63
1.69
1.742864
4.732828
7.722791
–
2.158564
5.148528
8.138491
–
0.562
0.168
1.18
0.494
0.148
0.0443
0.367
0.13
1.18
8.26
3.46
1.03
0.31
2.57
0.91
0.272
0.0815
0.841
5.9
0.505
3.54
Ni-apish complex
Ni-bpish complex
2.47
1.48
2.47
0.739
0.221
1.84
0.444
0.133
1.1
0.739
0.221
1.835
0.65
0.194
0.0582
20
40
60
0.65
0.39
2.573874
5.563838
8.553801
0.194
0.0582
0.117
0.0349
0.0389
0.0116
important strong bands at ࣈ388 and ࣈ230 nm due to
The second degradation step in the temperature range
ࣈ470–540 K may be attributable to the loss of the coor-
dinated water molecule (one water molecule), and the
third step of decomposition occurs within the temper-
ature range ࣈ730–790 K corresponding to the loss of
the organic moiety leaving NiCl2 as metallic residue.
*
*
π–π and n–π transitions [25] respectively. These
bands are altered to a greater extent on complexation.
The spectrum of Ni(II)-complexes shows characteristic
bands at λmax ࣈ 670–680 nm due to d–d transition.
Thermogravimetric Analysis of the Prepared Isatin-
Bishydrazone Complexes. Thermal data of the com-
plexes are given in Table III. The Ni(II) complexes
of the Isatin-bishydrazone ligands exhibited thermal
stability in the range of ࣈ25–50°C, and then degrade
in three steps (cf. Figs. 1a–1c in the Supporting In-
formation). The first degradation step in the tempera-
ture range ࣈ350–360 K may account for the loss of
the hydrated water molecules (two water molecules).
Acid-Catalyzed Aquation of Ni(II)
Complexes
The observed first-order rate constants (kobs) as a func-
tion on [H+] and in the presence of different water–
methanol and water–acetone ratios were calculated
from the dependence of absorbance on time at the ap-
propriate λmax for the investigated compounds using
Microcal Origin program version 7.5 (cf. Tables IV
International Journal of Chemical Kinetics DOI 10.1002/kin.20861

ACID-CATALYZED AQUATION OF NI (II)-HYDRAZONE COMPLEXES
457
Table V Observed First-Order Rate Constants, kobs (s−1), Derived Second-Order Rate Constants, k2 (103 dm3 mol−1
ࣔ
s
−1), and Transfer Free Energies of Activation, δmꢀG (KJ mol−1) for Acid-Catalyzed Aquation of Ni-cpish, Ni-apish,
and Ni-bpish complexes at 4.50 × 10−4 M in Different Water–Acetone Binary Mixtures and INaCl = 0.16 mol dm−3 at
T = 298 K
[H+] (mol dm−3
)
0.14
0.1
0.06
0.02
ࣔ
Acetone (vol%)
104 kobs (s−1
)
k2 103 (dm3 mol−1 s−1
)
δmꢀG (KJ mol−1
)
Ni-cpish complex
0
20
40
60
0
20
40
60
0
18.2
13
7.78
2.59
6.86
4.9
2.94
0.98
2.57
1.84
1.1
14.6
10.4
6.26
5.4
3.86
2.32
0.772
1.62
1.16
0.696
0.232
0.419
0.299
0.18
2.62
1.87
1.12
13
–
10.4
3.86
1.87
4.9
3.32
1.16
0.531
1.84
0.97
0.299
0.159
0.537016
3.001621
4.795782
–
0.964516
3.571156
5.50426
–
2.09
4.65
3.32
1.99
0.664
1.36
0.97
0.582
0.194
0.374
0.744
0.531
0.319
0.106
0.223
0.159
0.0955
0.0318
Ni-apish complex
Ni-bpish complex
20
40
60
1.580713
4.491745
6.057015
0.367
0.0599
and V), showing that the values of kobs decreased as
the proportion of co-organic solvent increases. This
may be due to the salting out of the hydrophilic H+
and stabilization of the studied compound. The de-
pendence of kobs on the acid concentration is linear
for all the Ni(II)-isatin-bishydrazone complexes in the
co-organic (methanol and acetone) solvent without a
significant intercept (cf. Figs. 2 and 3), hence the hy-
drolysis follows the rate law with
Figs. 2 and 3) and k2 (cf. Fig. 5) values as the methanol
or acetone content increases.
The Mechanism of the Acid-Hydrolysis. The char-
acteristic band for the investigated complexes in the
UV–vis spectra lies at λmax = 625–680 nm. But on
addition of the acid, a redshift occurred at the λmax
of the investigated complex (cf. Fig. 6), then this new
band decays by the first-order kinetics in [complex].
This spectrophotometric behavior illustrates with the
naked eye observation that a rapid change occurs in
the color of the complex, from dark red to violet, and
then fades slowly. These spectrophotometrically and
visually observable acts would suggest the following
mechanism for the acid aquation of the recent com-
plexes (cf. Scheme 2): In the recent acidic medium
(pH ࣘ 2), the aquation rates of these compounds in-
crease with decreasing pH. Thus, in these solutions,
the aquation mechanism would involve the conversion
of the azomethine group (–RC=N–) group to its con-
jugate acid (–RC=NH+–) by a rapid preequilibrium
followed by a slow attack of water. This argument has
a reasonable theoretical basis that the azomethine car-
bon assumes considerable carbonium ion characteris-
tics induced by conjugate acid formed in these acidic
media and that it is more susceptible to nucleophilic at-
tack by water [30]. After this, the protonated complex
decays rapidly into the final products of the reaction
(cf. Scheme 2).
kobs = k2[H+]
where k2 is the second-order rate constant at [H+] >>
[compound]. This equation indicates that the second-
order process is dominant in the present solvent mix-
tures. Values of k2 are cited in Tables IV and V.
The change in the activation barrier, δm G#, of the
investigated compounds from water to water–methanol
and from water to water–acetone mixtures is evaluated
and reported in Tables IV and V from the ratio of
rate constants of the acid hydrolysis in the aqueous-
solvent (k2S) to the corresponding values in the aqueous
solution (k2W) according to the following relation [29]:
ꢀ
ꢁ
k2s
δm G# = −RT ln
k2w
It is observed that the values of δm G# increase with
the increasing of the methanol content (cf. Fig. 4), and
these values matches with the decreasing of kobs (cf.
International Journal of Chemical Kinetics DOI 10.1002/kin.20861

458
MOHAMAD ET AL.
3.0 × 10−3
0.0020
0.0015
0.0010
0.0005
0.0000
(a)
(a)
(b)
(c)
0% Acetone
20% Acetone
40% Acetone
60% Acetone
0% Methanol
20% Methanol
40% Methanol
60% Methanol
2.5 × 10−3
2.0 × 10−3
1.5 × 10−3
1.0 × 10−3
5.0 × 10−4
0.0
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
0.16
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
[HCL], mol / dm3
[HCl] , mol/dm3
(b)
0.0007
0.0006
0.0005
0.0004
0.0003
0.0002
0.0001
0.0000
0% Acetone
20% Acetone
40% Acetone
60% Acetone
−
10 4
0% Methanol
8.0
6.0
4.0
2.0
×
×
×
×
20% Methanol
40% Methanol
60% Methanol
−
10 4
−
104
−
10 4
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
0.16
0.0
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
0.16
[HCL], mol / dm3
[HCl] , mol/dm3
0.00030
0.00025
0.00020
0.00015
0.00010
0.00005
0.00000
0% Acetone
20% Acetone
40% Acetone
60% Acetone
(c)
−
104
0% Methanol
20% Methanol
40% Methanol
60% Methanol
2.5
2.0
1.5
1.0
5.0
×
×
×
×
×
−
104
−
104
−
10 4
−
105
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
0.16
0.0
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
[HCL], mol / dm3
[HCl], mol/dm3
Figure 3 Plots of the observed first-order rate constants of
the reaction between HCl and Ni-complexes as a function of
[H+] in different water–acetone mixtures at [complex] = 1
× 10−4 M, I = 0.1 M, and 298 K: (a) Ni-cpish complex, (b)
Ni-apish complex, and (c) Ni-bpish complex.
Figure 2 Plots of the observed first-order rate constants of
the reaction between HCl and Ni-complexes as a function of
[H+] in different water–methanol mixtures at [complex] = 1
× 10−4 M, I = 0.1 M, and 298 K: (a) Ni-cpish complex, (b)
Ni-apish complex, and (c) Ni-bpish complex.
Solvent Effects. The solvent effect of the acid-
catalyzed aquation on the investigated complexes could
be presented by plotting logarithms of the rate con-
tone content for all Ni(II) complexes. The Ni-cpish
complex is considerably more reactive than the Ni-
apish complex and Ni-bpish complex toward the acid
aquation in aqueous media. This reactivity order could
be explained according to the hydrophobic/hydrophilic
nature of the ligands with substituents R groups, from
stant quotients in water–methanol mixtures, k2(s)/k2(w)
,
against volume% methanol (cf. Fig. 7). The rate con-
stants decrease with increasing of the methanol or ace-
International Journal of Chemical Kinetics DOI 10.1002/kin.20861

ACID-CATALYZED AQUATION OF NI (II)-HYDRAZONE COMPLEXES
459
9
8
7
6
5
4
3
2
1
0
0.020
cpish complex
apish complex
bpish complex
cpish complex
apish complex
bpish complex
0.015
0.010
0.005
δ
0
10
20
30
40
50
60
% Methanol
(a)
0.000
0
10
20
30
40
50
60
% Methanol
(a)
7
6
5
4
3
2
1
0
Ni-cpish complex
Ni-apish complex
Ni-bpish complex
0.014
0.012
0.010
0.008
0.006
0.004
0.002
0.000
Ni-cpish complex
Ni-apish complex
Ni-bpish complex
δ
0
10
20
30
40
50
60
% Acetone (v/ v)
0
10
20
30
40
50
60
(b)
% Acetone (v/v)
(b)
ࣔ
Figure 4 Dependence of the activation barrier δmꢀG on
the solvent composition for acid aquation of Ni(II)-isatin-
hydrazone complexes: (a) in different water–methanol mix-
tures and (b) in different water–acetone mixtures.
Figure 5 Dependence of the rate constant (k2) on the sol-
vent composition for acid aquation of Ni(II)-isatin-hydrazone
complexes: (a) in different water–methanol mixtures and (b)
in different water–acetone mixtures.
R = H, CH3 to ph in cpish, apish to bpish, respec-
tively. The more hydrophilic complex (Ni-cpish, R =
H) is more reactive than the less hydrophilic ligands
(Ni-apish and Ni-bpish, R = CH3 and ph, respectively)
in aqueous media. By increasing the methanol ratio in
the acid dissociation reaction, the reactivity of the more
hydrophilic complex, Ni-cpish, is highly decreased and
oppositely it took place for the less hydrophilic com-
plexes, Ni-apish and Ni-apish.
CONCLUSIONS
Some new Ni(II) Isatin-bishydrazone complexes have
been synthesized and characterized. To gain more in-
formation about the kinetic behavior of the acid aqua-
tion of these complexes, it was followed spectropho-
tometrically, with describing the effects of methanol
Figure 6 Repeated spectral scans of the Ni-bpish complex
at [H+] = 0.06 M, [Ni-bpish complex] = 1 × 10−4 M, I =
0.16 M, and 298 K, with an interval time = 2 min.
International Journal of Chemical Kinetics DOI 10.1002/kin.20861

460
MOHAMAD ET AL.
R
R
H
N
O
N
O
+
N
N
+
.
Cl nH
O
Ni
/
H
k
2
2
.
Cl nH
O
Ni
2
2
N
H
N
N
H
N
very fast
OH
B
2
OH
2
A
R
R
H
H
N
N
O
OH
+
N
N
.
Cl nH O
2
.
Cl nH
O
H O
2
2
Ni
/ k
Ni
2
2
N
H
N
N
H
N
O
slow
OH
OH
2
2
B
C
R
H
N
O
OH
N
.
k
Cl nH
O
2
2
NiCl + organic
2
Ni
fast
N
H
N
D
OH
2
C
Scheme 2 Schematic diagram of acid hydrolysis of the Ni(II)-isatin-bishydrazone complexes.
% Methanol
and acetone as organic cosolvents, on the kinetics of
the acid aquation of these complexes at 298 K, also
the second rate constant (k2) in addition to evaluate
the change in the activation barrier of the investigated
compounds. Finally, the mechanism of the acid aqua-
tion of these complexes was suggested. Methanol and
acetone have been chosen as a cosolvent due to the
availability previously measured chemical potentials,
which aided in interpretation of the rate constant data.
0
10
20
30
40
50
60
0.0
−0.2
−0.4
−0.6
−0.8
−1.0
−1.2
−1.4
−1.6
cpish complex
apish complex
bpish complex
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(a)
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