Synthesis and equilibrium studies of mixed-ligand complexes of Co(II), Ni(II), and Cu(II) with some aliphatic dicarboxylic acids and creatinine

Article abstract of DOI:10.1081/SIM-120030438

Ternary mixed-ligand metal complexes [M(L¹)(L²) ₂](H₂O)_n, where M = Co(II), Ni(II), or Cu(II), L¹ = the dianion of succinic, malic, or tartaric acid, and L ² = creatinine, were synthesized and characterized using microchemical and thermal analyses, molar conductance, infrared, and electronic spectral measurements. A square-planar structure is suggested for the Co(II) and Cu(II) ternary complexes, whereas Ni(II) ternary complexes have tetrahedral stereochemistry. Formation constants of the binary and ternary complexes in such systems were determined by pH-metric titrations at 25 ± 0.1°C and at a constant ionic strength I = 0.1 M (KNO₃). The stability of the various complexes was discussed in terms of the nature of both the ligands and metal ions. The thermodynamic functions (ΔH, ΔG, and ΔS°) associated with the complex formation of [Cu(II) + aliphatic acids + creatinine] were also determined and examined.

Full text of DOI:10.1081/SIM-120030438

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Synthesis and Equilibrium Studies of Mixed‐Ligand
Complexes of Co(II), Ni(II), and Cu(II) with Some
Aliphatic Dicarboxylic Acids and Creatinine
Iman T. Ahmed a
a Chemistry Department, Faculty of Science , El‐Minia University , El‐Minia, 61519, Egypt
Published online: 16 Nov 2010.
To cite this article: Iman T. Ahmed (2004) Synthesis and Equilibrium Studies of Mixed‐Ligand Complexes of Co(II), Ni(II),
and Cu(II) with Some Aliphatic Dicarboxylic Acids and Creatinine, Synthesis and Reactivity in Inorganic and Metal-Organic
Chemistry, 34:3, 523-540, DOI: 10.1081/SIM-120030438
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SYNTHESIS AND REACTIVITY IN INORGANIC AND METAL-ORGANIC CHEMISTRY
Vol. 34, No. 3, pp. 523–540, 2004
Synthesis and Equilibrium Studies
of Mixed-Ligand Complexes of
Co(II), Ni(II), and Cu(II) with
Some Aliphatic Dicarboxylic
Acids and Creatinine
*
Iman T. Ahmed
Chemistry Department, Faculty of Science,
El-Minia University, El-Minia, Egypt
ABSTRACT
Ternary mixed-ligand metal complexes [M(L1)(L2)2](H2O)n, where
M ¼ Co(II), Ni(II), or Cu(II), L1 ¼ the dianion of succinic, malic, or
tartaric acid, and L2 ¼ creatinine, were synthesized and characterized
using microchemical and thermal analyses, molar conductance, infrared,
and electronic spectral measurements. A square–planar structure is
suggested for the Co(II) and Cu(II) ternary complexes, whereas Ni(II) tern-
ary complexes have tetrahedral stereochemistry. Formation constants of the
binary and ternary complexes in such systems were determined by pH-
metric titrations at 25 + 0.1 8C and at a constant ionic strength I ¼ 0.1 M
(KNO3). The stability of the various complexes was discussed in terms
*
Correspondence: Iman T. Ahmed, Chemistry Department, Faculty of Science, El-
Minia University, El-Minia 61519, Egypt; E-mail: h_imanth124@yahoo.com.
523
DOI: 10.1081/SIM-120030438
0094-5714 (Print); 1532-2440 (Online)
www.dekker.com
Copyright # 2004 by Marcel Dekker, Inc.

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524
Ahmed
of the nature of both the ligands and metal ions. The thermodynamic func-
tions (DH, DGo, and DSo) associated with the complex formation of
[Cu(II) þ aliphatic acids þ creatinine] were also determined and
examined.
Key Words: Mixed-ligand complex; Creatinine; Aliphatic acids; Com-
plex formation.
INTRODUCTION
2-Amino-1-methyl-4-imidazolidinone (commonly known as creatinine) is
an important end product of nitrogen metabolism in vertebrates.[1] Moreover,
its levels in urine are clinically important as indicators of a variety of disease
states.[1] The investigation of the complexing ability of creatinine with various
metal ions might be of interest in connection with creatinine metabolism
studies.[1,2] The possibility of it existing in imino- or/and amino-tautomeric
forms and the presence of several donor groups determines its strong coordi-
nation capacity.[3,4] A literature survey indicates that very few pH-metric studies
on creatinine complexes were made.[5] Great interest was devoted to the syn-
thesis, characterization and structural studies of binary complexes of creatinine
with the following metal ions; Ag(I), Hg(II), Cd(II), Zn(II), Co(II), Ni(II),
Cu(II), Pt(II), and Pd(II).[6–12] The results obtained indicate that the complexa-
tion ability of this ligand and the types of complexes formed depend strongly on
the reaction conditions.[7,9–12] However, no studies have been made on ternary
systems containing divalent metal ions as well as creatinine, and aliphatic acids.
Therefore, extending the studies on ternary metal complexes,[13–23] this article
reports the synthesis of the ternary complexes of Co(II), Ni(II), and Cu(II) ions
with aliphatic acids (H2L1) as primary ligands and creatinine (L2) as a secondary
ligand, as shown in Fig. 1. The prepared complexes were characterized by
elemental, thermogravimetric analyses, molar conductance, infrared, and elec-
tronic spectral measurements. The formation constants of the various binary and
ternary complexes were also determined and discussed in terms of the nature of
both the ligand and metal ion. The thermodynamic functions (DH, DGo, and
DSo) associated with the complex formation of [Cu(II) þ aliphatic acids þ
creatinine] were also determined.
EXPERIMENTAL
Materials
The aliphatic acids and creatinine were of analytical grade and were pur-
chased from Aldrich. The purity of these compounds was verified by thin layer

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Mixed Ligand Complexes
525
Figure 1. Structures of the ligands.
.
chromatography (TLC). The metal salts [Co(NO3)2 6H2O, Ni(NO3)2 6H2O,
.
.
and Cu(NO3)2 6H2O] and all other chemicals used were also of analytical
grade. Spectroscopic grade DMSO purchased from Aldrich was used for the
electronic spectral measurements.
Synthesis of the Ternary Metal Complexes
In a round bottom flask an ethanolic solution (10 mL) containing 5 mmol
(0.566 g) of creatinine was added to an EtOH solution (10 mL) containing
.
5 mmol of each of the following metal salts: Co(NO3)2 6H2O (1.455 g); or
.
.
Ni(NO3)2 6H2O (1.454 g); or Cu(NO3)2 6H2O (1.163 g). To this mixture
was added slowly with stirring an EtOH solution (10 mL) containing
5 mmol of one of the following aliphatic acids: succinic (0.590 g), malic
(0.670 g), or tartaric (0.750 g). The ternary mixture was refluxed for 5 hr and
then evaporated to half of its volume and left to cool. The ternary complexes
precipitated out, were filtered and washed thoroughly with distilled H2O and
EtOH and dried in vacuo over P4O10.
Solutions
Stock solutions of the aliphatic acids and creatinine were prepared in
CO2-free, doubly distilled H2O. Solutions of the divalent transition metal

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526
Ahmed
ions were prepared from their nitrate salts in doubly distilled H2O, and the
metal ion concentrations were checked by compleximetric titration as rec-
ommended elsewhere.[24] Stock solutions of HNO3 and CO2-free (ꢀ0.2 M)
KOH were prepared and used after standardization. A stock solution of
KNO3 was also prepared in doubly distilled H2O.
pH-Metric Titration
Numerous titrations of M(II) þ aliphatic acids and/or creatinine mixture in
the 1 : 1 : 1 molar ratio (1.5 mL of 0.05 M of each) with a standard (0.1878 M)
KOH solution were performed at 25 + 0.1 8C. The titrations were carried out
in an inert atmosphere where nitrogen gas was bubbled through the titrated sol-
utions before and during the pH-measurements. The ionic strength was kept con-
stant (0.1 M) using a KNO3 solution, and a total mixture volume of 25 mL was
used for each titration. The titration was carried out in a closed titration jacket.
The titrations were repeated three times for each titration curve to ensure con-
sistency. More than 40 data points were collected for each curve.
The various solutions titrated were made according to the following
sequence: (a) 2.5 mL of 9.4 ꢁ 1022 M HNO3; (b) solution (a) þ 1.5 mL of
0.05 M creatinine; (c) solution (b) þ 1.5 mL of 0.05 M M(II); (d) solution
(a) þ 1.5 mL of 0.05 M aliphatic acids; (e) solution (d) þ 1.5 mL of 0.05 M
M(II); (f) solution (e) þ 1.5 mL of 0.05 M creatinine; and (g) solution
(f) þ 0.4 mL of KOH (0.1878 M) equivalent to the concentration of creatinine.
Apparatus
The C, H, and N contents of the prepared ternary complexes were deter-
mined by the Microanalytical Unit at Cairo University. Molar conductances
of DMSO solutions of the synthesized ternary complexes were measured at
25 8C using a model 31 YSI conductivity bridge with a conductivity cell
constant ¼ 0.10 M. Thermogravimetric (TG) analyses were performed auto-
matically using a DuPont 9000 thermal analyzer with a computer and at a
heating rate of 10 8C min21 in a dynamic air atmosphere. Infrared spectra
were recorded in the 4000–400 cm21 range on a Shimadzu IR-470 spectro-
photometer using KBr pellets. Electronic spectra of freshly prepared solutions
of the complexes in DMSO were recorded at 25 8C in the 200–1000 nm range
by a model UVA 1000E Unicam Scanning UV-VIS spectrophotometer with
an accuracy of +1 nm using matched silica cells of 1.0 cm path length. The
spectrophotometer and its accessories were controlled by software based on
Windows to provide advanced operational facilities. The pH-measurements

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Mixed Ligand Complexes
527
were performed by means of a model SM 702 Metrohm automatic titrator
(Switzerland) with a combined pH glass electrode equipped with a magnetic
stirrer. The accuracy of the pH-meter is +0.001 pH units. The glass electrode
and the pH-meter were calibrated before each series of measurements using
aqueous standard buffers of pH 4.00 (phthalate buffer) and 9.20 (borate buf-
fer). A Fischer Scientific Isotemp Refrigerator Circulator Model 9000 was
used for adjusting the temperature.
RESULTS AND DISCUSSION
Characterization of the Ternary Metal Complexes
Microanalysis
The microanalytical data for the prepared ternary complexes along with
their physical properties are listed in Table 1. The general molecular formula
of the ternary complexes was found to be [M(L1)(L2)2](H2O)n, where n ¼ 1
for the Co(II) and Cu(II) complexes, n ¼ 2 for the Ni(II) complexes.
The prepared ternary complexes were found to be insoluble in water as well
as in most common organic solvents (such as ethanol, methanol, chloroform,
acetonitrile). However, they are slightly soluble in DMF and DMSO. The
measured molar conductance values (Lm) of 1023 M DMSO solutions of the
ternary complexes shown in Table 1 are in the range 13–19ohm21 cm21 M21
indicating the nonelectrolytic nature of these complexes.[25]
,
TG Analysis
Thermogravimetric (TG) analyses of the various ternary complexes have
been carried out to obtain diagnostic structural evidence of the suggested mol-
ecular formula. Generally, the TG curves exhibit one smooth weight loss step
over the temperature range 75–115 8C. This weight loss is in good agreement
with the removal of one or two water molecules, depending on the nature of
the metal ion, indicating that the various synthesized ternary complexes contain
only water of hydration. This is in accordance with the elemental analyses data
as shown in Table 1, as well as the IR spectra of the ternary complexes shown in
the following section. At high temperature (.300 8C) a steep weight loss step is
observed pointing out to a rapid decomposition of the ternary complexes.

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Ahmed
l

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Mixed Ligand Complexes
529
IR Spectra
Table 2 shows the important IR bands of the prepared ternary complexes,
which provided good evidence for the coordination mode of the ligands. The
broad bands in the range of 3560–3500 cm21 in the IR spectra of the various tern-
ary complexes can be most likely attributed to the OH-stretching vibration of
water of hydration. This band overlapped with the stretching vibration of the
OH group of malate or tartarate. However, the absence of sharp band in the IR
spectra of free OH group belonging to malate or tartarate could likely be ascribed
to its being buried under the broad band due to the antisymmetric and symmetric
OH-stretching of hydrated water molecules. On the other hand, the IR spectra of
malate and tartarate complexes showed strong absorptions at 1370–1350 cm21
range due to the vibration bending of the free OH group. In addition, the IR spectra
displayed bands in the ranges 1075–1060 and 1125–1115 cm21 for C–O stretch-
ing of –CH2OH (malate) and CHOH (tartarate), respectively. This is another
evidence for the nonparticipation of hydroxyl groups in case of malate and tarta-
rate complexes. Furthermore, it had been reported earlier that the transition metal
ions Co(II), Ni(II), and Cu(II) formed seven-membered ring structures with malate
and tartarate with the oxygen of carboxyl groups.[15] A number of bands were
observed in the 3450–3400 cm21 range, in which n(NH2) symmetric and asym-
metric vibrations of creatinine appear.[26] These data show that creatinine is
mainly present in the complex as its amino-tautomer (A) as shown in Fig. 1.
The bands in the 2990–2900 cm21 range (medium to weak) could be assigned
to the C–H stretching vibrations in both primary and secondary ligands. The
n(C55O) bands of creatinine were observed in the range 1710–1690 cm21 show-
ing that the C55O groups of creatinine are not involved in the coordination. The
assignment of this band is in a good agreement with those previously reported for
creatinine on the basis of characteristic vibrations.[4,27] The stretching modes of
C55N of free creatinine observed[10] at 1680 cm21 is shifted to lower frequency
at the 1645–1665 cm21 range in the IR spectra of the complexes, revealing the
involvement of this group in the coordination with the metal ion. The suggested
structure of the complexes in Fig. 2 indicates the possible formation of intra-
molecular hydrogen bonding between C55O and exocyclic NH2 groups, which
may stabilize the structure.[10] A distinct band appearing in the 1640–
1600 cm21 region in all complexes is typical of coordination carboxylate
stretching.[26] This assignment is based on the fact that the ionized and coordi-
nated COO2 stretching band appears[26] at 1650–1590 cm21. The fact that
the band appeared in the 1450–1400 cm21 range is likely ascribed to the sym-
metric vibration of the coordinated carboxylates.[26] The above findings are
consistent with the conclusion that the divalent anions of aliphatic acids are
coordinated to the central metal ion as bidentate ligands, whereas, creatinine
coordinated as a monodentate ligand through the endocyclic N-atom.

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Ahmed

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Mixed Ligand Complexes
531
Figure 2. The suggested structures of the prepared ternary complexes
[M(L1)(L2)2](H2O)n, where n ¼ 1 for M ¼ Co(II) and Cu(II) and n ¼ 2 for
M ¼ Ni(II). L1 ¼ succinate (R1 ¼ R2 ¼ H), malate (R1 ¼ OH, R2 ¼ H), and tartarate
(R1 ¼ R2 ¼ OH).
Electronic Spectra
In order to shed some light on the geometrical structure of the ternary metal
complexes, the electronic absorption spectra of the complex solutions in DMSO
were recorded (Fig. 3). The wave length maxima and the molar extinction
coefficients of the various bands in the recorded spectra were collected in
Figure 3. Visible electronic absorption spectra of Co(II), Ni(II), and Cu(II) ternary
complexes containing creatinine as secondary ligand: (a) [Co(succinate)(creatinine)2]-
H2O, (b) [Ni(tartarate)(creatinine)2](H2O)2, and (c) [Cu(tartarate)(creatinine)2]H2O.

ORDER
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Ahmed
Table 1. The Co(II)-complexes are characterized by a broad absorption band
with lmax in the 490–524 nm range (1max ¼ 220–260 cm21 M21). In terms
of previous work, a square–planar structure may be suggested for the ternary
complexes of Co, Ni, and Cu with O2N2 ligands.[28–32] Accordingly, this
band can be assigned to an electronic transition from the lower orbital (eg) to
the empty (b1g) dx2 2 y2 s-anti-bonding orbital.[32]
The electronic absorption spectra of the Ni(II)-complexes display a broad
band in the visible region in the range 680–720 nm (1max ¼ 26.5–
30.4 cm21 M21), followed by a weak shoulder at the low-energy side.
Based on previous work, this is indicative of a tetrahedral stereochemical
configuration of the Ni(II) ion, where the recorded broad band is assigned
3
3
to the T1 ! T1(P) transition.[28]
It was reported previously that four-coordinate Cu(II)-complexes have d–d
transitions at about 1112–1250, 1175–740, and 699–917 nm for a regular
tetrahedral, pseudo-tetrahedral, or square–planar structure, respectively.[28]
Considering this fact that the absorption maximum related to four-coordinate
Cu(II)-complexes shifts to higher energy as the regular tetrahedral structure is
distorted towards square–planar.[28] Thus, one may suggest a square–planar
structure for the present synthesized Cu(II) complexes having an absorption
band in the range 708–770 nm (1max ¼ 37.50–44.00), where this band is
2
2
2
2
attributed to the B2g ! E1g and B2g ! A1g transitions.[28]
Formation Constants of the Various Complexes
Formation of the Complexes
A typical set of experimental titration curves obtained according to the
sequence described in the Experimental section for the systems under investi-
gation is shown in Fig. 4. Similar curves were obtained for the other metal ions.
Binary Complexes
The titration curves (c) in Fig. 4 corresponding to the 1 : 1 binary M(II)-
creatinine complex solutions indicate that the binary complexes are formed
at a pH range 5.0–6.5. This is concluded from the observed difference of
the titration curve (c) of the binary complex solution from the curve (b) of
the free ligand. The binary metal complex solutions show precipitation at a
pH range of 7.0–8.0 depending on the nature of metal ion. This behavior
can be attributed to the interference of hydrolysis reaction. Therefore,
beyond this point, the calculation of the binary stability constant could not
be made.

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533
Figure 4. Titration curves of [Co(II)–tartaric acid–creatinine] at 25 8C and constant
ionic strength I ¼ 0.10 M KNO3. (a) 2.5 mL of 9.4 ꢁ 1022 M HNO3; (b) solution
(a) þ 1.5 mL of 0.05 M of creatinine; (c) solution (b) þ 1.5 mL of 0.05 M of Co(II);
(d) solution (a) þ 1.5 mL of 0.05 M tartaric acid; (e) solution (d) þ 1.5 mL of 0.05 M
of Co(II); (f) solution (e) þ 1.5 mL of 0.05 M of creatinine; and (g) curve (f) after
addition of 0.4 mL of KOH (0.1878 M) equivalent to creatinine concentration present
in the solution.
The titration curve (e) in Fig. 4 of the 1 : 1 binary [M(II) þ aliphatic acids]
complex solutions clearly reveal that the various 1 : 1 binary complexes begin
to form at the lower pH range 3.0–4.5. Examination of the various titration
curves of the investigated binary M(II)–aliphatic acid complexes reveals
that the pH at which the binary complex is formed is largely dependent on
the nature of each of the aliphatic acids and metal ion. For the same metal
ion the pH value, at which the 1 : 1 binary complex of an aliphatic acid begins
to form, is lowered according to the following order: succinic . malic .
tartaric. This indicates a high tendency of the hydroxy aliphatic acid towards
complex formation along the same sequence.
Ternary Complexes
The titration curve (g) in Fig. 4 (obtained after addition of KOH
equivalent to the creatinine ligand present in the ternary solution) of the
mixed-ligand system under investigation overlaps with those corresponding

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534
Ahmed
to the 1 : 1 binary M(II)–aliphatic acids, (curve e) at the lower pH value. This
pH value is largely dependent on the nature of both the M(II) ion and aliphatic
acid. At a pH range 3.4–4.5, the curve (g) diverges from curve (e) indicating
the attachment of the secondary ligand creatinine producing the mixed-ligand
ternary complex. This revealed that the binary complex of the aliphatic acid
[ML1], is formed first and, accordingly, combines with creatinine in the
protonated form (HL2)þ as a secondary ligand producing the mixed-ligand
complex [ML1L2] in a stepwise manner as represented by the following
equations:
M(II) þ H2L1
½ML1ꢃ þ ½HL2ꢃ
½ML1ꢃ þ 2Hꢂ
½ML1L2ꢃ þ Hꢂ
ð1Þ
ð2Þ
where M(II) ¼ Co(II), Ni(II), Cu(II); H2L1 ¼ aliphatic acid (succinic, malic,
tartaric); and L2 ¼ creatinine.
Formation Constant Values
The formation constants for the various binary complexes [M(II)–ali-
phatic acid or M(II) þ creatinine] in the system under investigation were
also calculated by adopting the formula of Irving and Rossotti[33,34] to the
results of the pH-titration curves of the binary complex solutions as shown
in Fig. 4, curves (b), (c) and (d), (e) for creatinine and aliphatic acids, respect-
ively. The formation constant values of the various 1 : 1 : 1 ternary complexes
were calculated considering the titration curves (e) and (g) in the pH-range of
ternary complex formation. The following equation was used for the calcu-
lation of nmix (average number of creatinine molecules as a secondary ligand
associated with one molecule of binary complex specie [ML1])[33,34]
½ðVg ꢂ VeÞꢃ½Cb þ Ca þ CLðy ꢂ nHÞꢃ
nmix
¼
ð3Þ
½Vo þ VeÞꢃnHCM
Vg and Ve are the volumes of KOH consumed to reach the same pH values in
curves (g) and (e), respectively (Fig. 4). Cb and Ca are the concentrations of
KOH and HNO3, respectively. CM is the concentration of the binary [ML1]
complex, which is equal to the concentration of M(II) used, CL is the concen-
tration of the ligand L2, y is the number of dissociable protons of the secondary
ligand [HL2]2 (y ¼ 1 for such molecule), and Vo is the original volume
(25 mL). nH values are the average number of protons associated with the sec-
ondary ligand creatinine at different pH values and were available from the
determination of the formation constant. The nmix values do not exceed
unity, indicating that only one secondary ligand molecule combines with
the binary [ML1] complex forming the mixed-ligand complex [ML1L2].

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Mixed Ligand Complexes
535
Table 3. Formation constanta values for the various 1 : 1 binary [M(II) þ (aliphatic
acids)] and [M(II) þ (creatinine)] complexes formed at 25 8C and of I ¼ 0.1 M KNO3.
M(II) þ aliphatic acids (L1) or M(II) þ creatinine (L2)
Ligand
Cu(II)
Ni(II)
Co(II)
Succinic acid
Malic acid
4.65 + 0.05
4.01 + 0.07
3.90 + 0.11
3.18 + 0.10
4.14 + 0.09
3.35 + 0.10
3.06 + 0.10
3.07 + 0.12
3.65 + 0.12
3.08 + 0.11
2.85 + 0.12
3.05 + 0.11
Tartaric acid
Creatinine
aM(II) þ L1 or L2 O ML1 or ML2; K ¼ [ML1] or [ML2]/[M(II)][L1 or L2].
From the values of nmix the free secondary ligand exponent (pLmix) was
calculated using the following equation.[33,34]
(
)
P
y¼1 or 2 bHð1=10BÞ
Vo þ Vg
y¼0
y
pLmix ¼ log
ð4Þ
CL ꢂ nmixCM
Vo
Here bHy are the formation constant values of the secondary ligand creatinine,
and B is the pH meter reading.
ML1
ML1L2
The various mean log KMML1, log KMML2, and log K
values determined
along with the estimated errors using least-square fits are listed in Tables 3
and 4.
Examination of Tables 3 and 4 reveals that the stability constants of the
binary or mixed-ligand complex for the same metal ion vary in terms of the nature
of the primary ligand aliphatic acid according to the sequence: succinic .
malic . tartaric. This behavior may be ascribed to the decrease in the basicity,
as a measure of s-donor character, of the corresponding anions in the same
sequence.[35] Furthermore, it is evident that the mixed-ligand complex,
Table 4. Formation constanta values for the various 1 : 1 : 1 [M(II) þ (aliphatic
acids) þ (creatinine)] ternary complexes at 25 8C and I ¼ 0.1 M KNO3.
[M(II) þ (aliphatic acids) þ (creatinine, L2)]
Aliphatic acids (L1)
Cu(II)
Ni(II)
Co(II)
Succinic acid
Malic acid
Tartaric acid
3.35 + 0.11
3.29 + 0.08
3.23 + 0.11
3.26 + 0.10
3.22 + 0.11
3.16 + 0.09
3.20 + 0.09
3.16 + 0.11
3.14 + 0.10
aM(L1) þ L2 O ML1L2; K ¼ [ML1L2]/[ML1][L2].

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536
Ahmed
[ML1L2], formed is characterized by a high stability compared with that of the
corresponding binary complex of the secondary ligand [ML2]2þ. This result
can be plausibly attributed to the low electrostatic repulsion forces during
mixed-ligand complex formation f(ML1 þ [HL2]þ O [ML1L2] þ Hþ)g, com-
pared to that for binary complex formation (M2þ) þ [HL2]þ O [ML2]2þ þ Hþ).
The order of the stability of the various binary or mixed-ligand complexes
under investigation in terms of the nature of the central divalent metal ion
follows the usual sequence[36] Cu(II) . Ni(II) . Co(II).
Thermodynamic Function of Complex Formation
The formation constant of the 1 : 1 : 1 ternary [Cu(II) þ aliphatic acid þ
creatinine] complexes were determined by pH-titrations at 5, 15, 25, 35,
and 45 8C and at a constant ionic strength, I ¼ 0.1 M KNO3. The overall reac-
tion of the ternary complex formation can be represented by the following
equation:
½ML1ꢃ þ ½HL2ꢃþ
½ML1L2ꢃ þ Hþ
ð5Þ
Figure 5. Plot of log K values against 1/T for [Cu(II) þ (aliphatic acids) þ (creatinine)]
complex solutions at a constant ionic strength I ¼ 0.10 M KNO3.

ORDER
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Mixed Ligand Complexes
537
Table 5. Thermodynamic functionsa of [Cu(II) þ (aliphatic acid) þ (creatinine)]
ternary complexes at a constant ionic strength I ¼ 0.10 M KNO3.
log K values at T ( 8C)
2DH
(kJ
2DG8
(kJ
DS8
Aliphatic
acid
(J mol21
05
15
25
35
45
mol21
)
mol21
)
K21
)
Succinic
acid
3.48 3.39
3.43 3.37
3.36 3.29
3.35
3.26 3.20
3.22 3.17
3.15 3.10
11.82
11.50
11.32
19.11
24.42
24.30
32.86
Malic
acid
3.29
3.23
18.77
18.43
Tartaric
acid
aDG8 and DS8 values were calculated for 25 8C.
The values of the enthalpy change (DH) were determined graphically using
the van’t Hoff relationship by plotting log K values at different temperatures
vs. 1/T (Fig. 5) and equating the gradient with 2DH/19.15. The values of
the free energy (DG8) and entropy (DS8) changes associated with the com-
plex formation are calculated at 25 8C using the following equations:
DG8 ¼ 22.303 RT log K, and DS8 ¼ (DH 2 DG8)/T, respectively.
The formation constant values along with the DH, DG8, and DS8 values
are given in Table 5. The results obtained reveal that the metal–ligand com-
plex formation is less favored with increasing temperature, coincident with the
exothermic nature of the complex formation reaction. The negative DG8
values obtained are consistent with the spontaneity nature of the complex for-
mation processes. The values of entropy change DS8 related to the formation
of all complexes are positive indicating a favorable formation of the com-
plexes studied. This behavior is in agreement with the metal–ligand complex
formation reaction.[37]
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Received April 7, 2003
Accepted November 24, 2003
Referee I: T. J. McNeese
Referee II: Y. Z. Hamada

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