Nickel(II) complexes with 2-hydroxyethyliminodioacetic acid in aqueous solutions of dicarboxylic acids

Article abstract of DOI:10.1134/S0036023612040146

The equilibria in binary and ternary systems containing a nickel(II) salt, 2-hydroxyethyliminodi-acetic acid, and dicarboxylates were studied by spectrophotometric and potentiometric methods with NaClO₄ as the supporting electrolyte at I = 0.1 and T = 20 ± 2°C. The molar and protic composition and the pH regions of existence of the complexes were determined, the stability constants of complexes containing the same or different ligands were determined. The fractional distribution of the complexes as the function of acidity was elucidated. The experimental data were treated using mathematical models to estimate the possibility of existence of a broad range of complex species in solution and to distinguish the species that are sufficient to take into account for reproducing the experimental results.

Full text of DOI:10.1134/S0036023612040146

ISSN 0036ꢀ0236, Russian Journal of Inorganic Chemistry, 2012, Vol. 57, No. 4, pp. 616–621. © Pleiades Publishing, Ltd., 2012.
Original Russian Text © V.I. Kornev, M.G. Semenova, 2012, published in Zhurnal Neorganicheskoi Khimii, 2012, Vol. 57, No. 4, pp. 681–686.
PHYSICAL CHEMISTRY
OF SOLUTIONS
Nickel(II) Complexes with 2ꢀHydroxyethyliminodioacetic Acid
in Aqueous Solutions of Dicarboxylic Acids
V. I. Kornev and M. G. Semenova
Udmurt State University, Krasnoarmeiskaya ul. 71, Izhevsk, 426034 Russia
Received February 25, 2010
Abstract—The equilibria in binary and ternary systems containing a nickel(II) salt, 2ꢀhydroxyethyliminodiꢀ
acetic acid, and dicarboxylates were studied by spectrophotometric and potentiometric methods with
NaClO4 as the supporting electrolyte at I = 0.1 and T = 20 2°C. The molar and protic composition and the
pH regions of existence of the complexes were determined, the stability constants of complexes containing
the same or different ligands were determined. The fractional distribution of the complexes as the function of
acidity was elucidated. The experimental data were treated using mathematical models to estimate the possiꢀ
bility of existence of a broad range of complex species in solution and to distinguish the species that are sufꢀ
ficient to take into account for reproducing the experimental results.
DOI: 10.1134/S0036023612040146
Nickel(II) compounds (chlorides, sulfates, phosꢀ
(H2Ox), malonic (H2Mal), and succinic (H2Suc)
phates, formates, and so on) find wide use in various acids), as this would open up the way for developing
fields of industry as tanning extracts, nickelꢀplating
electrolytes, phosphate etching materials, and so on.
2ꢀHydroxyethyliminodiacetic acid, which forms comꢀ
plexes with many metals, is used as an effective comꢀ
plexing agent. The ability of this compound to form
metal ion complexes of various composition, charge,
and stability and its high water solubility served as preꢀ
requisites for the use of this chelating agent as an effecꢀ
tive eluent in the separation of lanthanides [1–4].
Dicarboxylic acids are used in various fields of
industry including the production of varnishes and
paints, synthetic fibers, and food products, while
dicarboxylic acid amides (dipeptides) coordinated to
micronutrient metals are known as biologically active
medicinal agents [5].
Nickel is also a biologically active metal. The
excessive “technologyꢀinduced” nickel(II) comꢀ
pounds that get into the body are toxic for the metabꢀ
olism and carcinogenic for cells [6].
Recent years have seen advantageous development
of studies into metal coordination compounds conꢀ
taining chelating agents. The formation of chelates in
the presence of additional ligands often markedly
enhances their functional activity, which may be
caused by electron density redistribution, increase in
the stability of the complex species, increase in the
reactivity, formation of new structural entities, etc.;
this considerably extends the scope of their applicaꢀ
tion.
complexing compositions with a preꢀspecified set of
properties.
The complexes of 2ꢀhydroxyethyliminodiacetic
acid with nickel(II) were studied repeatedly by various
methods. A potentiometric study at
[7], 25 [8, 9], and 30 [10] showed the presence of
[NiHeida] (log = 9.28 [7], 9.15 [8], 9.33 [9], 9.50
[10]) and [NiHeida2]2– complexes (log
= 14.25 [7],
14.18 [9], 14.65 [10]). According to spectrophotomeꢀ
I = 0.1 and T = 20
°
С
β
β
try at
(log
plexes exist [11].
I
= 0.1 (NaClO4) and
T
= 20
°
С
, the [NiHeida]
β
= 9.52) and [NiHeida2]2– (log
β = 13.90) comꢀ
Nickel(II) complexes with dicarboxylic acids have
also been repeatedly studied but the results are quite
contradictory as regards both the composition and the
stability constants of the complexes. To our knowlꢀ
edge, no data on differentꢀligand nickel(II) complexes
with H2Heida and dicarboxylic acids are present in the
literature.
For understanding of the complexation processes
in ternary systems, knowledge of the processes that
occur in the corresponding binary systems is needed.
Since the published data on the Ni(II) complexes conꢀ
taining one sort of the ligands are contradictory, we
studied in detail the complexation in binary systems
under identical experimental conditions in order to
determine more precisely the composition and the staꢀ
bility constants of Ni(II) complexes with these
reagents. Nickel(II) dicarboxylates were studied preꢀ
viously [12]. Also a review of publications dealing with
Ni(II) dicarboxylates was presented [12]. Here we
Thus, it appears pertinent to study the quantitative
characteristics of the reactions of nickel(II) with 2ꢀ
hydroxyethyliminodiacetic acid (H2Heida) in the
presence of saturated dicarboxylic acids (oxalic report the study of the Ni(II)–H2Heida system.
616

NICKEL(II) COMPLEXES
617
EXPERIMENTAL
α
1.0
3
2
4
A
The complexation was studied by spectrophotomeꢀ
try. The absorbance of the solutions was measured on
SFꢀ26 and SFꢀ56 spectrophotometers using a tailorꢀ
made Teflon cell with quartz walls and 5ꢀcm thick
absorption layer. This cell allows for simultaneous
measurement of the solution pH and absorbance. The
wavelengths were adjusted at 380–420 nm with an
0.2
1
0.5
error of 0.1 nm. All
A = f(pH) curves were obtained
by spectrophotometric titration. Doubly distilled
water was used as the reference solution. The hydrogen
ion activity was measured on an Iꢀ160 ion meter with
an ESꢀ10601/7 working electrode and an ESRꢀ10101
reference electrode. The instrument was calibrated
against standard buffer solutions prepared from voluꢀ
metric concentrates (fixanal) and tested at the UPKPꢀ1
bench. The desired pH was created by adding analytiꢀ
cal grade NaOH or HClO4 solutions. The constant
0.1
5
0
2
4
6
8
10
12
pH
ionic strength (I ≈ 0.1) was maintained by an analytiꢀ
Fig. 1. Absorbances (
A
) and mole fractions (
α
) of
cal grade solution of NaClO4. In the case of a large
excess of dicarboxylic acids, all equilibrium constants
were calculated for the basic ionic strength. The studꢀ
nickel(II) complexes vs pH for the Ni(II)–Heida system:
2+
–
(
1
4
) experimental curve
A
=
f
(pH), (
[Ni(OH)Heida]
10 mol/L.
2)
Ni , (
3) [NiHeida],
2–
–3
(
с
)
[NiHeida ] , (
5)
;
с
= 4 × 10 ,
2
Ni
–3
ies were performed at
t
= 20
2°С. A nickel(II) perꢀ
= 8
×
Heida
chlorate solution was prepared by dissolving reagent
grade nickel(II) oxide in perchloric acid. Solutions of
the chelating agents and dicarboxylic acids were preꢀ
pared by dissolving exact weighed amounts of reagent
grade chemicals (AO Reachim) in distilled water. The
mathematical treatment of the results was done by
CPESSP [13] and SolꢀEq[14] software.
of species chosen to describe the system, choosing the
appropriate set of complex species is of prime imporꢀ
tance. In the software we used, the expedience of
including one or another metal complex or ligand speꢀ
cies was determined based on minimization of the
Fisher criterion, which takes into account the discrepꢀ
ancy between the experimental and theoretical absorꢀ
RESULTS AND DISCUSSION
bances (А) for each component of the system. This
software provides a fairly reliable estimate of the equiꢀ
librium system parameters and the stoichiometry and
thermodynamics of the chemical processes. The
chemical equilibria in the binary and ternary systems
were described using a modification of the ion pair
model. This model implies the possible existence of a
broad range of complexes and associates in the soluꢀ
tion.
The absorption spectrum of nickel(II) hexaaqua comꢀ
plex exhibits three absorption bands: 400 (3A2g
→
3T1g
→ , F) →
3T1 , and 1111 nm (3A2 3T2g).
g g
, P),
714 (3A2
g
The investigation of the complexation in binary
and ternary systems was based on the fact that the
absorption spectra and the absorbance of solutions of
Ni(II) perchlorate change in the presence of the
chelating agent and dicarboxylic acids. In addition, we
constructed theoretical models of the complexation
for ternary systems neglecting the differentꢀligand
complexation. A comparison of the theoretical
(pH) curves and the experimental values revealed
deviations associated with the formation of differentꢀ
ligand complexes. The working wavelength was chosen
at 400 nm where the proper absorbance of the ligand is
insignificant at various pH, while the complexes show
a considerable hyperchromic effect. The pattern of the
By mathematical treatment of the
A = f(pH)
curves, the composition of the complex species and
the types of acid–base equilibria in binary and ternary
systems were determined. The pH regions of existence
of the complexes were derived from the diagrams of
distribution of the species as a function of pH. This is
exemplified in Fig. 1, which shows the experimental
dependence of absorbance and mole fraction distribuꢀ
A =
f
tion curves (α) of various complexes vs pH for the
Ni(II)–H2Heida system.
A
= f(pH) curves constructed for the binary and terꢀ
nary systems indicates that the complexation occurs
over a broad pH range for all of the systems.
As follows from Fig. 1, the
three buffer areas corresponding to the formation of
A = f(pH) curve has
It is quite a challenge to describe the equilbria in three complexes. The first one is formed at 1.0 < pH <
binary and, especially, in ternary systems containing 3.6 and exists at 3.6 < pH < 5.0. The second complex
various hydroxy complexes and various protonated is formed at 5.0 < pH < 8.6 and exists at 8.6 < pH <
forms of polydentate ligands. Since the values of the 10.0. When pH > 10.0, the third complex manifests
stability constants to be determined depend on the set itself. The mole fractions of the complexes that are
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 57 No. 4 2012

618
KORNEV, SEMENOVA
Some characteristics of identicalꢀ and differentꢀligand nickel(II) complexes with 2ꢀhydroxyethyliminodiacetic acid and
dicarboxylic acids at
Complex
I
= 0.1 (NaClO ) and
T
= 20 2°C
pHopt
4
pH regions
of existence
The maximum mole fraction
of the complex for pHopt, %
logβ
[NiHeida]
1.0–12.0
>3.7
3.6–4.7
9.33 0.05
14.20 0.06
12.05 0.11
98
94
2–
[NiHeida ]
8.6–11.6
2
–
[Ni(OH)Heida]
>10.0
–
[NiHeidaHOx]
0.6–4.8
>1.8
2.0
7.2
16.92 0.12
13.47 0.18
16.50 0.20
15.39 0.23
23
85
60
2–
[NiHeidaOx]
4–
[NiHeida Ox]
>5.9
9.5–11.3
2
3–
[Ni(OH)HeidaOx]
>11.2
–
[NiHeidaHMal]
1.0–7.0
>2.7
3.5
6.2
16.15 0.19
11.36 0.63
14.73 1.30
12.17 0.68
77
85
32
2–
[NiHeidaMal]
4–
[NiHeida Mal]
>5.1
9.1–12.0
2
3–
[Ni(OH)HeidaMal]
>10.6
–
[NiHeidaHSuc]
2.5–6.5
>3.3
4.6
5.9
13.83 0.79
9.77 0.26
13.44 0.47
4
39
6
2–
[NiHeidaSuc]
4–
[NiHeida Suc]
>5.4
8.5–12.5
2
formed in the Ni(II)–H2Heida systems and the releꢀ
vant stability constants are summarized in the table.
Note that the initial pH for the formation of most
complexes is rather clearꢀcut; however, the final pH of
the existence of one or another complex is not always
established because it is located in a highly alkaline
region (pH > 12).
Figure 2 shows the A = f(pH) curves for the acid–
base equilibria in the binary and ternary systems comꢀ
prising oxalic or malonic acid. In both cases, the forꢀ
mation of complexes with identical or different ligands
occurs over a broad pH range. In the Ni(II)–
H2Heida–H2Ox system, H2Ox functions obviously as
the primary ligand, while in other systems, this role is
played by H2Heida. This is indicated by the curves of
absorbance and fractional distribution of the comꢀ
plexes vs pH.
The complexes formed in binary and ternary sysꢀ
tems were identified taking into account three monoꢀ
meric nickel(II) hydrolysis constants [15], two dissoꢀ
ciation constants for each dicarboxylic acid [16, 17],
and three dissociation constants of 2ꢀhydroxyethylimꢀ
inodiacetic acid [18, 19]:
In the 1 : 2 : 2 Ni(II)–H2Heida–H2Ox system
(Fig. 3a), [NiHOx]+ and [NiOx]complexes exist in a
highly acidic medium. At pH 2.3, the complex
[NiHeidaHOx]– is formed in 52% yield; then it is
deprotonated to give [NiHeidaOx]2–. The yield of the
latter complex reaches 94% at pH 6.0. Simultaneously
with the differentꢀligand complex, nickel oxalate
[NiOx2]2– exists at 1.2 < pH < 6.0 (yield 35% at pH
4.0). On further decrease in the acidity, the complex
[NiHeidaOx]2– adds one more H2Heida molecule to
give mixed dichelate [NiHeida2Ox]4–. This species is
accumulated at 6.0 < pH < 14.0, the highest yield
being 38% (at 10.0 < pH < 11.0). The formation of
[NiHeida2Ox]4– is possible only through partial cleavꢀ
age of the chelate rings of the nickel dichelate. At pH >
11.0, this complex is hydrolyzed and the complex
[Ni(ОН)HeidaOx]3– is accumulated in 59% yield (at
pH 13.8).
Acid
H Ox
2
H Mal
2
H Suc
2
H Heida
2
p
K
–
–
–
1.60
2.20
8.73
I
1
2
pK
K
1.54
4.10
2.73
5.34
4.00
5.24
p
Note that that the yield of nickel(II) complexes
with either identical or different ligands depends not
only on the medium acidity but also on the ligand conꢀ
centration. The full yield of differentꢀligand comꢀ
plexes is not achieved even with considerable excess of
the ligands. It was also found that for the formation of
differentꢀligand complexes, the ligand concentration
should not be lower than their concentration in the
complexes with identical ligands.
As the oxalic acid concentration increases fivefold
(Fig. 3b), the fraction of [NiHeidaHOx]– decreases
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 57 No. 4 2012

NICKEL(II) COMPLEXES
619
A
0.3
A
(а)
(b)
3
7
2
0.2
5
6
2
0.2
8
4
1
1
0.1
0.1
0
2
4
6
8
10
12
pH
0
2
4
6
8
10
12
pH
Fig. 2. Absorbance of solutions vs pH for nickel(II) in the systems (
for ( Ni(ClO ) and its complexes with ( H Heida, ( H Ox, (4) H Mal, (5) H Heida + H Ox, and (6) H Heida + H Mal;
2 2 2 2 2
a) Ni(II)–H Heida–H Ox and (b) Ni(II)–H Heida–H Mal
2 2 2 2
1
)
2
)
3)
4 2
2
2
(7, 8) theoretically calculated curve neglecting the differentꢀligand complexation. The compositions are 2 (2, 3, 4), 1 : 2 : 25, (6,
–3
8
) 1 : 2 : 100;
с
= 4
×
10 mol/L,
λ
= 400 nm.
Ni
α
α
(а)
(b)
1.0
1.0
7
7
1
6
1
10
10
8
4
3
0.5
0.5
3
8
6
2
4
9
2
9
5
11
10 12 pH
0
2
4
6
8
10 12 pH
0
2
4
6
8
Fig. 3. Distribution of nickel(II) complexes vs. pH for the Ni(II)–H Heida–H Ox system for (a) 1 : 2 : 2 and (b) 1 : 2 : 10 comꢀ
2
–
2
2+
2–
2–
positions: (
1)
Ni , (
2
) [NiHOx], (
3
) [NiOx], (
4
) [NiHeidaHOx] , (
5
) [NiHeida], (
6
)
[NiOx ] , (
7
)
[NiHeidaOx] , (
= 4 × 10 mol/L, λ = 400 nm.
Ni
8)
2
4–
2–
3–
–
–3
[NiHeida Ox] , (
9
)
[NiHeida ] , (10
)
[Ni(OH)HeidaOx] , (11
)
Ni(OH)
;
с
2
2
3
but simultaneously the fractions of [NiHeida2Ox]4– ponent ratio is 1 : 2 : 10, the yields of the [NiHeidaHꢀ
(56% at pH 10.0) and [Ni(ОН)HeidaOx]3– (80% at Mal]– [NiHeidaMal]2–, and [NiHeida2Mal]4– comꢀ
,
pH 13.8) increase. The mole fractions of other species plexes are 46% (pH 4.0), 65% (pH 6.0), and 11%
change little. It was also noted that an increase in the (9.0 < pH < 12.0), respectively. A fivefold increase in
content of H2Heida in the system entails even greater the concentration of the secondary ligand results in
accumulation of [NiHeidaHOx]– and [NiHeida2Ox]4–
.
the yields of these complexes being 76, 84, and 31%,
For example, when the composition is 1 : 5 : 5, the respectively, at the same pH values (Fig. 4a, where
yields of these complexes are 65% (at pH 2.3) and 72% H2Dik stands for any dicarboxylic acid).
(at pH 10.0), respectively.
The differentꢀligand complexation in the Ni(II)–
The formation of differentꢀligand complexes in the H2Heida–H2Suc occurs only with large excess of succinic
Ni(II)–H2Heida–H2Mal system also depends appreciaꢀ acid, the yields still being low. For example, for 1 : 2 : 100
bly on the malonic acid concentration. When the comꢀ component ratio, the yields of the [NiHeidaHSuc]–
,
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 57 No. 4 2012

620
KORNEV, SEMENOVA
α
α
(а)
(b)
1.0
1.0
3
1
1
8
7
4
3
8
0.5
0.5
9
7
11
2
10
11
9
4
2
6
5
0
2
4
6
8
10 12pH
0
2
4
6
8
10 12 pH
Fig. 4. Distribution of nickel(II) complexes vs. pH for the Ni(II)–H Heida–H Mal and Ni(II)–H Heida–H Suc systems for
2
2
2
2
2+
+
–
(a) 1 : 2 : 50 and (b) 1 : 2 : 100 compositions: (
1
)
2
Ni , (2) [NiHDik] , (3) [NiHeida], (4) [NiHeidaHDik] , (5) [NiMal],
2–
2–
2–
4–
3–
–
[NiMal ] , (
7)
[NiHeidaDik] , (
8)
[NiHeida ] , (
9
)
[NiHeida Dik] , (10
)
[Ni(OH)HeidaMal] , (11) Ni(OH) ;
2
Ni
2
3
–3
(
11);
с
= 4 × 10 mol/L, λ = 400 nm.
[NiHeidaSuc]2–, and [NiHeida2Suc]4– complexes are or
4, 39, and 6% for pH 4.5, 6.0, and 9.0–13.0, respecꢀ
tively (Fig. 4b). It also follows from the Figure that
over a broad pH region, succinic acid solutions conꢀ
tain mainly the nickel chelates [NiHeida] and
−
NiHeidaHOx + Ox2−
[
]
ꢀ NiOx 2− + Heida2− + H+.
[
]
2
[NiHeida2]2–
.
In the pH region of formation of nickel dichelate,
one more mole of hydroxyethylimino diacetate adds
according to the equations
A comparison of the stability constants of neutral
nickel(II) chelate dicarboxylates results in the followꢀ
ing series of complexes in the order of increasing staꢀ
bility constants:
[NiHeidaDik]2− + Heida2−
ꢀ NiHeida 2− + Dik2−,
[NiHeidaSuc]2−< [NiHeidaMal]2−
< [NiHeidaOx]2−,
[
]
2
[NiHeidaDik]2– + Heida2–
[ NiHeida Dik]4–.
2
ꢀ
[NiHeida2Suc]4− < [NiHeida2Mal]4−
< [NiHeida2Ox]4−.
The yields of complexes formed according to these
equations are different for different systems. At the
same pH values, twoꢀligand complex predominates in
some systems and the differentꢀligand complex is the
major product in other systems.
The decrease in the stability of mixed nickel(II)
complexes is obviously related to the structure of
dicarboxylates. A decrease in the number of methylꢀ
ene groups in the dicarboxylic acid molecules is
known to suppress their complexing ability.
The latter complex is hydrolyzed in highly alkaline
media
Since in the [NiHOx]+ and [NiOx] complexes preꢀ
dominate in the Ni(II)–H2Heida–H2O system in
highly acidic media, the differentꢀligand complexꢀ
ation process can be represented as follows:
4−
NiHeida Dik +OH−
[
]
2
3−
ꢀ Ni OH HeidaDik + Heida2−.
(
)
[
]
[NiHOx]+ + Heida2–
[NiHeidaHOx]–.
The character of equilibria in these systems and the
composition and stability of the resulting complexes
can be interpreted by considering the structures of the
hydrated nickel(II) ion and the ligands, the symmetry,
and the type of bonds. In view of the fact that the
Ni(II) coordination number is 6 and the chelating
agent and dicarboxylic acids are triꢀ and bidentate
ligands, respectively, the structure of some complexes
can be represented by the following skeletal schemes:
ꢀ
In solutions of H2Mal and H2Suc, the opposite
process takes place, i.e., dicarboxylic acid (H2Dik
adds to nickel chelate:
)
[NiHeida] + HDik–
[NiHeidaHDik]–.
ꢀ
The subsequent reaction may follow two pathways:
[NiHeidaHDik]– [NiHeidaDik]2– + H+
ꢀ
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 57 No. 4 2012

NICKEL(II) COMPLEXES
~NCH2CH2OH
621
NCH2CH2OH
O
O
OH2
OH
O
O
O
O
O
O
Ni
O
O
O
O
Ni
Ni
Ni
O
O
O
O
NCH2CH2OH
[NiHeidaDik]2–
NCH2CH2OH
[NiHeida2Dik]4– [Ni(OH)HeidaDik]3–
~
NCH2CH2OH
NCH2CH2OH
[NiHeida2]2–
11. V. I. Kornev and V. A. Valyaeva, Zh. Fiz. Khim. 52 (7),
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