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CHAPTER 5
TERNARy COMPLEXES
197
SECTION – 1
STABILITY CONSTANTS OF TERNARY COMPLEXES
Introduction
Simultaneous equilibrium studies are of utmost important to
deal with biological, medical and environmental problems arising due
to advanced technical developments. Except in synthetic mixtures
encountered in laboratories, metal ions or ligands never exist in
isolated form1 necessitating an in depth investigation of multi-metal
and multi-ligand equilibria in solution. Thus the importance of such
studies is realized, as thorough investigation of all the species existing
in biosystem may not be possible due to the limitations of the present
available experimental methods to monitor the extent of complexation.
The metal ions exist in different forms2, 3 in different biofluids like
blood serum, intestinal fluid, cerebrospinal fluid, gastric juice etc.
They can be in non-exchangeable form, loosely bound to some
biological ligands and in equilibrium with a variety of bioligands with
similar metal ions in solution, and in aquated form. Thus, the
establishment of simultaneous equilibria involving a variety of metal
ions and ligands is possible in biological fluids.
Therefore, modeling studies involving ternary and quaternary
complexes gained importance. It means that the number and
concentrations of ligands outweigh those of the metal ions in many
biofluids. So the investigation of ternary complexes containing a metal
ion and two different ligands drew the attention of several researchers.
198
Mixed ligand complexes can be considered as models for apoenzyme-
metal ion-substrate complexes. In peptides, proteins and amino acids,
amide group is one of the important binding sites for the coordination
of the metal ions.
Another application of ternary complexes is in surgical cases
wherein it becomes necessary to depend on the Total Parenteral
Nutrition (TPN) for a sufficiently lengthy period. If proper mineral
balance is not maintained in the fluids, the metal ions that are
present in the physiological system will be gradually depleted. Hence,
it is a must that every nutritive solution should contain trace metal
ions such that their optimum concentrations in the body fluids are
maintained. The preparation of such a nutritive mixture requires the
knowledge of precise free metal ion concentration in body fluids and a
computer based reliable distribution of each metal ion in the presence
of a number of ligands and other metal ions present in different
concentrations.
Stability constants
Stability constants are based on the general equilibrium4 where
M is the central metal ion, H is the proton, L and X are first and
second ligands, respectively, in the complex and m, l, x, h and t are
stoichiometric coefficients.
mM + hH + lL + xX + tH2O Mm Hh Ll Xx (H2O)t
The corresponding stability constant is defined as
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x[X]l[L]h[H]m[M]
]t
O)2
(Hx
Xl
Lh
Hm
[Mβ =
Stability constants for the inorganic metal complexes and
minerals showed reasonable agreement between several independent
and critically reviewed sources.5, 6 In contrast to the inorganic metal
complexes and minerals, few data7 were available for metal complexes
with aspartic, citric, malonic, salicylic and tricarboxylic acids that
comprised the fatty acid models.
Literature survey
Literature survey shows that many of the earlier investigations
confined to the systems, whose pH regions for the formation of binary
and ternary species were widely different.8-10 In such cases the
concentrations of the binary complexes were assumed to be equal to
the total concentration of the metal ion. The formation constant of
ternary complexes was calculated in a similar way using modified
Irving equation.8a Simplified equations were arrived at by
Ramamoorthy and Santappa11 for the calculation of formation
constants of mixed ligand complexes, when the two ligands
simultaneously interacted with the metal ion. These are valid only if
the protonation constants of primary and secondary ligands are
comparable. But non-linear least squares algorithms were found to be
suitable for studying the unprotonated or protonated mixed ligand
200
complexes, even though the corresponding binary systems contain
hydroxylated and/or protonated species.
Formation constants of mixed ligand complexes of Cu(II), Ni(II),
Co(II) and Zn(II) with Aspartic or glutamic acid as primary ligand and
imidazole as secondary ligand were determined potentiometrically by
Sinha et al.12 Voltametric technique was used to study the binary and
ternary complexes of cadmium with L-lysine, L-ornithine, L-threonine,
L-serine, L-phenylalanine, L-glutamic acid and L-aspartic acid as
primary ligands and L-ascorbic acid as secondary ligand.13 Venkataiah
et al.14 studied the ternary complexes of Cu(II) with N-(1-naphthyl)
ethylenediamine, (N-N donors) and a series of amino acids (alanine,
phenylalanine, tryptophan, lysine, serine, threonine, aspartic acid or
histidine and ethylenediamine potentiometrically. Polarographic
technique was used to determine the stability constants of ternary
complexes of Zn(II) with L-lysine, L-ornithine, L-threonine, L-serine, L-
phenylglycine, L-phenylalanine, L-glutamic acid and L-aspartic acid as
primary ligands and γ-picoline as secondary ligand.15 Iman Ahmed16
determined the ternary systems of Zn(II), Ni(II), Co(II), Cd(II), Pb(II),
UO22+, Ce(III) and La(III) pH-metrically with N[tris(hydroxyl
methylmethyl]glycine(tricine), N-(2-acetamido)iminodi acetic acid and
dicarboxylic amino acids (aspartic and glutamic) as primary ligands
and 3-amino-5-mercapto-1, 2, 4-triazole as a secondary ligand.
Jussara and Judith17 studied ternary complexes of Cu(II) with
guanidinoacetic acid, glutamic and aspartic acids. Koteswar Rao and
201
Srinivas Mohan18 investigated ternary complexes of D- and L- aspartic
acid containing polyacrylamides and Cu(II) bovine serum albumin.
Potentiometric measurements19 have been made on the interaction of
glycine, serine, methionine, aspartic acid, glutamic acid and L-
histidine with Cu(II), Co(II), Ni(II), Mn(II) and Zn(II) in the presence of
biologically important secondary ligand zwitterionic buffers β-hydroxy-
morphocinepropanesulfonic acid (MOPS) and 3-bis(hydroxymethyl)
amino-2-hydroxypropanesulfonic acid (TAPSO). Christine and
Frantisek20 studied the ternary complexes of aspartic acid with Cu(II)
and phenanthroline ligands.
The speciation of selected environmentally relevant elements (H,
Na, K, Ca, Mg, Fe, Mn, U, Al, Pb, Zn, Cu, and Cd) in aqueous system,
with models of fulvic acid like aspartic, citric, malonic, salicylic and
tricarboxylic acids was studied.21 Venkataiah et al.22 studied the
interaction of adenosine-5–triphosphate (ATP) with a series of binary
Cu(II) complexes (MLn) [(where L = O-phenanthroline (Phen), 5-
nitrophenanthroline (NPhen), 5-methyl phenanthroline (Mphen), 2, 9-
dimethyl-phenanthroline (Dphen), 2, 9-dimethyl-4, 7-diphenyl
phenanthroline (Dphphen), oxalic acid (Ox), glycine (Gly), alanine
(Ala), valine (Val), phenylalanine (Phe), tryptophan (Trp), methionine
(Met), histidine (His) or aspartic acid (Asp)] to form ternary complexes
by pHmetric technique. With respect to the nitrogen donor ligands the
stability of the ternary complexes decreased in the order Nphen >
Phen > Mphe > Dphen > Dphphen, whereas in ternary complexes
202
containing amino acids the stability decreased in the order Phe > Trp
> Ala > Gly > Val > Met > His > Asp.
Mixed ligand complexation of Cu(II) with pyridoxine, isoleucine,
aspartic acid, glutamic acid and valine was studied by Saxena et al.23
who reported only MLX species. Janarthan et al.24 studied the mixed
ligand complexes of UO22+ with aspartic acid as primary ligand and
oxalic, succinic, malonic and lactic acids as secondary ligands
potentiometrically. Although the mixed ligand complexes reported
earlier were of the type, M(bipy)(amino acid) or M(amino acid)(catechol),
during the last decade there was increased interest in systems with two
amino acids, one serving as primary and the other as secondary ligand.
Sakurai et al.25 studied the ternary complexes of Asp, L-lysine,
L-ornithine, glumatic acid and L-arginine. Scheller et al.26 studied the
ternary complexes of Cu(II) with Asp as primary ligand and cytidine-5-
monophosphate (CMP) as secondary ligand. Venkatachalapathi et al.27
studied about the Cr(III) complexes with Asp as primary ligand and
Glu or L (+)-cysteine as secondary ligand. A brief account of some of
the typical systems reported in the literature is given in Table 5.1.
Both the protonation28, 29 and binary complexes30-33 of L-
aspartic acid and ethylenediamine in solvents Dox and PG were
already studied in our laboratory. Some of the present values are
comparable with those reported earlier studies. So the author has
presented ternary complexes of Asp and en in this chapter.
203
Table 5.1: Mixed ligand complexes reported in literature.
System 1110 1210 1111 1112 1120 1121 1122 (µ) Ref.
(Asp)-Cd(II)-(Glu) 4.37 - 18.59 - - - - 0.1 13
(Suc)-UO22+-(Asp) 9.60 - - - - - - 0.1 24
(L-Asp)-Cu(II)-(L-Lys) 15.82 - 26.32 - - - - 0.1 25
(L-Asp)-Cu(II)-(L-Orn) 15.29 - 25.65 - - - - 0.1 25
(L-Arg)-Cu(II)-(L-Asp) - - 27.43 - - - - 0.1 25
(L-Arg)-Cu(II)-(D-Asp) - - 27.47 - - - - 0.1 25
(L-Arg)-Cu(II)- (L-Glu) - - 26.61 - - - - 0.1 25
(L-Lys)-Cu(II)-(D-Asp) 15.67 - 26.27 - - - - 0.1 25
(Asp)-Cu(II)-H(CMP) - - 12.01 - - - - 0.1 26
(Asp)-Cu(II)-H2(CMP) - - - 27.25 - - - 0.1 26
(Arg)-Ni(II)-(His) 16.63 - 28.26 - - - - 0.16 27
(Asp)-Cr(III)-(Cys) 17.85 - - - - - - 0.1 27
(Asp)-Cr(III)-(Glu) 18.22 - 28.52 - - 35.74 - 0.1 27
(L-Asp)-Co(II)-(en) 11.57 14.17 - - 14.83 24.29 31.82 0.16 34
(L-Asp)-Ni(II)-(en) 13.89 16.92 - - 18.35 26.62 32.46 0.16 34
(L-Asp)-Cu(II)-(en) 17.81 20.98 - - 21.97 30.40 37.11 0.16 34
(L-Asp)-Cu(II)-(EDTA) - - 23.57 - - - - 0.1 35
(L-Arg)-Zn(II)-(Cys) - - 19.99 - - - - 0.15 36
(L-Arg)-Zn(II)-(L-His) - - - - - - - 0.1 36
(Asp)-Zn(II)-(Orn) 12.28 - 19.74 - - - - 0.1 37
204
Alkalimetric titration data
The alkalimetric titration curves of mixtures containing different
mole ratios of Asp and en in the presence of metal, mineral acid and
inert electrolyte are given in Figs. 5.1-5.7. A preliminary investigation
of the alkalimetric titration data inferred that these two ligands do not
form any condensed species.38, 39 Assuming that there is no expansion
of the coordination sphere and three bidentate ligands are sufficient to
satisfy the coordination number of the metal ion, the total number of
primary and secondary ligands together was restricted to a maximum
of three in generating the ternary species for modeling. The possible
primary and secondary ligand forms and resulting ternary complex
species are given in Table 5.2.
Table 5.2: Some of the possible ternary complex species of Asp- M(II)-en system. Constraints: 1. Maximum number of primary ligands = 2
2. Maximum number of secondary ligands = 2 3. Primary + Secondary ligands = 3
S. No. Ligand form Ligand number
Primary Secondary 2 3
1 LH3 XH2 MLXH3 ML2XH5 MLX2H4
2 LH3 XH MLXH2 ML2XH4 MLX2H2
3 LH2 XH2 MLXH2 ML2XH3 MLX2H3
4 LH2 XH MLXH ML2XH2 MLX2H
5 LH XH2 MLXH ML2XH MLX2H2
3 LH XH MLX ML2X MLX2
205
Fig. 5.1: Alkalimetric titration curves of Asp-M(II)-en ternary complexes
in aqueous medium. (A) Co(II), (B) Ni(II) and (C) Cu(II).
0 3 6 9
3
6
9
12321
(A)
pH
Vol. of NaOH (mL)
0 3 6 9
3
6
9
12321
(B)
pH
Vol. of NaOH (mL)
0 3 6 9
3
6
9
12321
(C)
pH
Vol. of NaOH (mL)
206
0 3 6 9
3
6
9
12 (F)321
pH
Vol. of NaOH (mL)
Fig. 5.2: Alkalimetric titration curves of Asp-Co(II)-en ternary
complexes in Dox-water mixtures (A) 10.0, (B) 20.0, (C) 30.0, (D) 40.0, (E) 50.0 and (F) 60.0% v/v. Number of mmols of ligands : 1) 0.25, 0.25 2) 0.25, 0.50 3) 0.50, 0.25
0 3 6 9
3
6
9
12 (A)321
pH
Vol. of NaOH (mL)
0 3 6 9
3
6
9
12 (B)321
pH
Vol. of NaOH (mL)
0 3 6 9
3
6
9
12 (C)321
pH
Vol. of NaOH (mL)
0 3 6 9
3
6
9
12 (D)321
pH
Vol. of NaOH (mL)
0 3 6 9
3
6
9
12 (E)321
pH
Vol. of NaOH (mL)
207
0 3 6 9
3
6
9
12 (F)321
pH
Vol. of NaOH (mL)
Fig. 5.3: Alkalimetric titration curves of Asp-Ni(II)-en ternary complexes
in Dox-water mixtures (A) 10.0, (B) 20.0, (C) 30.0, (D) 40.0, (E) 50.0 and (F) 60.0% v/v. Number of mmols of ligands : 1) 0.25, 0.25 2) 0.25, 0.50 3) 0.50, 0.25
0 3 6 9
3
6
9
12 (A)321
pH
Vol. of NaOH (mL)
0 3 6 9
3
6
9
12 (B)321
pH
Vol. of NaOH (mL)
0 3 6 9
3
6
9
12 (C)321
pH
Vol. of NaOH (mL)
0 3 6 9
3
6
9
12 (D)321
pH
Vol. of NaOH (mL)
0 3 6 9
3
6
9
12 (E)321
pH
Vol. of NaOH (mL)
208
0 3 6 9
3
6
9
12 (F)321
pH
Vol. of NaOH (mL)
Fig. 5.4: Alkalimetric titration curves of Asp-Cu(II)-en ternary
complexes in Dox-water mixtures (A) 10.0, (B) 20.0, (C) 30.0, (D) 40.0, (E) 50.0 and (F) 60.0% v/v. Number of mmols of ligands : 1) 0.25, 0.25 2) 0.25, 0.50 3) 0.50, 0.25
0 3 6 9
3
6
9
12 (A)321
pH
Vol. of NaOH (mL)
0 3 6 9
3
6
9
12 (B)321
pH
Vol. of NaOH (mL)
0 3 6 9
3
6
9
12 (C)321
pH
Vol. of NaOH (mL)
0 3 6 9
3
6
9
12 (D)321
pH
Vol. of NaOH (mL)
0 3 6 9
3
6
9
12 (E)321
pH
Vol. of NaOH (mL)
209
0 3 6 9
3
6
9
12 (F)321
pH
Vol. of NaOH (mL)
Fig. 5.5: Alkalimetric titration curves of Asp-Co(II)-en ternary
complexes in PG-water mixtures (A) 10.0, (B) 20.0, (C) 30.0, (D) 40.0, (E) 50.0 and (F) 60.0% v/v. Number of mmols of ligands : 1) 0.25, 0.25 2) 0.25, 0.50 3) 0.50, 0.25
0 3 6 9
3
6
9
12 (A)321
pH
Vol. of NaOH (mL)
0 3 6 9
3
6
9
12 (B)321
pH
Vol. of NaOH (mL)
0 3 6 9
3
6
9
12 (C)321
pH
Vol. of NaOH (mL)
0 3 6 9
3
6
9
12 (D)321
pH
Vol. of NaOH (mL)
0 3 6 9
3
6
9
12 (E)321
pH
Vol. of NaOH (mL)
210
0 3 6 9
3
6
9
12 (F)321
pH
Vol. of NaOH (mL)
Fig. 5.6: Alkalimetric titration curves of Asp-Ni(II)-en ternary complexes
in PG-water mixtures (A) 10.0, (B) 20.0, (C) 30.0, (D) 40.0, (E) 50.0 and (F) 60.0% v/v. Number of mmols of ligands : 1) 0.25, 0.25 2) 0.25, 0.50 3) 0.50, 0.25
0 3 6 9
3
6
9
12 (A)321
pH
Vol. of NaOH (mL)
0 3 6 9
3
6
9
12 (B)321
pH
Vol. of NaOH (mL)
0 3 6 9
3
6
9
12 (C)321
pH
Vol. of NaOH (mL)
0 3 6 9
3
6
9
12 (D)321
pH
Vol. of NaOH (mL)
0 3 6 9
3
6
9
12 (E)321
pH
Vol. of NaOH (mL)
211
0 3 6 9
3
6
9
12 (F)321
pH
Vol. of NaOH (mL)
Fig. 5.7: Alkalimetric titration curves of Asp-Cu(II)-en ternary
complexes in PG-water mixtures (A) 10.0, (B) 20.0, (C) 30.0, (D) 40.0, (E) 50.0 and (F) 60.0% v/v. Number of mmols of ligands : 1) 0.25, 0.25 2) 0.25, 0.50 3) 0.50, 0.25
0 3 6 9
3
6
9
12 (A)321
pH
Vol. of NaOH (mL)
0 3 6 9
3
6
9
12 (B)321
pH
Vol. of NaOH (mL)
0 3 6 9
3
6
9
12 (C)321
pH
Vol. of NaOH (mL)
0 3 6 9
3
6
9
12 (D)321
pH
Vol. of NaOH (mL)
0 3 6 9
3
6
9
12 (E)321
pH
Vol. of NaOH (mL)
212
Selection of best fit models
The qualitative evidence for the formation of mixed ligand
complexes was obtained from the shift of the precipitation point of mixed
ligand systems compared to those of the corresponding binary systems.
In all these systems, the pH for precipitation of the mixed ligand systems
was found to be more than that for any of the binary system.
The formation constants for acido-basic equilibria of both the
primary and the secondary ligands and those for the binary metal
complexes were fixed in testing various chemical models using
MINIQUAD7540 program. All the species cited in Table 5.2 were used to
generate different models.
The existence of these species was determined by performing an
exhaustive modeling study and the results of a typical system are given
in Table 5.3. The models were evaluated assuming the simultaneous
existence of different combinations of species. Models containing various
number and combinations of species were generated using an expert
system package CEES41 and these models were refined using
MINIQUAD75. As the number of species increased, the models gave
better statistical data denoting the best fit. This indicates that the final
model appropriately fits the experimental data. Such exhaustive
modeling was performed for all the systems and the final models are
given in Tables 5.4 and 5.5 for ternary complexes of Asp and en with
213
Co(II), Ni(II) and Cu(II) in Dox- and PG-water mixtures. The species
detected were MLX, ML2X2- and MLXH+ for Co(II), Ni(II) and Cu(II) in Dox-
and PG-water mixtures.
Table 5.3: Exhaustive modeling study performed on Asp-Ni(II)-en ternary
complexes in aqueous medium, pH range = 4.0-10.0, NP=96; Temperature= 303.0±0.1 K, µ= 0.16 mol L-1.
Model No.
log β(SD) Ucorr
x108
Skewness
χ2
Kurtosis
R-factor
MLX ML2X2- MLXH+
1 11.96(12) - - 3.84 3.46 0.81 4.82 .0008
2 - 22.18(15) - 3.44 5.32 0.20 10.65 .0012
3 - - 27.58(7) 2.70 4.70 -1.26 76.96 .0015
4 12.91(12) 21.30(20) - 2.04 1.15 -1.06 2.43 .0109
5 13.56(8) - 24.33(6) 1.80 2.46 0.97 4.26 .0006
6 - 20.73(16) 22.12(8) 1.69 3.42 1.29 28.91 .0075
7 15.01(32) 19.87(17) 21.15(10) 0.84 1.60 23.75 7.73 .0177
214
Table 5.4: Best fit chemical models of ternary complexes of Co(II), Ni(II) and Cu(II) with Asp and en in Dox-water mixture. Temperature= 303.0 K, µ= 0.16 mol L-1.
%v/v
Dox
log βmlxh(SD)
NP
Ucorr
x108
χ2
Skewness
Kurtosis
R-factor 1111 1110 1210
Co(II) (pH=4.0-10.0) 00.0 19.33(12) 11.88(8) 15.51(23) 23 0.82 12.35 -0.37 3.65 .0039 10.0 19.89(29) 12.22(34) 16.01(25) 52 6.81 51.08 1.74 9.72 .0177 20.0 19.98(36) 12.91(78) 16.36(56) 25 10.77 69.28 3.07 5.63 .0289 30.0 20.19(13) 13.21(18) 16.85(17) 33 8.73 59.03 2.49 8.97 .0169 40.0 20.27(11) 13.85(28) 17.52(15) 24 7.19 39.00 2.62 12.61 .0227 50.0 20.68(27) 14.03(22) 18.67(37) 25 11.66 42.72 2.44 10.44 .0346 60.0 21.01(8) 15.17(8) 19.328(8) 23 0.85 12.35 -0.37 3.65 .0039
Ni(II) (pH=4.0-10.0) 00.0 21.15(10) 15.01(32) 19.87(17) 96 0.84 23.75 1.60 7.73 .0177 10.0 21.89 (12) 15.39(11) 20.44(13) 30 0.72 9.07 1.49 6.31 .0040 20.0 22.00(14) 15.81(14) 20.78(14) 77 3.00 14.55 -0.50 3.72 .0099 30.0 22.44(12) 16.17(24) 20.91(18) 24 0.75 6.50 0.01 3.56 .0044 40.0 22.85(15) 16.64(18) 21.18(24) 29 1.38 22.43 2.04 9.30 .0061 50.0 23.06(12) 17.29(12) 22.23(20) 22 0. 85 5.82 2.14 3.53 .0066 60.0 23.89(16) 18.91(13) 22.98(19) 24 0. 82 19.67 0.40 3.06 .0064
Cu(II) (pH=2.5-11.0) 00.0 28.07(20) 23.49(55) 30.39(53) 33 2.66 7.15 0.64 4.62 .0115 10.0 27.25(12) 22.38(24) 26.30(31) 27 1.91 2.07 -0.53 4.06 .0059 20.0 27.81(13) 22.25(17) 25.57(16) 98 3.49 34.98 -1.22 4.45 .0221 30.0 26.95(24) 21.64(55) 24.93(68) 42 9.53 14.76 -0.88 3.65 .0126 40.0 26.22(43) 21.50(41) 25.25(52) 25 6.64 31.20 -2.34 10.45 .0168 50.0 26.08(20) 21.25(14) 25.63(18) 40 1.21 30.40 -1.45 3.86 .0173 60.0 25.55(62) 21.12(19) 23.73(44) 30 2.73 67.70 -2.27 8.87 .0101
215
Table 5.5: Best fit chemical models of ternary complexes of Co(II), Ni(II) and Cu(II) with Asp and en in PG-water mixture. Temperature= 303.0 K, µ= 0.16 mol L-1.
%v/v
PG
log βmlxh(SD)
NP
Ucorr
x108
χ2
Skewness
Kurtosis
R- factor
1111 1110 1210
Co(II) (pH=5.0-10.0) 00.0 19.33(12) 11.88(8) 15.51(23) 23 0.85 12.35 -0.37 3.65 .0039 10.0 19.46(12) 12.15(12) 16.16(12) 37 1.52 116.4 2.33 7.97 .0109 20.0 19.67(22) 12.64(28) 16.70(20) 35 2.68 69.28 3.07 12.75 .0289 30.0 19.87(12) 12.83(12) 16.86(12) 48 1.90 74.00 3.46 17.27 .0526 40.0 19.99(12) 12.86(12) 16.93(12) 32 0.16 34.88 1.10 3.71 .0432 50.0 20.18(12) 13.34(12) 17.10(12) 21 1.14 12.94 2.01 8.80 .0064 60.0 20.78(12) 13.64(12) 18.37(12) 23 1.52 12.36 1.75 7.55 .0132 Ni(II) (pH=4.0-10.0)
00.0 21.15(10) 15.01(32) 19.87(17) 96 7.84 23.75 1.60 7.73 .0177 10.0 21.95(70) 15.39(49) 19.97(42) 34 9.38 22.09 2.12 8.57 .0163 20.0 22.28(24) 15.59(22) 20.01(39) 36 6.18 14.55 -0.15 3.01 .0134 30.0 22.33(25) 15.72(28) 20.24(36) 35 3.14 32.33 2.47 11.20 .0094 40.0 22.60(18) 16.02(15) 20.40(19) 25 3.28 4.32 -0.22 3.09 .0093 50.0 22.61(11) 16.33(12) 20.71(15) 26 0.60 23.08 2.79 15.08 .0041 60.0 23.27(32) 17.11(63) 22.60(54) 27 9.54 12.59 2.61 10.07 .0178 Cu(II) (pH=2.0-8.0)
00.0 30.31(24) 25.56(45) 32.65(27) 30 2.66 5.87 -0.19 3.36 .0096 10.0 30.09(10) 25.61(25) 33.12(22) 37 3.29 4.63 -0.36 3.07 .0085 20.0 30.83(30) 25.75(35) 33.43(33) 33 2.83 16.11 -0.42 3.38 .0117 30.0 31.00(42) 26.28(49) 33.58(47) 59 4.58 48.07 -0.38 3.58 .0199 40.0 31.22(14) 26.49(40) 34.88(23) 36 1.07 32.44 2.01 10.93 .0083 50.0 31.38(13) 27.66(13) 35.01(13) 37 0.20 17.51 -0.06 4.84 .0016 60.0 32.24(4) 28.18(8) 35.24(7) 39 0.95 10.15 -0.46 3.09 .0034
216
A very low standard deviation in log β values indicates the
precision of the parameters. The small values of Ucorr indicate the
consistency of the model with the experimental data.42 The kurtosis
values are around 3 for most of the systems. Hence the residuals form
mesokurtic pattern. Since the kurtosis values are more than 3 for most
of the systems they form leptokurtic pattern. The skewness values -2.34
to 3.07 and -0.46 and 3.46 in Dox- and PG-water mixtures, respectively,
show that the residuals form a part of normal distribution and hence a
least squares method can be applied to the present data. The sufficiency
of the model is further evident from the low crystallographic R factor
values which indicate the need for inclusion of additional species in the
model. χ2 is a special case of distribution which measures the probability
of residuals forming a part of standard normal distribution.43 The
reasons for the existence of different species are ascribed under the head
distribution diagrams.
217
Effect of systematic errors
Errors were introduced into the concentrations of the ingredients
intentionally to assess their effect on the perturbation of the stability
constants. If the concentrations of ingredients determined and the
experimental conditions maintained are appropriate, any variation in the
concentrations of ingredients will affect the magnitudes of stability
constants and worsen the statistical parameters. Even the species shall
be rejected some times. The results of typical samples given in Table 5.6
emphasize that the errors in the concentrations of alkali and acid affect
the stability constants more than those of the ligands and metal ions.
The increased SD’s and rejection of some species on the introduction of
errors in the concentrations clearly infer the appropriateness of the
experimental conditions.
218
Table 5.6: Effect of errors in influential parameters on the stability constants of Asp-Co(II)-en ternary complexes in 50% v/v of co-solvent-water mixtures.
Ingredient %
error
log βmlxh(SD)
1111 1110 1210
Dox
0 20.68(27) 14.03(22) 18.67(37)
Alkali
-5
Rejected
13.20(12)
Rejected
-2 19.23(7) 12.12(16) 16.11(4)
+2 19.92(41) Rejected Rejected
+5 Rejected Rejected Rejected
Acid -5 20.01(5) 14.02(9) 18.30(6)
-2 18.90(23) Rejected 18.96(20)
+2 Rejected 14.71(35) 14.82(99)
+5 18.35(43) Rejected Rejected
Asp -5 19.73(5) 13.92(11) 17.90(2)
-2 19.15(8) 14.07(4) 17.05(27)
+2 18.20(25) 14.02(42) Rejected)
+5 17.97(12) Rejected 15.83(9)
en -5 18.56(3) 13.06(7) 17.87(2)
-2 18.12(26) Rejected 17.23(32)
+2 Rejected 13.99(7) Rejected
+5 19.08(8) 13.69(5) 17.29(3)
Metal -5 19.99(3) 14.26(5) 18.09(2)
+2
+2
19.27(4)
19.27(4)
14.06(5)
14.06(5)
18.14(12)
18.14(12)
+5 19.69(5) 14.163) 18.24(11)
219
PG
0
20.18(12)
13.34(12)
17.10(12)
Alkali -5 Rejected 13.71(29) Rejected
-2 19.11(48) 15.70(38) 16.23(27)
+2 Rejected 15.00(35) 19.92(41)
+5 19.15(29) Rejected Rejected
Acid -5 19.31(16) 12.82(29) 18.33(29)
-2 18.90(23) Rejected 18.96(20)
+2 Rejected 14.71(35) 14.82(99)
+5 18.35(43) Rejected Rejected
Asp -5 18.73(35) 14.52(67) 16.30(30)
-2 18.25(26) 14.17(46) 17.85(27)
+2 18.20(25) 14.02(42) 16.15(36)
+5 17.97(28) 13.98(38) 15.83(79)
en -5 17.56(30) 13.86(17) 17.74(42)
-2 18.12(26) 13.97(35) 17.23(32)
+2 18.59(41) 13.98(78) 17.82(43)
+5 19.98(38) 13.69(58) 17.29(40)
Metal -5 19.55(30) 14.20(53) 18.99(28)
-2 19.43(29) 14.04(48) 18.02(31)
+2 19.27(54) 14.06(57) 18.14(62)
+5 19.14(24) 14.64(37) 18.59(39)
220
Stability of ternary complexes
The formation of mononuclear unprotonated binary and ternary
complexes from a mixture of metal ion (M) and primary (L) and secondary
(X) ligands is shown in Eq. 5.1.
.….5.1
The change in the stability of the ternary complexes as compared
to their binary analogues was quantified45-48 based on the
disproportionation constant (log X) given by Eq. 5.2.
which corresponds to the equilibrium
ML2 + MX2 2 MLX ….5.3
Under the equilibrium conditions one can expect 50% ternary
complex and 25% each of the binary complexes to be formed and the
value of log X was reported49 to be 0.6. A value greater than this,
accounts for the extra stability of MLX.
M + L+ X ML + X
K ML
M
MX + L MLX
M
MX K
M
MLX K
M
ML K
M K
X
X
L MLX
…..5.2 log log log 2 log 2 2
M MX
M ML
M MLX K K K X − − =
221
Another approach47, 50 to quantify the stability of ternary complexes
was based on the difference in stability (Δ log K) for the reactions ML with
X and M(aq) with L and X, where L is the primary ligand and X is the
secondary ligand. It is compared with that calculated purely on the
statistical grounds. Eq. 5.4 can be formulated based on the properties of
the cyclic systems reported earlier50, 51, from which it is clear that both
the ligands in the ternary complex influence mutually to the same extent.
....5.4
The electrostatic theory of binary complex formation and statistical
arguments suggest the additional coordination positions of given
multivalent hydrated metal ion available for the first ligand than for the
second. Hence, the usual order of stability MMLK > ML
MLXK applies. This
suggests that Δ log K should be negative, although several exceptions52
have been found. The statistical values of Δ log K for bidentate L and X
are -0.4, -0.6 and between -0.9 and -0.3 for octahedral, square planar
and distorted octahedral complexes, respectively. Negative values of Δ log
K can be understood as the secondary ligand forms a more stable
complex with hydrated metal ion than with ML.
Whenever the experimental values of Δ log K exceed the statistical
values, it can be inferred that the ternary complex is formed as a result
of interaction of ML with X or MX with L. Δ log K values of ternary
MMX
MML
MMLX KKKK loglogloglog −−=∆
222
complexes containing bipyridyl as the primary ligand are positive47 for O-
donors (malonic acid, pyrocatechol etc.), negative50 for N-donors
(ethylene diamine) and intermediate or negative53 for amino acids with
both N and O co-ordination sites. However, a very high negative value (-
2.3) for Cu(en)(iminodiacetic acid) and a positive value (0.82) for
Cu(phen)(6,7-dihydroxynaphthalene-2-sulphonate) was also observed.
The equations for the calculation of Δ log K and log X are given in
Table 5.7. These values calculated from the binary and ternary
complexes are shown in Table 5.8. The values for some of the systems
could not be calculated due to the absence of relevant binary species. In
the present study, the values of Δ log K range from -15.56 to 3.67 and -
6.67 to 7.06 for Dox- and PG-water mixtures, respectively.
Table 5.7: Equations for the calculation of Δ log K and log X values from the overall stability constants
Δ log K1110 Δ log K1210 Δ log K1111
=log β1110 =log β1210 =log β1111
-log β1100 -log β1200 -log β1101
-log β1010 -log β1010 -log β1010
log X1110 =2log β1110 -log β1200 -log β1020 log X1210 =2log β1210 -log β1400 -log β1020 log X1111 =2log β1111 -log β1202 -log β1020
223
Table 5.8: Δ log K and log X values of ternary complexes of Co(II), Ni(II) and Cu(II)-Asp and en in Dox- and PG-water mixtures.
% v/v Dox Δ log K log X
Co(II) 1110 1210 1110 1111
0.0 -0.23 -5.78 2.47 2.71 10.0 -1.02 -6.99 1.44 3.13 20.0 -0.36 -6.54 2.08 3.87 30.0 -0.35 -7.31 2.56 3.46 40.0 -0.24 -7.51 1.39 3.62 50.0 -0.09 -6.39 2.92 4.79 60.0 0.09 -7.59 3.13 3.83 Ni(II) 0.0 0.13 -6.47 3.68 4.69 10.0 0.16 -6.86 3.48 5.90 20.0 0.26 -6.49 4.35 5.89 30.0 0.07 -7.82 3.61 5.88 40.0 -0.06 -8.58 3.52 5.71 50.0 0.87 -7.61 4.74 5.04 60.0 1.69 -8.52 6.32 3.94 Cu(II) 0.0 3.67 -5.50 11.09 11.29 10.0 2.34 -10.0 8.46 9.57 20.0 1.63 -10.95 7.98 10.50 30.0 0.10 -12.39 5.96 6.92 40.0 -0.15 -13.17 4.58 4.90 50.0 0.06 -11.74 5.13 5.77 60.0 -1.35 -15.56 2.95 2.20
% v/v PG Δ log K log X Co(II) 1110 1210
0.0 -0.47 -5.77 2.48 2.71 10.0 -0.39 -5.75 2.60 3.27 20.0 -0.37 -6.38 2.20 2.73 30.0 -0.29 -5.75 3.05 3.15 40.0 -0.51 -6.67 2.12 3.69 50.0 -0.03 -6.50 2.86 5.05 60.0 -0.19 -6.22 2.69 4.67
Ni(II) 0.0 0.13 -5.19 3.68 4.69 10.0 -0.58 -5.56 3.27 4.40 20.0 -0.33 -5.12 3.78 4.76 30.0 -0.22 -5.49 3.62 5.65 40.0 -0.41 -6.24 3.20 5.69 50.0 -0.34 -6.25 3.62 6.12 60.0 0.51 -6.08 4.87 7.64
Cu(II) 0.0 3.13 -3.24 15.23 15.77 10.0 6.28 -2.49 15.61 15.78 20.0 5.21 -4.93 13.14 - 30.0 4.96 -4.50 14. 48 16.14 40.0 5.38 -2.76 15.34 16.46 50.0 6.21 -3.22 17.09 15.51 60.0 7.06 -2.46 18.66 17.55
224
Co(II), Ni(II) and Cu(II) ions form octahedral complexes with Asp
and en. For most of the systems the values of Δ log K are found to be
higher (Table 5.9) than those expected on statistical grounds (-0.4).
These higher values account for the extra stability of the ternary
complexes compared to the corresponding binary complexes.
In the present study the values of log X range from 1.39 to 11.29
and 2.12 to 18.66 in Dox- and PG-water mixtures (Table 5.9),
respectively. The log X values indicate that ternary complexes are more
stable than the corresponding binary complexes because most of these
values are higher than the theoretical value (0.6).
225
SECTION - 2
EFFECT OF CO-SOLVENT
Variation of logarithmic values of stability constants (log β) are
shown for both Dox- and PG-water mixtures in Figs. 5.8 and 5.9. These
plots indicate the nature of electrostatic and non-electrostatic forces
operating in the equilibria.
The linear trend indicates that either the dielectric constant or the
long range interactions are responsible for the trend in stability. This
linear trend in log β values indicates the dominance of the structure
forming nature of Dox/PG over the complexing ability.
The Dox-water mixtures are the combination of aprotic and protic
solvents with a wide range of dielectric constant and with high solubility
for polar as well as non-polar solutes. The increased basicity of
Dox54 water mixtures, induced by co-solvent, stabilizes the protons. At
the same time the coordinating solvent (Dox) competes with the ligands
for coordination with the metal ions and this property of Dox decreases
the stability of the complexes. Hence, the stability of the complex is
expected to either increase or decrease.
PG is an amphiprotic solvent. It is a structure former and it
enhances the water structure in PG-water mixtures; hence, it removes
water from the coordination sphere of metal ions, making them more
226
reactive towards the ligands. As a result, the stability of the complexes is
expected to increase. At the same time, it is a coordinating solvent and it
competes with the ligands for coordinating the metals. This decreases the
stability of the complexes. Hence, the stability of the complexes is
expected to either increase or decrease linearly.
The variation of overall stability constants with co-solvent content
depends upon electrostatic and non-electrostatic factors. Born’s classical
treatment holds good in accounting for the electrostatic contribution to
the free energy change.55 According to this treatment, the energy of
electrostatic interaction is related to dielectric constant. Hence, the log β
values should vary linearly as a function of reciprocal of the dielectric
constant (1/D) of the medium.
227
Fig. 5.8: Variation of stability constants of Asp-metal-en ternary complexes in Dox-water mixtures.(A) Co(II), (B) Ni(II) and (C) Cu(II); (□) log βMLX, (○) log βML2X and (Δ) log βMLXH.
0.012 0.024 0.03620
22
24
26
28
30C
log β
1/D
0.012 0.018 0.024 0.03014
16
18
20
22
24B
log β
pH
0.012 0.018 0.024 0.030 0.03612
14
16
18
20
22 A
log β
pH
228
0.012 0.016 0.020
12
14
16
18
20
A
log β
1/D
0.012 0.016 0.020
16
18
20
22
24B
log β
1/D
Fig. 5.9: Variation of stability constants of Asp-metal-en ternary
complexes in PG-water mixtures. (A) Co(II), (B) Ni(II) and (C) Cu(II); (□) log βMLX, (Δ) log βML2X and (○) log βMLXH.
0.012 0.016 0.020
27
30
33
36 C
log β
1/D
229
SECTION - 3
DISTRIBUTION DIAGRAMS
The distribution diagrams indicate the relative abundance of
various forms of metal ions (chemical speciation) at different pH’s and
dielectric conditions. A stable ternary complex shall be responsible for
metal ion transportation in bio-systems and the weak binary metal
complexes make the essential metal ions bioavailable. The increased
concentrations of complexing agents make the essential metal ions
unavailable due to the formation of stable metal complexes.
In PG- and Dox-water mixtures
The distribution diagrams were drawn using the formation
constants of the best fit model (Figs. 5.10-5.16). They reveal that the
concentrations of binary complexes are less than those of the ternary
species, due to the extra stability of the ternary complexes. The ternary
complex species of Asp (L) and en (X) in Dox- and PG-water mixtures are
MLX, ML2X2- and MLXH+. The protonated ternary species, MLXH+ exist at
lower pH than the unprotonated ternary species, MLX and ML2X2-. The
formation of these complex species can be represented by the following
equilibria.
230
Fig. 5.10: Distribution diagrams of Asp-M(II)-en complexes in aqueous medium. (A) Co(II), (B) Ni(II) and (C) Cu(II).
6 8 100
20
40
60
80
100(A)
% s
peci
es
pH
FM
MLXML
ML2H2
MLXH
ML2X
4 6 8 100
20
40
60
80
100
ML
(B)
ML2H2 MLXH
MLX
% s
peci
es
pH
MX
ML2X
FM
2 4 6 80
20
40
60
80
100 (C)
MLX
MLXH
FM
MX
ML2X
% s
peci
es
pH
231
Fig. 5.11: Distribution diagrams of Asp-Co(II)-en complexes in PG-
water mixtures % v/v (A) 0.0 (B) 10.0, (C) 20.0, (D) 30.0, (E) 40.0, (F) 50.0 and (G) 60.0.
6 8 100
20
40
60
80
100
(A)
MLXH ML2XMLX
ML
FMML2H2
pH
% s
peci
es
6 8 100
20
40
60
80
100(B)
%
spe
cies
pH
FM
MLX
MLML2H2
MLXHML2X
6 8 100
20
40
60
80
100(C)
% s
peci
es
pH
FM
MLXML
ML2H2
MLXH
ML2X
6 8 100
20
40
60
80
100(D)
% s
peci
es
pH
FM
MLXML
ML2H2
MLXH
ML2X
6 8 100
20
40
60
80
100(E)
% s
peci
es
pH
FM
MLXML
ML2H2
MLXH
ML2X
6 8 100
20
40
60
80
100(F)
ML2
% s
peci
es
ML2H2
ML
MLXH
MLX
ML2X
FM
pH
232
Fig. 5.12: Distribution diagrams of Asp-Ni(II)- en complexes in PG-
water mixtures % v/v (A) 0.0 (B) 10.0, (C) 20.0, (D) 30.0, (E) 40.0, (F) 50.0 and (G) 60.0.
4 6 8 100
20
40
60
80
100 (B)
MLXMLXHML
ML2H4
FM
ML2X
MX2
% s
peci
es
pH
4 6 8 100
20
40
60
80
100
ML
(A)
ML2H2 MLXH
MLX
% s
peci
es
pH
MX
ML2X
FM
4 6 8 100
20
40
60
80
100
MX2
(C)
MLXMLXHML
ML2H4
FM
ML2X
% s
peci
es
pH4 6 8 10
0
20
40
60
80
100(D)
MLX
ML2H4
ML
ML2X
MLXH
FM%
spe
cies
pH
4 6 8 100
20
40
60
80
100(E)
pH
% s
peci
es
MLX
FM
ML2H2
ML MLXH
ML2X
4 6 8 100
20
40
60
80
100(F)
% s
peci
es
ML2
MLXH
MLX
FM
ML2H2
ML
ML2X
pH
233
Fig. 5.13: Distribution diagrams of Asp-Cu(II)-en complexes in PG-water
mixtures % v/v (A) 0.0 (B) 10.0, (C) 20.0, (D) 30.0, (E) 40.0, (F) 50.0 and (G) 60.0.
2 4 6 80
20
40
60
80
100 (A)
MLX
MLXH
FM
MX
ML2X
% s
peci
es
pH2 4 6 8
0
20
40
60
80
100(B)
pH
% s
peci
es
FMMLX
MLXH ML2X
2 4 6 80
20
40
60
80
100(C)
pH
% s
peci
es
FM
MLX
MLXH ML2X
2 4 6 80
20
40
60
80
100(E)
pH
% s
peci
es
MLX
FM
MLXHML2X
2 4 6 80
20
40
60
80
100(D)
pH
% s
peci
es
FM
MLX
MLXH ML2X
2 4 6 80
20
40
60
80
100(F)
pH
% s
peci
es
MLX
FM
MLXHML2X
234
Fig. 5.14: Distribution diagrams of Asp-Co(II)-en complexes in Dox-water
mixtures % v/v (A) 10.0, (B) 20.0, (C) 30.0, (D) 40.0, (E) 50.0 and (F) 60.0.
4 6 8 100
20
40
60
80
100
FM
(A)
% o
f spe
cies
pH
ML2XMLXMLXH
4 6 8 100
20
40
60
80
100(B)
FM
pH
ML2X
MLX
MLMLXH
% o
f spe
cies
4 6 8 100
20
40
60
80
100 (C)
FM
pH
ML2X
MLX
MLMLXH
% o
f spe
cies
4 6 8 100
20
40
60
80
100 (D)
FM
pH
ML2X
MLX
MLMLXH
% o
f spe
cies
4 6 8 100
20
40
60
80
100 (E)
FM
pH
ML2X
MLX
ML MLXH
% o
f spe
cies
4 6 8 100
20
40
60
80
100 (F)
FM
pH
ML2X
MLX
MLMLXH
% o
f spe
cies
235
2 4 6 8 100
20
40
60
80
100 (E) ML2X
MLXH
MLX
ML2H2
ML
FM
% s
peci
es
pH
Fig. 5.15: Distribution diagrams of Asp-Ni(II)-en complexes in Dox-water
mixtures % v/v (A) 10.0, (B) 20.0, (C) 30.0, (D) 40.0, (E) 50.0 and (F) 60.0.
2 4 6 8 100
20
40
60
80
100 (A) ML2X
MLXH
MLX
ML2H2
ML
FM%
spe
cies
pH2 4 6 8 10
0
20
40
60
80
100(B)
MLXH
MLXML
FMML2X
pH
% s
peci
es
2 4 6 8 100
20
40
60
80
100(C)
ML2H2MLXH
ML2
MLXML
ML2H3
FM ML2X
pH
% s
peci
es
2 4 6 8 100
20
40
60
80
100(D)
ML2H2MLXH
ML2
MLXML
ML2H3
FM ML2X
pH
% s
peci
es
2 4 6 8 100
20
40
60
80
100(F)
ML2H2
MLXH
ML2
MLX
ML
ML2H4
FM ML2X
pH
% s
peci
es
236
Fig. 5.16: Distribution diagrams of Asp-Cu(II)-en complexes in Dox-water
mixtures % v/v (A) 10.0, (B) 20.0, (C) 30.0, (D) 40.0, (E) 50.0 and (F) 60.0.
2 4 6 8 100
20
40
60
80
100
% s
peci
es
(F)
pH
MLXH
ML2X
MLX
MLML2H3
ML2H4
FM
2 4 6 8 100
20
40
60
80
100 (A)%
spe
cies
pH
MX
MLXH
MLX
ML2X
ML2H4
FM
2 4 6 8 100
20
40
60
80
100 (C)
ML2H2 MX2
% s
peci
es
pH
MLXH
ML2XMLX
ML2H4ML2H3
FM
2 4 6 8 100
20
40
60
80
100(B)
MLXHML2X
MLX
% s
peci
es
pH
FM
2 4 6 8 100
20
40
60
80
100 (D)
MX
MX2
% s
peci
es
pH
MLXH
ML2X
MLXML
ML2H3
FM
2 4 6 8 100
20
40
60
80
100 (E)
ML2H2% s
peci
es
pH
MLXH
ML2X
MLX
MLML2H3
FM
237
ML2H2 + XH22+ MLXH++ H+ (5.5)
M(II) + LH- +XH22+ MLXH+ + 2H+ (5.6)
MLXH+ MLX+ H+ (5.7)
M(II) + LH- +XH22+ MLX + 3H+ (5.8)
ML + XH22+ MLX+ 2H+ (5.9)
ML2H2 + XH22+ ML2X2- + 4H+ (5.10)
ML + LH- + XH22+ ML2X2- + 3H+ (5.11)
MLX + LH- ML2X2- + H+ (5.12)
The active forms of the ligands are LH3+, LH2, LH-, L2- and XH22+,
XH+ and X. The binary complexes of Asp for Co(II), Ni(II) and Cu(II) are
ML, ML22-, ML2H2, ML2H3+ and ML2H42+. All the metal ions with en form
unprotonated complexes MX2+, MX22+ and MX32+ in both the media. The
formation of ternary species is represented in Equilibria 5.5-5.12. The
protonated ternary species, MLXH+ was formed at lower pH i.e
MLXH+ was formed by the interaction of protonated ligands with
ML2H2 (Equilibrium 5.5) and with free metal ion (Equilibrium 5.6). In the
pH range 4.0-10.0, the protonated ligands interact with metal ion
(Equilibria 5.7 and 5.8) to form MLX which is also formed by the
interaction of ML with XH22+ (Equilibrium 5.9). Similarly ML2X2- is
238
formed by the interaction of ML2H2 with XH22+ (Equilibrium 5.10), ML,
LH- with XH22+ (Equilibrium 5.11) and MLX with LH (Equilibrium 5.12),
because the concentrations of both MLX and LH- are decreasing with
increasing concentration of ML2X2- in the pH range 8.0-10.0.
Structures of ternary complexes
The literature suggests that Co(II), Ni(II) and Cu(II) complexes shall
be octahedral56-61. Amino nitrogen can associate with hydrogen ions in
physiological pH ranges. Hence, there is often significant competition
between hydrogen and metal ion for this donor site. This situation
results in the simultaneous existence of a number of equilibria producing
an array of protonated complexes, which are detected in the present
study. Asp acts as bidentate or tridentate ligand depending on the
experimental conditions and en acts as bidentate ligand. Thus based on
the above equilibria the speculative structures of the complexes are
presented in Fig. 5.17.
239
H2N
NH2
M
S
S
O
O
OO
H3N H2N
NH2
M
S
OH2N
O
OO
H2N
NH2
M
O
O-
O
O
OH2N
MLXH MLX
ML2X
+
O NH2
O-
O
Fig. 5.17: Speculative structures for ternary complexes of Asp and en, where M = Co(II), Ni(II) and Cu(II).
240
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