3.1. Chemical models of mixed ligand complexes
Reliable and best representative models for the experimental data were obtained by increasing the number of species to be refined, satisfying statistical refinement parameters. This indicates that the final model appropriately fits with the experimental data. The plausible chemical models for Orn-M(II)-Phen mixed ligand systems in aqueous TBAB or PEG-400 medium for Co(II), Ni(II) and Cu(II) along with the magnitude of the corresponding stability constants are given (Tables 2 and 3).
The species successfully converged and finally refined in the active pH ranges were MLX, MLXH, MLX2H and ML2X for all Co(II), Ni(II) and Cu(II) in PEG-400- and TBAB-water mixture. The results of refinement under the parameters of the best-fit models are presented in Tables 2 and 3. The best fit models were found to be chemically consistent with better statistical fits to the electrometric titration data without showing systematic trends in the overall magnitude of the residuals. The standard deviations in log β values were also very low and Ucorr were very small, indicating the precision of the parameters; the good agreement of the models converged to the titration data and consistency of the model, respectively [39]. The slight left or right distribution of errors has been clearly shown by the magnitude of the skewness close to zero and hence a least squares method may be applied to the generated data. The values of kurtosis were virtually observed to be greater than 3 for which the residuals form part of leptokurtic pattern of distribution errors [8, 40].
The representativeness of the chemical models and its sufficiency has been indicated by the very low values of crystallographic R factor given in Tables 2 and 3. This enabled to consider additional species in the proposed and converged models. The gamma (γ) distribution, χ2, measures the probability of residuals to form part of standard normal distribution curve [8,
Table 2
Best fit chemical models of Orn-M-Phen complexes in PEG- 400-water media. Temperature = 298 K, Ionic strength = 0.16 mol L− 1. M stands for Co(II), Ni(II) or Cu(II).
% v/v
PEG-400
|
log βmlxh(SD)
|
|
NP
|
Ucorr
x108
|
χ2
|
Skewness
|
Kurtosis
|
R-factor
|
MLX
|
MLXH
|
ML2X
|
MLX2H
|
Co(II) (pH 1.67–10.5)
|
0.0
|
14.45(23)
|
21.12(19)
|
19.21(21)
|
28.07(16)
|
159
|
1.96
|
25.08
|
1.34
|
6.23
|
0.0110
|
0.5
|
13.13(13)
|
20.85(12)
|
18.86(15)
|
27.28(18)
|
148
|
1.35
|
63.54
|
-0.06
|
9.49
|
0.0016
|
1.0
|
13.10(11)
|
20.01(12)
|
18.43(11)
|
26.65(12)
|
116
|
2.34
|
9.25
|
0.30
|
6.25
|
0.0012
|
1.5
|
12.84(14)
|
19.88(15)
|
17.89(16)
|
27.18(10)
|
99
|
2.04
|
6.30
|
-0.96
|
6.30
|
0.0036
|
2.0
|
12.69(10)
|
19.54(11)
|
17.31(10)
|
26.23(08)
|
78
|
1.13
|
19.14
|
-0.75
|
4.48
|
0.0101
|
2.5
|
12.43(8)
|
19.28(10)
|
17.14(12)
|
26.03(10)
|
57
|
1.98
|
7.04
|
-0.58
|
5.38
|
0.0017
|
Ni(II) (pH 1.6–6.5)
|
0.0
|
27.12(38)
|
29.45(34)
|
28.37(42)
|
31.63(37)
|
240
|
3.85
|
72.82
|
-0.33
|
3.91
|
0.0028
|
0.5
|
27.41(10)
|
29.14(11)
|
27.93(12)
|
30.28(12)
|
119
|
6.16
|
102.62
|
-0.44
|
2.70
|
0.0016
|
1.0
|
26.33(18)
|
28.52(27)
|
27.19(28)
|
29.98(26)
|
71
|
1.49
|
44.18
|
-0.60
|
5.42
|
0.0025
|
1.5
|
25.59(23)
|
27.49(24)
|
26.21(32)
|
30.13(52)
|
35
|
0.18
|
25.75
|
-0.60
|
4.37
|
0.0023
|
2.0
|
25.12(22)
|
27.64(21)
|
26.23(21)
|
28.84(41)
|
53
|
0.24
|
4.34
|
-0.03
|
4.57
|
0.0032
|
2.5
|
25.00(17)
|
26.41(18)
|
26.08(15)
|
28.53(12)
|
61
|
1.35
|
2.50
|
-0.49
|
3.42
|
0.0069
|
Cu(II) (pH 1.6–10.0)
|
0.0
|
28.80(10)
|
34.78(13)
|
30.32(12)
|
44.65(12)
|
134
|
7.55
|
9.36
|
-0.82
|
5.04
|
0.0014
|
0.5
|
28.07(7)
|
33.26(14)
|
28.38(13)
|
42.28(32)
|
131
|
6.76
|
14.05
|
-0.45
|
2.54
|
0.0017
|
1.0
|
27.18(6)
|
30.93(23)
|
27.43(16)
|
40.89(16)
|
98
|
1.91
|
3.80
|
-0.81
|
6.50
|
0.0082
|
1.5
|
26.83(8)
|
29.84(34)
|
27.58(33)
|
37.78(35)
|
74
|
1.05
|
5.38
|
-1.00
|
7.02
|
0.0091
|
2.0
|
24.97(6)
|
27.37(24)
|
26.63(10)
|
33.33(12)
|
56
|
0.73
|
8.84
|
-0.05
|
5.21
|
0.0093
|
2.5
|
24.46(9)
|
26.78(10)
|
25.49(11)
|
23.49(14)
|
91
|
0.83
|
3.43
|
-0.83
|
2.96
|
0.0042
|
Ucorr = U/(NP-m); where m = number of species; NP = Number of experimental points
|
Table 3
Best fit chemical models of Orn-M-Phen complexes in TBAB-water media. Temperature = 298 K, Ionic strength = 0.16 mol L− 1. M stands for Co(II), Ni(II) or Cu(II).
% w/v
TBAB
|
log βmlxh(SD)
|
|
NP
|
Ucorr
x108
|
χ2
|
Skewness
|
Kurtosis
|
R –factor
|
MLX
|
MLXH
|
ML2X
|
MLX2H
|
Co(II) (pH 2.0–10.0)
|
0.0
|
14.45(23)
|
21.12(19)
|
19.21(21)
|
28.07(16)
|
159
|
1.96
|
25.08
|
1.34
|
6.23
|
0.0110
|
0.5
|
13.10(04)
|
20.25(07)
|
18.36(16)
|
27.00(19)
|
143
|
2.25
|
51.71
|
-0.12
|
3.18
|
0.0014
|
1.0
|
12.95(12)
|
19.92(10)
|
17.91(14)
|
26.25(13)
|
104
|
0.78
|
6.36
|
-0.42
|
5.45
|
0.0012
|
1.5
|
12.74(14)
|
19.68(12)
|
17.39(18)
|
27.03(21)
|
90
|
1.52
|
8.93
|
0.38
|
4.00
|
0.0044
|
2.0
|
12.59(19)
|
18.76(17)
|
17.15(15)
|
26.12(16)
|
81
|
0.89
|
12.21
|
-0.72
|
8.43
|
0.0059
|
2.5
|
11.48(38)
|
18.43(20)
|
16.84(32)
|
25.71(41)
|
52
|
1.26
|
9.12
|
-0.76
|
9.98
|
0.0019
|
Ni(II) (pH 2.0–6.0)
|
0.0
|
27.12(38)
|
29.45(34)
|
28.37(42)
|
31.63(37)
|
240
|
3.85
|
72.82
|
-0.33
|
3.91
|
0.0028
|
0.5
|
27.11(10)
|
29.05(13)
|
27.54(18)
|
29.93(23)
|
98
|
2.23
|
53.62
|
-0.26
|
4.36
|
0.0021
|
1.0
|
26.22(17)
|
28.32(27)
|
27.03(20)
|
29.52(26)
|
59
|
1.55
|
26.86
|
-0.82
|
6.89
|
0.0012
|
1.5
|
25.27(23)
|
27.10(19)
|
26.12(32)
|
28.66(55)
|
46
|
0.62
|
14.8
|
-0.03
|
4.59
|
0.0053
|
2.0
|
24.94(25)
|
27.14(24)
|
25.79(21)
|
27.94(41)
|
53
|
0.37
|
4.5
|
-0.03
|
4.57
|
0.0039
|
2.5
|
24.83(14)
|
25.87(18)
|
25.15(15)
|
27.48(12)
|
48
|
1.03
|
7.80
|
-0.56
|
13.2
|
0.0090
|
Cu(II) (pH 1.4–10.0)
|
0.0
|
28.80(10)
|
34.78(13)
|
30.32(12)
|
44.65(12)
|
134
|
7.55
|
9.36
|
-0.82
|
5.04
|
0.0014
|
0.5
|
27.21(07)
|
33.12(14)
|
29.38(11)
|
41.88(19)
|
131
|
4.60
|
12.0
|
-0.54
|
5.34
|
0.0021
|
1.0
|
26.32(09)
|
30.93(21)
|
28.29(16)
|
39.88(18)
|
87
|
1.18
|
3.60
|
0.65
|
4.57
|
0.0079
|
1.5
|
25.36(08)
|
29.74(12)
|
27.73(30)
|
37.68(26)
|
70
|
0.98
|
7.46
|
-0.12
|
7.0
|
0.0093
|
2.0
|
24.57(05)
|
28.37(14)
|
26.77(15)
|
35.33(12)
|
63
|
0.76
|
5.64
|
-1.03
|
2.73
|
0.0078
|
2.5
|
23.48(07)
|
27.78(09)
|
25.79(10)
|
23.29(11)
|
76
|
0.94
|
4.35
|
-0.73
|
6.58
|
0.0033
|
Ucorr = U/(NP-m); where m = number of species; NP = Number of experimental points
|
3.2. Validation of the chemical models and interpretation of systematic errors
The sufficiency and quality of the best fit chemical model have been evaluated through the introduction of pessimistic errors in the concentrations (±2 % and ±5 %) of acid, alkali, ligand molecules and metal ions of interest. The validation was made in order to test the reliability and accuracy of data acquisition under varied experimental conditions. The results of refinements for the data subjected to pessimistic errors were found to show high standard deviation in log β values, significant deviation in the magnitude of the stability constants and rejection of some of the proposed models by MINIQUAD75 algorithm. The effects of errors were more profoundly observed in alkali and acids than ligands and the metal ions as their concentration changed systematically. This shows the sufficiency of the models and accuracy of the method. The results of typical data in 1.5 % PEG-400 and TBAB are given in Table 4.
Table 4
Effect of errors in concentrations of influential parameters on the stability constants of Orn-Ni(II)-Phen complexes in 1.5 % (v/v) surfactant-water mixtures.
Ingredient
|
% of error
|
log βmlxh(SD)
|
|
|
|
|
Aqua-PEG-400
|
Aqua-TBAB
|
MLX
|
MLXH
|
ML2X
|
MLX2H
|
MLX
|
MLXH
|
ML2X
|
MLX2H
|
|
0
|
25.27(23)
|
27.10(19)
|
26.12(15)
|
30.33(12)
|
25.36(6)
|
29.74(12)
|
27.73(10)
|
37.68(12)
|
|
-5
|
Rejected
|
Rejected
|
28.33(8)
|
35.15(12)
|
Rejected
|
Rejected
|
29.39(08)
|
Rejected
|
Alkali
|
-2
|
Rejected
|
28.54(23)
|
28.68(29)
|
Rejected
|
Rejected
|
28.54(23)
|
28.72(29)
|
Rejected
|
|
+ 2
|
26.43(21)
|
27.75(25)
|
24.98(53)
|
33.62(43)
|
26.35(21)
|
27.75(25)
|
26.26(53)
|
35.57(44)
|
|
+ 5
|
25.12(27)
|
26.78(28)
|
25.00(53)
|
30.81(49)
|
25.19(27)
|
26.78(28)
|
24.23(53)
|
30.45(49)
|
|
-5
|
24.40(20)
|
26.35(21)
|
27.11(26)
|
32.78(43)
|
24.42(15)
|
28.35(21)
|
27.11(26)
|
38.92(18)
|
Acid
|
-2
|
25.19(18)
|
26.06(20)
|
26.53(48)
|
29.56(65)
|
25.23(28)
|
29.06(20)
|
26.53(48)
|
29.76(28)
|
|
+ 2
|
Rejected
|
25.19(21)
|
27.39(32)
|
28.93(34)
|
Rejected
|
26.19(21)
|
Rejected
|
30.93(16)
|
|
+ 5
|
Rejected
|
Rejected
|
26.23(23)
|
28.56(18)
|
Rejected
|
Rejected
|
26.23(23)
|
29.55(25)
|
|
-5
|
26.10(7)
|
27.40(8)
|
26.38(6)
|
31.03(10)
|
26.23(7)
|
29.40(9)
|
26.38(6)
|
38.3(11)
|
Orn(L)
|
-2
|
25.89(7)
|
27.42(8)
|
26.52(16)
|
30.36(9)
|
26.67(10)
|
29.50(9)
|
26.45(18)
|
37.26(8)
|
|
+ 2
|
25.71(7)
|
27.35(7)
|
26.31(9)
|
30.53(17)
|
25.56(17)
|
29.42(6)
|
26.31(9)
|
36.64(9)
|
|
+ 5
|
25.73(8)
|
27.29(7)
|
26.55(21)
|
30.27(12)
|
25.32(11)
|
29.33(7)
|
26.15(12)
|
36.48(11)
|
|
-5
|
27.05(9)
|
27.80(8)
|
26.83(21)
|
30.53(8)
|
26.16(19)
|
29.71(12)
|
26.44(21)
|
37.89(13)
|
Phen(X)
|
-2
|
26.58(8)
|
27.65(6)
|
26.45(18)
|
30.18(7)
|
25.61(10)
|
29.12(9)
|
26.57(15)
|
37.33(10)
|
|
+ 2
|
25.81(7)
|
27.52(5)
|
26.13(10)
|
30.37(7)
|
25.49(8)
|
29.15(6)
|
26.24(7)
|
36.30(08)
|
|
+ 5
|
25.75(7)
|
27.41(6)
|
26.15(12)
|
29.94(10)
|
25.36(6)
|
29.74(12)
|
27.73(10)
|
37.68(12)
|
|
-5
|
25.98(6)
|
27.46(8)
|
26.40(10)
|
30.27(7)
|
Rejected
|
28.54(23)
|
28.72(29)
|
Rejected
|
Metal
|
-2
|
25.77(5)
|
27.42(8)
|
26.52(14)
|
30.36(10)
|
26.35(21)
|
27.75(25)
|
26.26(53)
|
35.57(44)
|
|
+ 2
|
25.66(5)
|
27.35(9)
|
26.24(12)
|
30.47(9)
|
25.19(27)
|
26.78(28)
|
24.23(53)
|
30.45(49)
|
|
+ 5
|
25.43(6)
|
27.31(7)
|
26.25(21)
|
30.45(8)
|
25.12(21)
|
26.52(23)
|
24.02(41)
|
30.32(35)
|
3.3. Effect of surfactant
The additions of PEG-400 and TBAB lowered the dielectric constant [6, 41, 42] of the aqueous medium, thereby decreasing log β values. Moreover, PEG-400 (neutral) and TBAB (cationic) surfactants have destabilizing effects on the positively charged complexes formed due to electrostatic repulsive effects which synergizes with the lower dielectric constants causing log β values to decrease. Thus, in the present study, log β values of the mixed ligand system were found to be linearly decreasing with increasing percentage of PEG-400 or TBAB (Fig. 1), which causes destabilization of the complexes with net predominant effect of electrostatic factors.
3.4. Prediction of extra stability of the mixed ligand complexes
The relative stabilities of mixed ligand complexes, compared to their binary counterparts, can be predicted from disproportionation equilibria [13–15; 34–35; 43], and the change in the Log values of stability constants (Δlog K) [8]. The disproportionation constant (log X) [44–47] which corresponds to equilibrium 3 and the change in the Log values of stability constants (Δlog K) [8] of mixed ligand complexes and their parent binary complexes are given by Eqs. 2 and 3, respectively. The two different ligands have mutually the same degree of influence in the formation of mixed ligand complex [14, 15, 34, 43].
The formation of a mixed ligand complex of two different ligands, MLX, is statistically more favored than the formation of the parent ML2 and MX2 binary complexes when equal concentrations of M, L and X are available in solutions [12]. Additionally, ML2 (25%) and MX2 (25%) complexes can readily form mixed ligand complexes, 2MLX (50%), and the value of log X was reported to be 0.6 [17, 18; 21, 47, 48].
The value of Δlog K should be negative when either of the ligands coordinates with the free metal ion in comparison to the simple complexes in binary system, where the log value of first stepwise stability constant K1 is always greater than the log value of second stepwise stability constant K2. The change in log values of the mixed ligand constant (Δlog K) is usually positive. Thus, the Δlog K could be taken as better measure of the tendency towards the formation of mixed ligand complexes than severely criticized log values of overall stability constants [13, 43].
Based on the electrostatic theory of binary complex formation and statistical arguments, the additional site of coordination of given multivalent hydrated central metal ion could easily be available for the first ligand than for the second, probably due to steric hindrance. Hence, the usual order of stability> applies. This suggests that Δlog K should be negative although several exceptions [46] have been found. In the case of bidentate ligands, there are twelve edges of normal octahedron available for first coordinating ligand leaving only five for the second entering ligand and accordingly, (-5/12 ∼ -0.4). The statistical values of Δ log Koh, Δ log Kpl, and Δ log Kdisoh 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. Whenever the experimental values of Δlog K exceed the critical values, the mixed ligand complex is supposed to be formed due to interaction of ML with X or MX with L. Some of Δlog K values of mixed ligand complexes reported containing bipyridyl as the primary ligand are positive [13, 43] for O-donor (malonic acid, pyrocatechol, etc.), negative for N-donors (ethylenediamine) and intermediate or negative [40] for amino acids with N and O binding sites.
∆log KMLXH
|
=
|
log βMLXH - log βMLH - log βMX
|
In the present study, it was impossible to calculate the values of Δ log K for Ni(II), Co(II) except for MLX2H model (1.57–3.14 in PEG-400, 3.39–5.51 in TBAB) and some models of Cu(II) due to the absence of some of the binary chemical models [26]. For the mixed ligand complexes of Cu(II), the values of Δlog K were found to range from − 8.19 to 6.96 in PEG-400 and − 8.7 to 10.18 in TBAB-water mixtures, respectively. For all the metal complexes, the values of log X and Δlog K were observed to be higher than the statistical values (0.6) and (-0.4), respectively, suggesting extra stability of the mixed ligand complexes.
In the Orn-M(II)-Phen system, the central metal ion forms octahedral complexes, where M stands for Co(II), Ni(II) and Cu(II). For all the M-L systems (Table 6), the Δlog K values were found to be higher than statistical grounds (-0.4), indicating the extra stabilities of the mixed ligand complexes compared to their parent binary model complexes. The interactions of the system outside the coordination sphere like the formation of H-bonding between the coordinated ligands, chelated ring effect, charge neutralization, stacking interactions and electrostatic interactions between non-coordinated charge groups of the ligands could be listed as the plausible reasons for the extra stabilities of the mixed ligand complexes [11, 49].
The values of Δlog K and log X are calculated from the magnitude of stability constants of binary and mixed ligand models using the selected equations given in Table 5. The values of Δlog K for the formation of MLX, MLXH, ML2X and MLX2H model species for Cu(II) are very near to or on positive sides of the statistically allowed values, indicating the tendency of the formation of MLX, MLXH, ML2X and MLX2H complex species with different relative stabilities. With respect to mixed ligand complexes of Co(II), the values of Δlog K of MLX2H complex was also found to be on the positive side of the statistically expected values, indicating the preference of mixed ligand complex formation due to favoured entropy change, arisen mainly from charge neutralization with release of solvent molecules. However, the negative Δlog K values do not preclude the tendency towards the formation of mixed ligand species.
When the charges of the two different ligands are not equal, electrostatic factor also contributes for the formulation of log X. The log X values for all Orn-M(II)-Phen system were observed to be higher than the known table value of the disproportion constant (usually 0.6) which reveals the extra stability of the mixed ligand complex (Table 6). It also indicates that statistical and electrostatic factors are applied favourably for the formation of mixed ligand complexes.
Table 5
Selected equations used to calculate Δlog K and log X values from the magnitude of corresponding overall stability constants.
Δ log K
|
Δ log K1110
|
|
=log β1110
|
|
-log β1100
|
|
-log β1010
|
|
Δ log K1210
|
|
=log β1210
|
|
-log β1200
|
|
-log β1010
|
|
Δ log K1121
|
|
=log β1121
|
|
-log β1101
|
|
-log β1020
|
|
|
|
=log β1121
|
|
-log β1100
|
|
-log β1021
|
|
Δ log K1111
|
|
=log β1111
|
|
-log β1101
|
|
-log β1010
|
|
log X |
Table 6
Δ log K and log X values of mixed ligand complexes of Co(II), Ni(II) and Cu(II)-Orn and Phen in PEG-400- and TBAB-water mixtures.
log X1110
|
= 2log β1110
|
-log β1200
|
-log β1020
|
log X1210
|
= 2log β1210
|
-log β1400
|
-log β1020
|
log X1121
|
= 2log β1121
|
-log β1201
|
-log β1041
|
|
= 2log β1121
|
-log β1200
|
-log β1042
|
logX1111
|
= 2log β1111
|
-log β1202
|
-log β1020
|
|
Δ log K
|
log X
|
1110
|
1111
|
1210
|
1121
|
1110
|
% v/v PEG-400
|
Co(II)
|
0.0
|
------
|
------
|
------
|
1.86
|
-1.39
|
0.5
|
------
|
------
|
------
|
1.57
|
-2.53
|
1.0
|
------
|
------
|
------
|
1.80
|
-2.42
|
1.5
|
-----
|
-----
|
-----
|
3.14
|
-1.63
|
2.0
|
------
|
------
|
------
|
2.79
|
-1.10
|
2.5
|
------
|
------
|
------
|
2.97
|
-0.85
|
Ni(II)
|
0.0
|
------
|
------
|
------
|
------
|
26.38
|
0.5
|
------
|
------
|
------
|
------
|
28.20
|
1.0
|
-----
|
-----
|
-----
|
-----
|
27.38
|
1.5
|
------
|
------
|
------
|
------
|
26.21
|
2.0
|
------
|
------
|
------
|
------
|
25.30
|
2.5
|
------
|
------
|
------
|
------
|
25.92
|
Cu(II)
|
0.0
|
3.51
|
6.96
|
-8.19
|
10.11
|
12.37
|
0.5
|
4.03
|
6.36
|
-6.72
|
8.57
|
14.23
|
1.0
|
4.07
|
4.53
|
-5.96
|
8.34
|
14.82
|
1.5
|
3.63
|
4.66
|
-7.02
|
6.41
|
12.87
|
2.0
|
1.64
|
2.31
|
-7.49
|
1.90
|
09.45
|
2.5
|
2.25
|
1.11
|
-6.79
|
-8.52
|
10.30
|
% w/v TBAB
|
Co(II)
|
0.0
|
------
|
------
|
------
|
3.39
|
-1.39
|
0.5
|
------
|
------
|
------
|
4.28
|
-2.01
|
1.0
|
-----
|
-----
|
-----
|
4.96
|
-0.95
|
1.5
|
------
|
------
|
------
|
5.03
|
-1.23
|
2.0
|
------
|
------
|
------
|
5.00
|
0.06
|
2.5
|
------
|
------
|
------
|
5.51
|
-0.04
|
Cu(II)
|
0.0
|
------
|
------
|
------
|
10.18
|
12.44
|
0.5
|
------
|
------
|
------
|
9.49
|
15.15
|
1.0
|
-----
|
-----
|
-----
|
7.73
|
14.21
|
1.5
|
------
|
------
|
------
|
6.49
|
12.95
|
2.0
|
------
|
------
|
------
|
1.80
|
09.35
|
2.5
|
------
|
------
|
------
|
-8.7
|
10.12
|
3.5. Chemical Speciation and species distribution plots
The species distribution diagrams for the Orn-M(II)-Phen system plotted from SIM run best fit model data are presented in Figs. 2 and 3. In the plots, both binary, with relatively low percentage composition [26], and mixed ligand complexes, with higher percentage compositions, co-exist in the active pH regions due to inbuilt provision of MINIQUAD75 program. This indicates that mixed ligand complexes have extra stabilities compared to their parent binary complexes of the same metal ion under exactly the same experimental conditions. The plausible chemical models converged for mixed ligand complexes of Orn(L) and Phen(X) in PEG-400- and TBAB-water mixtures are MLXH, MLX, MLX2H and ML2X for all the metals. MLXH and MLX2H model species exist at lower pH while MLX and ML2X were found at higher pH (cf. Figures 2 and 3).
As shown in Equilibrium (4), MLXH is formed by direct combination of the free metal ion, M(II), with LH and XH2 forms of primary and secondary ligands in the pH ranges of 1.5–11.0; 1.5–7.0 and 1.5–11.0 with max extent of formation, resulting in the formation of complexes with different amounts, e.g. 60–80% for Co(II); 35–80% for Ni(II) and 55–80% for Cu(II) system, respectively.
Deprotonation of MLXH at higher pH gives MLX chemical model, Equilibrium (5). MLX could also be formed by the interaction of free metal ion M(II), primary ligand (LH) and secondary ligand (XH2) (cf. Equilibrium (6) and ML with XH2, Equilibrium (7), in the pH ranges of 7.0–11.5; 1.5–8.0 and 2.0–8.5 with maximum extent of formation varying as low as 15 to max of 90% for Co(II), 30–80% for Ni(II) and 40–90% for Cu(II) system, respectively.
The MLX2H type species might be formed by the interaction of MLXH with secondary ligand XH2, Equilibrium (8), in the pH range of 3.0–11.0; 1.5–8.0 and 2.0–9.0 with max percentage range of 15–40%; 20–90% and 35–50% for Co(II), Ni(II) and Cu(II) system, respectively.
The other species which exists predominantly at higher pH region is ML2X. In the present study, ML2X might be formed by the interaction of ML simultaneously with LH and XH2 (Equilibrium (9)) and MLX with LH (Equilibrium (10)) above the pH of 8.0 for Co(II); 5.0 for Ni(II) and 6.0 for Cu(II) with max relative concentrations of 40 to 100% for all Orn-M(II)-Phen system in PEG-400- and TBAB-water mixture. The formation of these complex species can be represented by the following equilibria.
M(II) + LH + XH2
|
MLXH + 2H+
|
(4)
|
MLXH
|
MLX + H+
|
(5)
|
M(II) + LH + XH
|
MLX + 3H+
|
(6)
|
ML + XH2
|
MLX + 2H+
|
(7)
|
MLXH + XH2
|
MLX2H + 2H+
|
(8)
|
ML + LH + XH2
|
ML2X + 3H+
|
(9)
|
MLX + LH
|
ML2X + H+
|
(10)
|
3.6. Structures of mixed ligand complexes
The stability of a complex is directly dependent on the nature of the central metal ion (charge density and size) in readily accepting electron pairs from the donor atom, the ligand (its basicity to easily coordinate with metal ion via its electron pairs), the hard-soft interaction of the ligand and metal ion; and the complex formed (chelates vs open chain). Based on the knowledge of lone pair/bond pair interactions to minimize repulsive strain with maximum bond separation and other contributing factors, chemical structures for the mixed ligand complexes were deduced. The rigid framework of Phen and the relative stability of the five member L-ornithine ring complexes with central metal ion could also be considered as contributors to the stability of the synthesized complexes. Considering these, the following most likely octahedral structures [21], corresponding to the MLX, MLXH, ML2X and MLX2H chemical models, are proposed for all the metal ions but Cu(II) complexes have also Jahn-Teller distorted structure [26] (Fig. 3).