CdI2 Extraction with 18-Crown-6 Ether into Various Diluents: Classification of Extracted Cd(II) Complex Ions Based on the HSAB Principle

CdI2 in water was extracted with 18-crown-6 ether (L) into 10 diluents at 298 K. The following equilibrium constants were determined or evaluated: some extraction constants (Kex/mol∙dm & Kex,ip/mol∙dm for CdLI2, Kex±/mol∙dm for CdLI with I, & Kex2±/mol∙dm for CdL with 2I), conditional distribution constants (KD,I for I, KD,CdLI for CdLI, & KD,CdL for CdL) between the two phases, and an ion-pair formation constant (K1,org/mol∙dm) for CdLI and that (K2,org/mol∙dm) for CdLI2 in the organic (org) phases. Using the K1,org and K2,org values, acidities of the complex ions, CdL and CdLA (A = I, Br, & Cl), in the 11 diluents were classified by applying the HSAB rule. Especially, the CdLA ions were classified as the soft acids in 9 diluents. Also, molar volumes (Vj/cm∙mol) of j = CdLI2 and CdL were determined with the regular-solution-theory plot of logKex,ip vs. logKD,L and its pseudo-plot of logKD,CdL, respectively. Here, KD,L denotes the distribution constant of L between the two phases. So, sizes among CdLA2 and CdL were compared by using the Vj values. Additionally, some distribution equilibrium potentials (dep/V) between the water and org bulk phases were topically calculated from an equation of KD,I with S D,I K , where the symbol S D,I K shows a standard distribution constant of I at dep = 0 V for a given diluent.


Introduction
It is well known that crown ethers (L) extract Cd(II) and Pb(II) salts, such as metal picrates (MPic 2 ) [1] [2] [3] [4] [5], the former chloride [6], and bromides [1] [6], into various diluents.Similar extraction behaviors into benzene (Bz) and nitrobenzene (NB) have been reported for Ca(II), Sr(II), and Ba(II) picrates with L [7] [8].In these studies, the distribution equilibrium potentials (dep or Δϕ eq ) for monovalent anions (A − ) between the water and diluent bulk phases and the ion-pair formation for ML 2+ and MLA + in the diluent phases saturated with water have been examined and clarified, respectively [1]- [6] [8].For the latter [1] [2] [4] [6] [8], the reactivities of CdL 2+ and CdLA + with A − = Cl − , Br − , and picrate ion Pic − in various organic (org) phases have been quantitatively discussed at L = 18-crown-6 ether (18C6).The complex ions composed of a soft Cd 2+ and hard L, Cd18C6 2+ and CdB18C6 2+ , have been classified in terms of the HSAB rule [9] as the hard acids in water [10], where B18C6 refers to benzo-18C6.This classification would make the studies on reactivity of the Cd(II) complexes and properties of the diluent molecules in the extraction interesting.However, there were few comprehensive studies for the M(II) extraction systems with L and various diluents [11].
In the present paper, by doing extraction experiments of CdI 2 with 18C6 into ten diluents, we determined extraction constants, K ex and K ex± , and their related equilibrium constants, K D,I and K Cd/CdL , [4] [5] at 298 K. Here, K ex , K ex± , K D,I , and K Cd/CdL were defined as [ [1]- [6], respectively.From these values and the thermodynamic relations, K 1,org and K 2,org values were evaluated: ( ) and K 2,org = K ex /K ex± [4] [5] (see the Section 2.4).Using these evaluated K 1,org and K 2,org values, reaction properties of CdLA + and CdL 2+ with mainly A − = I − , Br − , and Cl − in the org or diluent phases were also classified based on the HSAB principle [9] [10] [11].Moreover, molar volumes (V/cm 3 •mol −1 ) of the ion-pair complex CdLI 2 and complex ion CdL 2+ were determined at 298 K with the plots based on the regular solution theory (RST) [1] [2] [3] [4] [6] and then their comparable sizes were estimated from these V. On the basis of these data, the HSAB acidic and structural properties of the Cd(II) complexes with 18C6 were discussed independently.

Composition Determination of Cd(II) Species Extracted into Various Diluents
According to previous papers [1]- [6], the following equation was employed for the determination of the composition of Cd(II) species extracted into the org phases.

Determination of KD,I, Kex±, and Kex by Using the Parameter K mix ex
For the determination of K D,I , K ex± , and K ex , we employed the parameter ( ) as similar to the previous papers [1]- [6].Therefore, we can determine the K D,I and K ex values from the plot of while can do the K ex± and K ex ones from that of at L = 18C6; see the Section 3.3 for the detailed derivation of Equations ( 2)-(2b).
Figure 1 and Figure 2 show examples of these plots for the present Cd(II) extraction systems and logarithmic values of these equilibrium constants were listed in Table 1.The K ex values determined with Equation (2a) in the 5 diluent systems (Table 1) were in accordance with those with Equation (2b) within their experimental errors.

Estimation of Dep for Some Diluent Systems
The relation between dep or Δϕ eq and K D,A has been reported for these extraction ) log K values at 298 K were used for these calculations: −4.0 [15] for NB, −4.56 [15] for DCE, and −3.790 [16] for DCM.The dep presences were clarified at least for these diluents systems, as similar to the results [1]- [6] [8] reported previously.

Determination of K1,org and K2,org
Referring to the previous papers [1]- [6] [8] [17], the K 1,org and K 2,org values were obtained from ( ) for a given ionic strength (I org ) in the org phase.Here, the equilibrium constant K Cd/CdL has been assumed to be equal to D/[L] org [3] [5] [6] [17].The thus-calculated values are listed in Table 2, together with the K Cd/CdL and their corresponding I org values.Figure 3 shows the K 1,org and K 2,org values with ten kinds of the diluents described in Table 2.The x-axis indicates the decrease of the diluent's polarities from No. 1 (NB) to 10 (mX).Except for the DCE and mX systems, there was the relation [3] [5] [6] [17] of K 1,org ≥ K 2,org .This trend seems to be similar to that   2.

Determination of Kex2±, KD,CdLI, and KD,CdL and Their Characterization
The extraction constant K ex2± {see Equation (4)} and the two conditional distribution constants, K D,CdLI and K D,CdL , were calculated from the following thermo- Cd/CdL D,I org org and ( ) with called the ion-pair formation constant for water, and β 2,org = K 1,org K 2,org , which is an overall ion-pair formation constant for the org phase.As described in Equation (3a), K D,CdLI and K D,CdL are expressed as functions of Δϕ eq and called the conditional distribution constants: Assuming that the relation of (7) were estimated approximately from the experimental I (Table 1) and 0 1 K (≈y II+ K 1 ) values.Here, the 0 CdA K and 0 CdLBr K refer to an ion-pair (or a complex) formation constant [19] of Cd 2+ with A − (=I − & Br − ) and that [10] of Cd18C6 2+ with Br − in water at I → 0, respectively.The activity coefficient (y II+ ) of CdL 2+ in water was evaluated from the Davies equation [20].These calculated values, with the K D,18C6 values available from references [13] [14] were listed in  8); e. Refs.[13] & [14]; f.The values were employed for the plots of Figure 4.
smallest in the three extraction constants determined: logK ex2± < logK ex± < logK ex (see Table 2 for K ex & K ex± ).Equations ( 7) and ( 8) are related with pseudo-RST plots described in the Section 2.8.
As shown in Figure 4, the K D,j values were in the order j = I − (<18C6) < Cd18C6 2+ < Cd(18C6)I + .This order is basically different from that [3] for the CdPic 2 -B18C6 extraction system: j = Pic − {<Cd(B18C6)Pic 2 0 } < Cd(B18C6)Pic + < CdB18C6 2+ .Equations ( 7) and (8) predict that a difference between K D,CdLA and K D,CdL is proportional to that between can cause the K D,j order of j = Cd18C6 2+ < Cd(18C6)I + , while that of ( ) These experimental results of the CdI 2 -18C6 extraction systems were in the −logK 1 range of −0.0 3 to 0.0 3 and in the −log(K averaged in the Bz system) and in the −log(K 1,org K D,Pic ) one of −2.7 to −0.1 [3].
These experimental orders are in good agreement with the orders predicted above.

For Relative Concentrations of CdLI2, CdLI + , and CdL 2+ Extracted into the Diluents
We have defined distribution ratios D 0 , D + , and D 2+ as described below [3] [5] at each experimental point.Here, the K ex , K ex± , K D,I , and K Cd/CdL values at L = 18C6 in Table 1 and Table 2 were used for the calculations.From the three equations, we can calculate relative concentrations (or molar fractions), such as f 0 /% = 100D 0 /D t and f + = 100D + /D t with D t = D 0 + D + + D 2+ [5].The mean values of f 0 , f + , and f 2+ were listed in Table A1 of the Appendix, where the symbols f 0 , f + , and f 2+ (=100D 2+ /D t ) denote the relative concentrations of CdLI 2 , CdLI + , and CdL 2+ , respectively.
As can be seen from Figure 5 and Table A1, the f + values were the largest in the extraction into the many diluents, except for the values of the DCE and mX systems.Especially, the f + values exceeded 50% in the NB, oDCBz, DCM, BBz, CF, and Bz systems.These behaviors in Figure 5 can be explained as follows.
Considering a homogeneous reaction defined as K 1,org /K 2,org .
[ ] ( ) From the K 1,org and K 2,org values in Table 2, the log (K 1,org /K 2,org ) values were calculated to be negative (namely K 1,org < K 2,org ) for the org = DCE and mX systems, while their values to be positive (namely K 1,org > K 2,org ) for the other systems.
Therefore, we can easily see that the formation of CdLI 2 is dominant, , in the DCE and mX phases, while that of CdLI + is dominant, 2CdLI + org , in the other diluents.The diluent dependence of the f values in Figure 5 reflects mainly the difference between K 1,org and K 2,org (see the Section 2.4).
Considering these phenomena from ion-pair-formation point of view [3] [6], the systems dominant for the distribution of CdLI + can be a major case in the present extraction systems.

Classification of the Acidity of CdL 2+ and CdLA + in the Org Phases Based on the HSAB Rule
According to our previous paper [10], the complex ions Cd18C6 2+ and CdB18C6 2+ in water have been classified as the hard acids in their reactions with A − = Cl − , Br − , (I − ,) or Pic − .As standards of the HSAB classification, we assumed that 1) trends in the hardness and softness of the anions A − in the org phases are the same as those [9] [10] in water.That is, I − and Br − are soft bases [9], while Cl − and Pic − are hard bases [9] [10].
2) The reactions with the halogen ions are primarily employed for the classification.Only when one of the reactions with the three halogen ions lack, the reaction of Pic − was used for it.In the classification, 3) we neglected effects of the I org values on K 1,org , K 2,org , and β 2,org , because the I org values were in the lower ranges [1] [2] [3] [6]: see Table 2 as an example.For example, the K 1,NB and β 2,NB (=K 1,NB K 2,NB ) values were in the order Pic − < I − < Cl − (see Table A2 in Appendix).These orders suggested that the Cd18C6 2+ is a borderline acid in the NB phase, because the order between the hard and soft bases is random.The K 1,oDCBz values were in the order Br − < I − < Cl − , while the β 2,oDCBz ones were Cl − < I − < Br − (see Table A2).The former order suggested that Cd18C6 2+ in the oDCBz phase is a hard acid.On the other hand, the latter one indicated that Cd18C6 2+ is a soft acid.This discrepancy in the classification between K 1,org and β 2,org can reflect the soft acidity of the intermediate ion-pair complex ion, Cd(18C6)A + ; namely the effect of K 2,org .A similar trend was observed in the Bz systems: they were classified as the hard acid from K 1,Bz (Br − < I − < Cl − ) and as the borderline acid from β 2,Bz (I − < Cl − < Br − ).The Cd18C6 2+ ions in the other diluents were classified as the soft acids for the DCE, DCM, CBz, BBz, and CF systems, the borderline acid for CBu, and the hard acid for mX and TE: see Table A2 in Appendix.In these systems, the HSAB classifications by K 1,org were in accordance with those by β 2,org .
On the basis of the above results, it could be considered that Cd18C6 2+ in water almost changes from the hard acid to the soft or borderline acids in the extraction into the org phases.This indicated that the hardness and softness of Cd18C6 2+ might be changed with species of the diluents, according to the criteria of the A − basicity.
The following measure can be also considered for the HSAB classification of Cd(18C6)A + in the each phase, because there were no data for the reactions, such as ] org terms at unity, the ratio virtually can become the [CdLA 2 ] org /[CdLCl 2 ] org ratio.Hence, we considered that if the logK 2,org (A/Cl) value is positive, the formation of CdLA 2 in the org phase becomes dominant and if it is negative, that of CdLCl 2 does dominant.The former case means the softer complex ion, while the latter one does the harder ion.So, this K 2,org (A/Cl) value gives us a criteria for evaluating the HSAB acidity of Cd(18C6)A + in the org phases (water).Consequently, the order of K 2,org among A − yields the magnitude in the formation of CdLA 2 in the org phase under the assumption for the above ratio.
From the above, all Cd(18C6)A + change from the hard acids in water to the soft and borderline ones in the org phases.
Thus, the changes of the diluents (or the org phases) are reflected into the HSAB acidities of these complex ions in the extraction of Cd18C6 2+ and Cd(18C6)A + .In other words, this means that the HSAB acidity of the complex ion or the ion-pair cation varies with the kinds of the diluents, if the HSAB basicity of the A − can be considered to be the standard.It can be seen that it is easier for the monovalent CdLA + to become the soft acid than for the divalent CdL 2+ to do it with the extraction into the diluents.This can be supported by the fact that Cd(18C6)A + in the 9 diluents among the 11 ones is classified as the soft acids, compared with Cd18C6 2+ in the 4 diluents done as the hard acids (Table A2).We can see it particularly from this comparison that the six diluents, DCE, oDCBz, DCM, CBz, BBz, and CF, are the higher effect than the others in softening the acidity of the complex ions.It is interesting that these diluents contain the Cl-or Br-group(s) in their molecules, though CBu, Cl-CH 2 CH 2 CH 2 CH 3 , does not clearly show its effect.

Comparisons of Molar Volumes among the Ion-Pair Complexes
We obtained the regression line from the RST plot vs. logK D,18C6 for the present Cd(II) extraction systems, except for the points of the NB and CF ones [3] [13]: logK ex,ip = (0.7 5 ± 0.2 1 )logK D,18C6 + (6.8 0 ± 0.2 5 ) at R = 0.800.Also, using V 18C6 = 214 ± 47 cm 3 mol −1 [13] reported by Takeda, the V CdLI2 value was calculated to be 160 ± 57 from the slope of the RST plot.Adding the data of previous papers, the V j values became in order At least, there is a tendency in the order of V CdLA2 among A = Cl, Br, and I.
In general, the RST plot for the M(II) extraction system is expressed as log-K ex,ip = (V MLA2 /V L )logK D,L + C + log β 2 in the form of a linear equation, where the constant C shows solute-solvent (or non-electrostatic) interactions term with cohesive energy densities [1] [2] [3] [6] [13].From the thermodynamic relation of Equation ( 8), we can derive the following equation: with ( ) . Hence, one can see that the C' term includes the β 2 /β 2,org term corresponding to the ion-ion interactions in addition to the solute-solvent interactions term C. The plot of logK D,CdL vs. logK D,L for the CdI 2 -18C6 extraction systems is shown in Figure 6.Its regression line was logK D,CdL = (0.5 6 ± 0.1 5 )logK D,L + (2.4 7 ± 0.1 7 ) at R = 0.774, where the data of the NB and CF systems were added in the estimation, because of the plot for the ionic species.This slope was somewhat smaller than that (≈0.8) of the RST plot.If this difference reflects a difference in V j between j = CdLI 2 and CdL 2+ , then the ratio between the slopes can directly express that between V j .So, the ratio of slope(CdLI 2 )/slope(CdL 2+ ) (=1.3) is equivalent to V CdLI2 /V CdL at a fixed V L .Therefore, the V CdL value was estimated to be 120 ± 64 cm 3 •mol −1 from the V CdLI2 one (=160).This value was smallest in the V j with j = CdLCl 2 , CdLBr 2 , CdLI 2 , and CdLPic 2 .This is in good agreement with the image that the size of CdL 2+ is smaller than those of CdLA 2 .
The same trend as above can be seen in a plot of logK D,CdLI vs. logK D,L (see Table 3 for their data): the V CdLI value was 155 ± 46 cm 3 •mol −1 at L = 18C6.Similarly, V CdLBr /cm 3 •mol −1 was estimated to be 225 ± 55 from the slope (=1.0 5 ± 0.1 1 [1]) of the logK D,CdLBr vs. logK D,L plot reported previously.These values satisfy the following relations: V CdLI2 ≥ V CdLI ≥ V CdL and V CdLBr2 ≥ V CdLBr ≥ V CdL .

Estimation of Apparent Sizes for the Cd(II) Complexes
From the V j data, we can evaluate apparent sizes of Cd(18C6)A 2 or Cd18C6 2+ .Assuming 3 4π 3 , namely that shapes of the ion pairs and complex ion are close to spheres, we can easily calculate apparent radii (R j ) from the V j .Their  gestion is correct, then both the R j and V j values can strongly reflect the structural properties of the complexes "in the water phase".On the basis of the above results, the V j values obtained in the section 2.8 and those reported before seem to be self-consistent.

Extraction Procedure
Basic operations and equipment were the same as those described before [1]- [6].That is, the operations were constructed of original Cd(II) extraction, its back one, and Cd(II) analyses with the AAS measurements at 228.8 nm.The calibration curves of Cd(NO 3 ) 2 in the aqueous 0.1 mol•dm −3 HNO 3 solutions were employed for the AAS determination of Cd(II).Here, differences in the calibration curve between pure water and the aqueous HNO 3 solution were experimentally negligible.So, the back extraction was operated with pure water instead of 0.1 mol•dm −3 HNO 3 [1]- [6] as the back extraction phase, because the Cd(II) amounts in the latter acidic solutions analyzed by the AAS deviated largely.

Extraction Equilibrium Model and Its Data Handlings
The following extraction model [4] was employed for the analysis of the present extraction system with L = 18C6: 1) Cd 2+ + L ⇌ CdL 2+ [12] and 2) Cd 2+ + I − ⇌ CdI + [19] in the water phase; 3) CdL CdL + +  , and 6) L ⇌ L org [13] [14] between the water and org phases; 7)  ], where the latter three concentrations were determined with a successive approximation procedure, using the equilibrium constants of the processes 1), 2), and 6) [2] [4].When a negative value for K ex had been obtained from the analysis with Equation (2b), its analysis was performed again by fixing the K ex value to that determined by the analysis with Equation (2a) [1]- [6] (see the footnotes c & e in Table 1).

Conclusions
The ion-pair formation in the 11 diluents saturated with water was classified in terms of the HSAB principle, although the hardness and softness of the simple A − in the diluents were assumed to be the same as those in water.This classification mainly showed us the two results.1) CdL 2+ and CdLA + with L = 18C6 and A − = Cl − , Br − , and I − change from the hard acids in water to almost the soft or borderline acids in the extraction into the org phases at least.2) The charge effects on CdLA + and CdL 2+ in the org phases are remarkable.Namely, CdLA + softens more its acidity than CdL 2+ does in the extraction.Especially, DCE, oDCBz, DCM, CBz, BBz, and CF have the higher ability to soften the HSAB acidity of the complex ions.
The presence of dep was also observed in the CdI 2 -18C6 extraction into NB, DCE, and DCM.The relation of f + < f 0 simply reflects that of K 1,org < K 2,org , about which the structural changes around Cd(II) were suggested, while the relation of f + > f 0 does that of K 1,org > K 2,org .
The molar volumes V j obtained from the RST plots indicated the size-dependence on the Cd(18C6)A 2 (=j) ion pairs.Additionally, the V Cd18C6 value was evaluated from the pseudo-RST plots and then was the smallest of the V j ones of all the Cd(18C6)A 2 examined.At the same time, it was demonstrated that the apparent radii R j , estimated from the V j values, reflects inversely the bond lengths of Cd-A with A − = Cl − , Br − , and I − in the crystallographic and DFT studies.These V j and R j results proved validities for the analyses of the RST and pseudo-RST plots about such extraction systems and thereby indicated a possibility that the two plots give the structural information about some complexes, although it is unclear which of org or water phase is the corresponding phase.
CdLI 2 ] org /P, [CdLI + ] org [I − ] org /P with P = [Cd 2+ ][L] org [I − ] 2 , [I − ] org /[I − ], and [CdL 2+ ] org /[Cd 2+ ][L] org with D defined as [Cd(II)] org /([Cd(II)] t − [Cd(II)] org ) at A − = I − and L = 18C6.American Journal of Analytical Chemistry This equation was derived approximately from the definition of K ex[7] described in the introduction.Here, the symbols D, [Cd(II)] org , and [Cd(II)] t denote an experimental distribution ratio for Cd(II), a measurement concentration of all the Cd(II) species extracted into the org phase determined by AAS, and a total concentration of CdI 2 included in the water phase at the beginning of the extraction experiment, respectively.When slopes obtained from the plots of log (D/[A − ] 2 ) vs. log [L] org are unity, they would mean that the extracted species have the composition of Cd(II): L = 1:1 [1]-[7].The experimental slopes were 0.95 at correlation coefficient (R) = 0.813 for the NB system, 1.03 at 0.989 for 1,2-dichloroethane (DCE), 0.97 at 0.939 for o-dichlorobenzene (oDCBz), 1.03 at 0.940 and 0.96 at 0.754 for dichloromethane (DCM), 1.07 at 0.883 for chlorobenzene (CBz), 1.08 at 0.959 for bromobenzene (BBz), 1.02 at 0.769 for chloroform (CF), 1.01 at 0.871 for Bz, 0.90 at 0.827 for toluene (TE), and 1.05 at 0.924 for m-xylene (mX).Here, the R values were obtained from the regression lines determined with the log(D/[I − ] 2 ) vs. log[18C6] org plots.Also, the composition of I(−I) was speculated from the formal charge of Cd(II).This speculation was based on the experimental data plots of the log(D/[L] Bz ) vs. log [Pic − ] with the slope of two[7].These results indicated that the complexes composed of Cd(II):18C6:I(−I) = 1:1:2 were extracted into the employed ten diluents.

Figure 2 .
Figure 2. Plots of 298 K, where the symbols z A and S D,A K denote the formal charge of A − with its sign and a standard distribution constant at dep = 0 V for an A − transfer across the interface between the water and org bulk phases, respectively.The value of K D,A called a conditional distribution constant of A − into the org phase changed in depending on species of M(II), L, and diluent molecule [1]-[6] [8].Estimated dep values at z I = −1 were −0.01 0 V for the NB system, −0.06 4 for DCE, and 0.02 4 and 0.02 1 for DCM.Here, the following S D,I

[ 6 ]
[17].Using the experimental data sets of [L] org and [I − ], these values were calculated from

Figure 4 .
Figure 4. Variation of logK D,j with kinds of diluents.Here, j = I − (full circle), L (full diamond), CdL 2+ (square), and CdLI + (triangle) at L = 18C6.For the DCM system, the logK D,I value of −4.2 in Table 1 was used for this plot.

Table 1 .
Logarithmic values of K D,I , K ex± , K ex , and K ex,ip for the CdI 2 extraction with 18C6 into various diluents at 298 K.

Table 2
. Logarithmic values of K Cd/CdL , K 1,org , and K 2,org for the CdI 2 extraction with L = 18C6 into various diluents at 298 K. No. Diluent a I org /10 −7 mol•dm −3 a. .See the footnote a in Table 1; b.Average values calculated from experimental D/[L] org ones; c.Calculated from Equation (4); d.Calculated from Equation (5); e.The values were employed for the plots of Figure 3. Y. Kudo et al.DOI: 10.4236/ajac.2018.911041566 American Journal of Analytical Chemistry

Table 3 .
Logarithmic values of K ex2± , K D,CdLI , K D,CdL , and K D,L for the CdI 2 extraction with L = 18C6 into various diluents at 298 K.
a. See the footnote a inTable 1; b.Calculated from Equation (6); c.Calculated from Equation (7); d.Calculated from Equation ( 1,org K D,I ) one of −2.6 3 to −2.0 4 .On the other hand, the values of the CdPic 2 -B18C6 systems were evaluated to be in the −logK 1 range of −4.60 to −4.39 (see Appendix for the calculation of logK 1 j values were 5.4 Å for j = CdLCl 2 , 4.6 for CdLBr 2 , 4.0 for CdLI 2 , 4.1 for CdLPic 2 , and 3.6 for CdL 2+ at L = 18C6.As similar to the results of V CdL (see the Section 2.8), the R CdL value was smallest of the R j ones.The R Cd18C6 value (=3.6 Å) was larger than the following data of bond lengths 18C6 are in the order d Cd-Cl < d Cd-Br < d Cd-I , while the R j values are in that j = CdLCl 2 > CdLBr 2 > CdLI 2 .Additionally, it was shown that d Cd-Pic is apparently close to d Cd-I .Also, the bond lengths d Cd-Cl and d Cd-O in a Cd(18C6)Cl 2 crystal have been reported to be 2.364 Å and 2.752, respectively [24].The same trend is also observed in Cd(18C6)Br 2 and Cd(18C6)I 2 crystals [25]: d Cd-Br = 2.506 Å and d Cd-O = 2.752 for CdLBr 2 and d Cd-I = 2.692 and d Cd-O = 2.768 for CdLI 2 .Interestingly, the three d Cd-O values have been almost constant among the crystals.These results suggested that CdLCl 2 , CdLBr 2 , and CdLI 2 with L = 18C6 are close to solvent-separated or -shared ion pairs, such as CdL(OH 2 ) x A 2 , in phases.If this sug- R CdLI + ] org ≈ [I − ] org which was approximately derived from the charge balance equation for the org phase [1]-[6].At least, the conditions of of the CdI 2 extraction without L = 18C6.On the other hand, in the case of [CdLI + ] org /P ≈ K ex± /[I − ] org , we can immediately obtain was calculated from the experimental [Cd(II)] org , [Cd 2+ ], [18C6] org , and [I −