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COMPLEXES OF THE RARE EARTHS
CHAPTER 2
COMPLEXES OF THE RARE EARTHS IN RECENT years the complexing tendencies of the trivalent rareearth ions towards a variety of chelating agents have been investi gated and there has been a rapid accumulation of information regarding stepwise formation constants, factors influencing the formation as well as the magnetic moment of the chelates. These data proved to be particularly valuable with a view to answering several questions, such as, (1) whether it is pertinent to invoke the participation of the 4/electrons of lanthanides in chemical bonding during complex formation like the involvement of the d electrons of the first d-type transition series; (2) how does the coordination number vary within the rare-earth series? (3) can a suitable ligand stabilize the oxidation states other than the characteristic tripositive ones? (4) is there any possibility of charge transfer in case of rareearth complexes comparable to the 3d transition ones, where it is so frequently observed? A wealth of experimental, specially spectroscopic, evidence may be put forward against (1), and the unavailability of the wellshielded 4/electrons for bond formation is confirmed.f No profound changes, other than the perturbation of environmental type, were observed. Because of the pronounced shielding of the 4/ electrons, the spectral characteristics of the complex species are invariably the same as the central metal ion, which are observed in case of both absorption and luminescence spectra. Moreover, the occurence of the intramolecular energy transfer phenomenon within the chelates makes their study interesting, yielding additional information re garding the ions themselves. A detailed treatment of the luminescence t Evidence for weak covalent bonding in rare-earth complexes is given in ref. 7.9. 17
18
COMPLEXES OF THE RARE EARTHS
spectra and the mechanism of the intramolecular energy migration process is given in Chapter 8. Studies on the chelating tendencies of rare earths with various multidentate ligands favor the original suggestion of Anderegg, Nägeli, Müller and Schwarzenbach (21) that the rare-earth ions can expand their coordination beyond six and perhaps to eight or nine as in case of Nd(OH 2 ) 9 (Br0 3 ) 3 and Nd(OH 2 ) 9 (C 2 H 5 S0 4 ) 3 . Some evidence regarding this fact is discussed with individual chelates. Gadolinium chloride hexahydrate has been shown to contain [Cl2Gd(OH2)6]+, an eight-coordinated cation(2 2) which has a completely unsymmetrical configuration. This type of structure appears to be common in the rare earth and the actinide halide hexahydrates. The role of certain carboxylic acids in the separation of individual rare earths is important. Thus various hydroxy carboxylic acids have been employed to separate the trivalent lanthanides and actinides by ion-exchange techniques.(2·3~2·7) A trend of hydroxyisobutyrate > lactate > glycolate has been observed depending on the increase in efficiency of separation. The observations in the Oak Ridge Laboratories during 1947 suggested that the proper choice of complexing agents may prove fruitful for the separation of tripositive rare earths and extensive tracer scale experiments were carried out in the above laboratories. One phase of research was centered for obtaining the individual fission products in very pure state, so that they can be used in biological research, with particular interest focused on zirconium and its daughter, niobium, cerium and other rare earths, yttrium, barium, strontium, tellurium and iodine. Boyd et α/. (2 · 8 - 211) have found that these ions could be adsorbed on organic resins and could be selectively eluted by complexing agents under carefully controlled pH. They have extended their studies in many aspects of the method and the rate of exchange reaction. The observed series with decreasing adsorbability of ions by organic zeolites is as follows: La 3+ > Ce3+ > Pr 3+ > Nd 3+ > Sm3+ > Eu3+ > Gd 3+ > Tb 3+ > Dy 3+ > Y 3+ > Ho 3+ > Er3+ > Tm 3+ > Yb 3+ > Lu3+ > Sc3+. As the size of the ion increase the affinity of being adsorbed on resin decreases. We may consider an equilibrium of the type Mi(+aquo) +
.YH(IE)
^ ΜΛΙΕ), + xW
(10)
COMPLEXES OF THE RARE EARTHS
19
to exist in a solid ion exchanger, where H(IE) represents the protonated form of the ion exchanger. When a second metal (M 2 + ) is present this may undergo exchange and a second equilibrium M£(:quo) + M ^ E ) , ^ M a (IE), + Μ Γ
(11)
may be established. It has been shown that with a mixture of tripositive lanthanides adsorbed on a cation-exchange column, the separation may be affected with an eluent say 5 per cent citric acid at pH 3.2 and the lanthanides are obtained in a reverse sequence of the atomic numbers from the eluted portions. From eqns. (10) and (11) the distribution coefficients kx and k2 of the lanthanides Mx and M 2 may be written as kl
=
[M^my [Ml]
(I2a)
and
K
=
[M 2 (iE)j [M2]
(12b)
and the separation factor ß is defined as the ratio of the second distribution coefficient to the first one.
ß = K1K
(13)
Now the addition of a chelating agent (CA) in the above equilibriated system will re-establish new equilibria which may be represented by the following equations: Mi(aquo) + wCA(aquo) ^ M1(CA)„(aquo) M2(aquo) +
W
CA(aquo) ^ M2(CA)„(aquo)
(14a) (14b)
At this stage both complexed and uncomplexed species are present in solution and their distribution between solution and ion-exchange resin invokes another distribution coefficient, say k'19 for a parti cular lanthanide Mx and the modified relation is k,
1
=
[Mx(IE)] [M 1(aquo) ] + [MiiCA^uo)]
The usual formation constant KFi for the reaction given by the eqn. (14 a) may be expressed as v
—
[M1(CA)n(aquo)] [^Kaquo)] [CA(aquo)Jn
and can be rearranged to give the value of [M1(CA)/l(aquo)] (eqn. 16). [Mi(CA)e(eQU0)] = ^ [ M 1 ( g q u o ) ] [CA(aquo)]»
(16)
20
COMPLEXES OF THE RARE EARTHS
Substitution of eqn. (16) in eqn. (15) yields [M^IE)] [M1(aquo)] {1 + #Fl[CA(aquo)]"}
K=
which can be reduced to k {1 + KFi[CAimuo)]"}
k 1=
'
(17)
and if KF, the formation constant is very very large, eqn. (17) may be approximated to ArFi[CA(aquo)]" Equation (18) is an expression of the relation between the distribu tion coefficient and the formation constant (stability constant) of the complex species and how they are affected. A similar expression can be written for the second lanthanide (M2) where /Co
κ<% AF 2 L^A (aquo )J
Here again we may define the new separation factor β' as P
K
V*F.
P
KF2
W
In case of a strong chelating agent, the chance for the presence of unchelated species in solution is relatively small and, if the chelated species of two or more cations have different stability, the separa tion of the cations can be achieved. Thus a proper choice of chelat ing agent is necessary which will form complexes having widely varying stabilities. In the subsequent chapters the trends in stability of the complexed species are discussed. Tompkins et α/.(2·12) have separated some cationic species (fissionproduced radioisotopes including rare earths) by using Amberlite IR-1 resin and complexing agent followed by elution under carefully adjusted pH. Later, in their studies on the equilibrium of rare-earth complexes with ion-exchange resins, Tompkins et α/., ( 2 · 1 3 ' 2 1 4 ) by taking into account the various interdependent variables such as total rare-earth ions concentration, NH^ ion concentration, citrate ion concentration and the pH, have shown that the optimum condi tion for the separation with Dowex 50 resin is that the concentration of the rare-earth ions in solution should not exceed 3 x 10~4 M, and
COMPLEXES OF THE RARE EARTHS
21
with Amberlite IR-1 the concentration must be of the order of IO -6 M or less. The marked success of the ion-exchange process of separation using citrate ion as complexing agent depends on the fact that at optimum pH a competition between citrate ion and the resin is set up for the uptake of rare-earth ions. Spedding and his coworkers (215 ~ 217) were able to separate Ce and Y at an optimum pH 2.6 and Nd and Pr at pH 2.55. A high degree of perfection in separating the fission products of rare earths on a tracer scale using 5 per cent citrate as eluent (218_2 · 20) has been achieved recently. The experimental results*210) indicate that the primary factors determining the nature of the rate controlling step in the ionexchange mechanism are (i) distribution constant and (ii) particle size (radius). Large values of distribution constants and smaller particle size favor the rate determining step of film diffusion when the flow rate and temperature are held constant. Besides citrate ions, a number of other eluents have been tried and they may be classified as (a) hydroxy acids, (b) amino acids and (c) aminopolycarboxylic acids. Among the hydroxy acids glycolic, (2 · 3 ' 2 · 19 · 2 · 21) lactic/ 2 · 4 ' 2 · 5 · 2 · 19 · 2 · 22 · 223) *-hydroxyisobutyric,(2-7) malic ( 2 1 9 ' 2 2 4 ) and tartaric (2,25) have been successfully used to resolve the mixtures of rare earths. Aminoacetic acid, (225) aminopolyacetic acid, ethylenediaminetetraacetic acid (EDTA), (2,26 ~ 2,33) and nitriloacetic acid (NTA) (234 ~ 3,36) are also of extensive use in both micro and macro scale of separation. Although many eluents have been tried, none has proved as effective as EDTA for resolving the rare-earth mixtures.
FACTORS INFLUENCING THE FORMATION OF COMPLEXES
In aqueous medium the tripositive rare-earth ions exhibit strong hydrolysis and a distinct lowering of pH is noted when the salts of these elements are dissolved in water. The formation of aquo complex [M(OH2)]!J+ (where n is larger than six, perhaps eight or nine) takes place. The extent of lowering of pH depends on the concentration of the salt and the nature of particular rare-earth ion. Because of the lanthanide contraction, the heavier rareearth ions possessing small ionic radii have greater tendency to hydrolyze.
22
COMPLEXES OF THE RARE EARTHS
The aquo complex, having n H 2 0 molecules surrounding the central ion, has a definite structure and the cloud of water molecules has another geometry than the rest of the water. Thus when say MCI3 salts are dissolved in water there will be very little attraction between [M(OH2)„]3+ and the solvated Cl~ ion. Unless the other ions or ligands have a strong structure-breaking influence, the "sheath" (or "iceberg" according to Frank and Evans (237) ) of water molecule will so to say protect the rare-earth ions from the influence of other anions or ligands. The observations of Frank and Evans (237) and Freed ( 2 3 8 ) show that nitrate ions have large struc ture-breaking tendency over Cl~ ions. When complexes are formed, the approach of a ligand will inter fere with the sheath (the hydration shell) and the ordered geometry will break down. The multidentate ligands will presumably have a stronger influence than the unidentate ones. J. F. Duncan (2,39) has pointed out that, provided no changes in the structure of complex ion or the hydrated ion occur, the enthalpy of complex formation varies linearly with \jr (r = ion radii of the metal ions). However, in case of the rare earths no such linear relationship was observed and indicates that some structural change has occurred. Not only the nature of the metal ions and the complexing agents can affect the formation of the complexes, the environmental factors such as solvent, temperature and pressure are also important. The concentration factor may play some role in the stability of the com plexes. Substitutions on the ligands markedly influence the stability and formation of the complexes. Multidentate ligands often form stable complexes, possibly due to the formation of a chelate ring.
REFERENCES 2.1.
ANDEREGG, NÄGELI, MÜLLER and SCHWARZENBACH, Helv. Chim. Acta
2.2.
MAREZIO, PLETTINGER and ZACHARIASEN, Acta Cryst. 14, 234 (1961).
2.3.
STEWART, Proc. First Int. Conf. Peaceful use of Atomic Energy, Geneva 1955, vol. 7, paper 837 U N (1956) and Anal Chem. 27, 1279 (1955).
2.4.
THOMPSON, HARVEY, CHOPPIN and SEABORG, / . Amer. Chem. Soc. 76,
42, 827 (1959).
6229 (1954). 2.5.
CUNINGHAME, SIZELAND, WILLIS, EAKINS and MERCER, / . Inorg. Nucl.
Chem. 1, 163 (1955). 2.6. CHOPPIN, HARVEY and THOMPSON, / . Inorg. Nucl. Chem. 2, 66 (1956). 2.7. CHOPPIN and SILVA, / . Inorg. Nucl. Chem. 3, 153 (1956).
COMPLEXES OF THE RARE EARTHS 2.8.
23
KETELLE and BOYD, / . Amer. Chem. Soc. 69, 2800 (1947).
2.9. BOYD, SCHUBERT and ADAMSON, J. Amer. Chem. Soc. 69, 2818 (1947).
2.10. 2.11. 2.12. 2.13. 2.14.
BOYD, ADAMSON and MYERS, / . Amer. Chem. Soc. 69, 2936 (1947). BOYD, MYERS and ADAMSON, / . Amer. Chem. Soc. 69, 2849 (1947). TOMPKINS, KHYM and COHN, / . Amer. Chem. Soc. 69, 2769 (1947). HARRIS and TOMPKINS, / . Amer. Chem. Soc. 69, 2792 (1947). TOMPKINS and MAYER, / . Amer. Chem. Soc. 69, 2859 (1947).
2.15. SPEDDING, VOIGT, GLADROW and SLEIGHT, / . Amer. Chem. Soc. 69, 2777
(1947). 2.16. SPEDDING, VOIGT, GLADROW, SLEIGHT, POWELL, W R I G H T , BUTLER and
FIGARD, / . Amer. Chem. Soc. 69, 2786 (1947). 2.17. SPEDDING, FULMER, BUTLER, GLADROW, GOBUSH, PORTER, POWELL and
WRIGHT, / . Amer. Chem. Soc. 69, 2812 (1947). 2.18. KETELLE and BOYD, / . Amer. Chem. Soc. 73, 1862 (1951). 2.19. MAYER and FREILING, / . Amer. Chem. Soc. 75, 5647 (1953). 2.20. CORNISH, PHILLIPS and THOMAS, Canad. J. Chem. 34, 1471 (1956).
2.21. STEWART and FARIS, / . Inorg. Nucl. Chem. 3, 64 (1956). 2.22. FREILING and BUNNEY, / . Amer. Chem. Soc. 76, 1021 (1954). 2.23. PREOBRAZHENSKII, KALYAMIN and LILOVA, Zhur. Neorg. Khim. 2, 1164
(1957). 2.24. KAMENEV,
2.25. 2.26. 2.27. 2.28.
MARTYNENKO
and EREMIN,
RUSS.
J.
Inorg.
Chem.
2.29. SPEDDING, POWELL and WHEELWRIGHT, J. Amer.
Chem. Soc.
2557 (1954). 2.30. MARSH, / . Chem. Soc. 1957, 978.
2.31. 2.32. 2.33. 2.34. 2.35. 2.36. 2.37. 2.38. 2.39.
6, 880
(1961). VICKERY, / . Chem. Soc. 1952, 4357. HOLLECK and HARTINGER, Angew. Chem. 66, 586 (1954). LORIERS and QUESNEY, Compt. rend. 239, 1643 (1954). LORIERS, Compt. rend. 240, 1537 (1955).
FUGER, Bull. Soc. Chim. Beiges 66, 151 (1957). BRUNISHOLZ, Helv. Chim. Acta 40, 2004 (1957). LORIERS and LENOIR, Compt. rend. 247, 468 (1958). HOLLECK and HARTINGER, Angew. Chem. 68, 411, 412 (1956). LORIERS and CARMINATI, Compt. rend. 237, 1328 (1953). WOLF and MASSONNE, Chem. Tech. {Berlin) 10, 290 (1958). FRANK and EVANS, / . Chem. Phys. 13, 507 (1945). FREED, Rev. Mod. Phys. 14, 105 (1942). DUNCAN, Aust. J. Chem. 12, 356 (1959).
76, 612,
COMPLEXES OF THE RARE EARTHS IN RECENT years the complexing tendencies of the trivalent rareearth ions towards a variety of chelating agents have been investi gated and there has been a rapid accumulation of information regarding stepwise formation constants, factors influencing the formation as well as the magnetic moment of the chelates. These data proved to be particularly valuable with a view to answering several questions, such as, (1) whether it is pertinent to invoke the participation of the 4/electrons of lanthanides in chemical bonding during complex formation like the involvement of the d electrons of the first d-type transition series; (2) how does the coordination number vary within the rare-earth series? (3) can a suitable ligand stabilize the oxidation states other than the characteristic tripositive ones? (4) is there any possibility of charge transfer in case of rareearth complexes comparable to the 3d transition ones, where it is so frequently observed? A wealth of experimental, specially spectroscopic, evidence may be put forward against (1), and the unavailability of the wellshielded 4/electrons for bond formation is confirmed.f No profound changes, other than the perturbation of environmental type, were observed. Because of the pronounced shielding of the 4/ electrons, the spectral characteristics of the complex species are invariably the same as the central metal ion, which are observed in case of both absorption and luminescence spectra. Moreover, the occurence of the intramolecular energy transfer phenomenon within the chelates makes their study interesting, yielding additional information re garding the ions themselves. A detailed treatment of the luminescence t Evidence for weak covalent bonding in rare-earth complexes is given in ref. 7.9. 17
18
COMPLEXES OF THE RARE EARTHS
spectra and the mechanism of the intramolecular energy migration process is given in Chapter 8. Studies on the chelating tendencies of rare earths with various multidentate ligands favor the original suggestion of Anderegg, Nägeli, Müller and Schwarzenbach (21) that the rare-earth ions can expand their coordination beyond six and perhaps to eight or nine as in case of Nd(OH 2 ) 9 (Br0 3 ) 3 and Nd(OH 2 ) 9 (C 2 H 5 S0 4 ) 3 . Some evidence regarding this fact is discussed with individual chelates. Gadolinium chloride hexahydrate has been shown to contain [Cl2Gd(OH2)6]+, an eight-coordinated cation(2 2) which has a completely unsymmetrical configuration. This type of structure appears to be common in the rare earth and the actinide halide hexahydrates. The role of certain carboxylic acids in the separation of individual rare earths is important. Thus various hydroxy carboxylic acids have been employed to separate the trivalent lanthanides and actinides by ion-exchange techniques.(2·3~2·7) A trend of hydroxyisobutyrate > lactate > glycolate has been observed depending on the increase in efficiency of separation. The observations in the Oak Ridge Laboratories during 1947 suggested that the proper choice of complexing agents may prove fruitful for the separation of tripositive rare earths and extensive tracer scale experiments were carried out in the above laboratories. One phase of research was centered for obtaining the individual fission products in very pure state, so that they can be used in biological research, with particular interest focused on zirconium and its daughter, niobium, cerium and other rare earths, yttrium, barium, strontium, tellurium and iodine. Boyd et α/. (2 · 8 - 211) have found that these ions could be adsorbed on organic resins and could be selectively eluted by complexing agents under carefully controlled pH. They have extended their studies in many aspects of the method and the rate of exchange reaction. The observed series with decreasing adsorbability of ions by organic zeolites is as follows: La 3+ > Ce3+ > Pr 3+ > Nd 3+ > Sm3+ > Eu3+ > Gd 3+ > Tb 3+ > Dy 3+ > Y 3+ > Ho 3+ > Er3+ > Tm 3+ > Yb 3+ > Lu3+ > Sc3+. As the size of the ion increase the affinity of being adsorbed on resin decreases. We may consider an equilibrium of the type Mi(+aquo) +
.YH(IE)
^ ΜΛΙΕ), + xW
(10)
COMPLEXES OF THE RARE EARTHS
19
to exist in a solid ion exchanger, where H(IE) represents the protonated form of the ion exchanger. When a second metal (M 2 + ) is present this may undergo exchange and a second equilibrium M£(:quo) + M ^ E ) , ^ M a (IE), + Μ Γ
(11)
may be established. It has been shown that with a mixture of tripositive lanthanides adsorbed on a cation-exchange column, the separation may be affected with an eluent say 5 per cent citric acid at pH 3.2 and the lanthanides are obtained in a reverse sequence of the atomic numbers from the eluted portions. From eqns. (10) and (11) the distribution coefficients kx and k2 of the lanthanides Mx and M 2 may be written as kl
=
[M^my [Ml]
(I2a)
and
K
=
[M 2 (iE)j [M2]
(12b)
and the separation factor ß is defined as the ratio of the second distribution coefficient to the first one.
ß = K1K
(13)
Now the addition of a chelating agent (CA) in the above equilibriated system will re-establish new equilibria which may be represented by the following equations: Mi(aquo) + wCA(aquo) ^ M1(CA)„(aquo) M2(aquo) +
W
CA(aquo) ^ M2(CA)„(aquo)
(14a) (14b)
At this stage both complexed and uncomplexed species are present in solution and their distribution between solution and ion-exchange resin invokes another distribution coefficient, say k'19 for a parti cular lanthanide Mx and the modified relation is k,
1
=
[Mx(IE)] [M 1(aquo) ] + [MiiCA^uo)]
The usual formation constant KFi for the reaction given by the eqn. (14 a) may be expressed as v
—
[M1(CA)n(aquo)] [^Kaquo)] [CA(aquo)Jn
and can be rearranged to give the value of [M1(CA)/l(aquo)] (eqn. 16). [Mi(CA)e(eQU0)] = ^ [ M 1 ( g q u o ) ] [CA(aquo)]»
(16)
20
COMPLEXES OF THE RARE EARTHS
Substitution of eqn. (16) in eqn. (15) yields [M^IE)] [M1(aquo)] {1 + #Fl[CA(aquo)]"}
K=
which can be reduced to k {1 + KFi[CAimuo)]"}
k 1=
'
(17)
and if KF, the formation constant is very very large, eqn. (17) may be approximated to ArFi[CA(aquo)]" Equation (18) is an expression of the relation between the distribu tion coefficient and the formation constant (stability constant) of the complex species and how they are affected. A similar expression can be written for the second lanthanide (M2) where /Co
κ<% AF 2 L^A (aquo )J
Here again we may define the new separation factor β' as P
K
V*F.
P
KF2
W
In case of a strong chelating agent, the chance for the presence of unchelated species in solution is relatively small and, if the chelated species of two or more cations have different stability, the separa tion of the cations can be achieved. Thus a proper choice of chelat ing agent is necessary which will form complexes having widely varying stabilities. In the subsequent chapters the trends in stability of the complexed species are discussed. Tompkins et α/.(2·12) have separated some cationic species (fissionproduced radioisotopes including rare earths) by using Amberlite IR-1 resin and complexing agent followed by elution under carefully adjusted pH. Later, in their studies on the equilibrium of rare-earth complexes with ion-exchange resins, Tompkins et α/., ( 2 · 1 3 ' 2 1 4 ) by taking into account the various interdependent variables such as total rare-earth ions concentration, NH^ ion concentration, citrate ion concentration and the pH, have shown that the optimum condi tion for the separation with Dowex 50 resin is that the concentration of the rare-earth ions in solution should not exceed 3 x 10~4 M, and
COMPLEXES OF THE RARE EARTHS
21
with Amberlite IR-1 the concentration must be of the order of IO -6 M or less. The marked success of the ion-exchange process of separation using citrate ion as complexing agent depends on the fact that at optimum pH a competition between citrate ion and the resin is set up for the uptake of rare-earth ions. Spedding and his coworkers (215 ~ 217) were able to separate Ce and Y at an optimum pH 2.6 and Nd and Pr at pH 2.55. A high degree of perfection in separating the fission products of rare earths on a tracer scale using 5 per cent citrate as eluent (218_2 · 20) has been achieved recently. The experimental results*210) indicate that the primary factors determining the nature of the rate controlling step in the ionexchange mechanism are (i) distribution constant and (ii) particle size (radius). Large values of distribution constants and smaller particle size favor the rate determining step of film diffusion when the flow rate and temperature are held constant. Besides citrate ions, a number of other eluents have been tried and they may be classified as (a) hydroxy acids, (b) amino acids and (c) aminopolycarboxylic acids. Among the hydroxy acids glycolic, (2 · 3 ' 2 · 19 · 2 · 21) lactic/ 2 · 4 ' 2 · 5 · 2 · 19 · 2 · 22 · 223) *-hydroxyisobutyric,(2-7) malic ( 2 1 9 ' 2 2 4 ) and tartaric (2,25) have been successfully used to resolve the mixtures of rare earths. Aminoacetic acid, (225) aminopolyacetic acid, ethylenediaminetetraacetic acid (EDTA), (2,26 ~ 2,33) and nitriloacetic acid (NTA) (234 ~ 3,36) are also of extensive use in both micro and macro scale of separation. Although many eluents have been tried, none has proved as effective as EDTA for resolving the rare-earth mixtures.
FACTORS INFLUENCING THE FORMATION OF COMPLEXES
In aqueous medium the tripositive rare-earth ions exhibit strong hydrolysis and a distinct lowering of pH is noted when the salts of these elements are dissolved in water. The formation of aquo complex [M(OH2)]!J+ (where n is larger than six, perhaps eight or nine) takes place. The extent of lowering of pH depends on the concentration of the salt and the nature of particular rare-earth ion. Because of the lanthanide contraction, the heavier rareearth ions possessing small ionic radii have greater tendency to hydrolyze.
22
COMPLEXES OF THE RARE EARTHS
The aquo complex, having n H 2 0 molecules surrounding the central ion, has a definite structure and the cloud of water molecules has another geometry than the rest of the water. Thus when say MCI3 salts are dissolved in water there will be very little attraction between [M(OH2)„]3+ and the solvated Cl~ ion. Unless the other ions or ligands have a strong structure-breaking influence, the "sheath" (or "iceberg" according to Frank and Evans (237) ) of water molecule will so to say protect the rare-earth ions from the influence of other anions or ligands. The observations of Frank and Evans (237) and Freed ( 2 3 8 ) show that nitrate ions have large struc ture-breaking tendency over Cl~ ions. When complexes are formed, the approach of a ligand will inter fere with the sheath (the hydration shell) and the ordered geometry will break down. The multidentate ligands will presumably have a stronger influence than the unidentate ones. J. F. Duncan (2,39) has pointed out that, provided no changes in the structure of complex ion or the hydrated ion occur, the enthalpy of complex formation varies linearly with \jr (r = ion radii of the metal ions). However, in case of the rare earths no such linear relationship was observed and indicates that some structural change has occurred. Not only the nature of the metal ions and the complexing agents can affect the formation of the complexes, the environmental factors such as solvent, temperature and pressure are also important. The concentration factor may play some role in the stability of the com plexes. Substitutions on the ligands markedly influence the stability and formation of the complexes. Multidentate ligands often form stable complexes, possibly due to the formation of a chelate ring.
REFERENCES 2.1.
ANDEREGG, NÄGELI, MÜLLER and SCHWARZENBACH, Helv. Chim. Acta
2.2.
MAREZIO, PLETTINGER and ZACHARIASEN, Acta Cryst. 14, 234 (1961).
2.3.
STEWART, Proc. First Int. Conf. Peaceful use of Atomic Energy, Geneva 1955, vol. 7, paper 837 U N (1956) and Anal Chem. 27, 1279 (1955).
2.4.
THOMPSON, HARVEY, CHOPPIN and SEABORG, / . Amer. Chem. Soc. 76,
42, 827 (1959).
6229 (1954). 2.5.
CUNINGHAME, SIZELAND, WILLIS, EAKINS and MERCER, / . Inorg. Nucl.
Chem. 1, 163 (1955). 2.6. CHOPPIN, HARVEY and THOMPSON, / . Inorg. Nucl. Chem. 2, 66 (1956). 2.7. CHOPPIN and SILVA, / . Inorg. Nucl. Chem. 3, 153 (1956).
COMPLEXES OF THE RARE EARTHS 2.8.
23
KETELLE and BOYD, / . Amer. Chem. Soc. 69, 2800 (1947).
2.9. BOYD, SCHUBERT and ADAMSON, J. Amer. Chem. Soc. 69, 2818 (1947).
2.10. 2.11. 2.12. 2.13. 2.14.
BOYD, ADAMSON and MYERS, / . Amer. Chem. Soc. 69, 2936 (1947). BOYD, MYERS and ADAMSON, / . Amer. Chem. Soc. 69, 2849 (1947). TOMPKINS, KHYM and COHN, / . Amer. Chem. Soc. 69, 2769 (1947). HARRIS and TOMPKINS, / . Amer. Chem. Soc. 69, 2792 (1947). TOMPKINS and MAYER, / . Amer. Chem. Soc. 69, 2859 (1947).
2.15. SPEDDING, VOIGT, GLADROW and SLEIGHT, / . Amer. Chem. Soc. 69, 2777
(1947). 2.16. SPEDDING, VOIGT, GLADROW, SLEIGHT, POWELL, W R I G H T , BUTLER and
FIGARD, / . Amer. Chem. Soc. 69, 2786 (1947). 2.17. SPEDDING, FULMER, BUTLER, GLADROW, GOBUSH, PORTER, POWELL and
WRIGHT, / . Amer. Chem. Soc. 69, 2812 (1947). 2.18. KETELLE and BOYD, / . Amer. Chem. Soc. 73, 1862 (1951). 2.19. MAYER and FREILING, / . Amer. Chem. Soc. 75, 5647 (1953). 2.20. CORNISH, PHILLIPS and THOMAS, Canad. J. Chem. 34, 1471 (1956).
2.21. STEWART and FARIS, / . Inorg. Nucl. Chem. 3, 64 (1956). 2.22. FREILING and BUNNEY, / . Amer. Chem. Soc. 76, 1021 (1954). 2.23. PREOBRAZHENSKII, KALYAMIN and LILOVA, Zhur. Neorg. Khim. 2, 1164
(1957). 2.24. KAMENEV,
2.25. 2.26. 2.27. 2.28.
MARTYNENKO
and EREMIN,
RUSS.
J.
Inorg.
Chem.
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