Werner Centennial - ACS Publications

21

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Coordination Chemistry of the Lanthanide Elements — One Hundred Years of Development and Understanding THERALD MOELLER Noyes Chemical Laboratory, University of Illinois, Urbana, Ill.

Coordination compounds of the lanthanide elements are distinguished by profound property differences from the derivatives of the d-transition elements and similarities to those of the alkaline-earth elements. Predominantly ionic bonding and lack of interactions of the 4f orbitals are significant in establishing these differences and similari ties. Coordination numbers of at least six through 12 are observed. Irregularities in thermodynamic stability in aqueous solution are traceable to changes in coordina tion number or differences in degree of penetration of the primary hydration sphere. Complexation alters to a limited degree properties dependent upon the 4f electrons but changes significantly and quite regularly properties dependent upon ion concentrations and useful in separa tions. Absorption of energy by aromatic ligands leads to important fluorescent or line-like emission with certain cations.

t the time of Alfred Werner's birth, the only lanthanide elements that had been identified positively were lanthanum and cerium. Y t t r i u m , a lanthanide element by all criteria except electronic configuration, was known also. However, in 1891 when Werner proposed the substance of the coordination theory, all of the elements of the lanthanide series except promethium, europium, and lutetium had been clearly identified and quite well characterized. Only promethium remained undiscovered at the time of Werner's demise. 306 In Werner Centennial; Kauffman, G.; Advances in Chemistry; American Chemical Society: Washington, DC, 1967.


21.

MOELLER

307

Lanthanide Elements

Although it might be assumed quite reasonably that the coordination chemistry of the some 15 elements known during Werner's lifetime would have developed significantly during that period, the literature contains only a few accounts of synthesis and characterization. Indeed, the 1920 edition of Werner's classic monograph (50) records essentially the same information as the 1908 edition, namely the compositions of a limited number of double nitrates, sulfates, and oxalates. Both Spencer (45) and Little (28), in their comprehensive compilations of data at about the same time, describe only certain lanthanide double salts and adducts, but not within the framework of complex compounds or coordination chemistry. Subsequent developments prior to the 1940% were meager, and even as late as 1953 the significant aspects of this area were reviewed in terms of only 60 literature citations (88). Yet a 1965 review covering published information only through 1962 required in excess of 500 references (86), and the current literature contains literally a flood of accounts of various aspects of the coordination chemistry of the lanthanide elements. Certainly the problems involved in separation and purification limited severely most early attempts to prepare and study the complex species that would have permitted logical extensions of Werner's ideas to this series of elements. Paradoxically, it was the utilization of differences i n the proper ties of certain complex species, i n the 1940's and subsequently, that per mitted the development of the large-scale fractionation procedures that have made the lanthanide elements available in sizeable quantities and high purities at not unreasonable prices and have thus permitted more ex tensive and comprehensive investigations of their compounds. However, lack of availability cannot account completely for the paucity of data i n the early literature. Significant also is the general reluctance of the lanthanide ions to form complex species of substantial thermodynamic stability and the profound differences between the properties of those species that do form and the corresponding derivatives of the d-type transi tion metals. The synthesis of new and strongly complexing ligands, i n particular those of the chelating type, has done much to prompt more recent studies in this broad area.

Coordination Chemistry of the Lanthanide Elements— General Survey The formation and properties of the lanthanide complex species can be best understood by summarizing first some of the pertinent general charac teristics of these elements. I n their ground states, the lanthanide atoms have the characteristic valence-shell electronic configurations 4 / 5 d 6 s or 4 / 6 s , where n = 0 for lanthanum and 14 for lutetium, overlying the closed-shell xenon arrangement. The atoms are large and readily oxidized. In both aqueous or nonaqueous systems and the solid state, oxidation n

n + 1

2

In Werner Centennial; Kauffman, G.; Advances in Chemistry; American Chemical Society: Washington, DC, 1967.

x

2

308

WERNER CENTENNIAL

yields the tripositive ions ( L n ) preferentially, probably as a consequence of a fortuitous balance between ionization energy and solvation or lattice energy, respectively (10). Only a limited number of strongly reducing dipositive ( S m , E u , Yb+ ), or strongly oxidizing tetrapositive ( C e ) ions, can be distinguished in solution. A l l of the cations are fundamentally of the noble-gas type, the distinguishing 4/ electrons being essentially completely shielded from direct environmental and chemical effects. As a consequence of the increase in nuclear charge without the compensating effect of adding balancing electrons to higher energy levels, crystal radius in a given oxidation state decreases with increasing atomic number (the lanthanide contraction). Among the tripositive ions, this decrease in size accounts for the differences in the degree to which any property that per mits fractional separation is exhibited. That the size of the yttrium (III) ion is duplicated in the general dysprosium-erbium region accounts for the lanthanide-like properties that are responsible both for the natural occur rence of yttrium with and for the experimental difficulties encountered i n separating yttrium from the heavier lanthanide elements. Such unusual properties (e.g., paramagnetism, radiant energy absorption, and emission), as are characteristic of certain of the lanthanide ions, can be traced to the presence of unpaired 4/ electrons. In most other respects, these ions are remarkably similar to the alkaline-earth metal ions. +3


+2

+ 2

2

+4

A comparison of the coordination chemistry of the lanthanide elements with that of the d-type transition metals shows some striking differences in thermodynamic stability, magnetic and spectral properties, and bonding. Appreciably stable species, other than the hydrated cations, can be ob tained only with the most strongly chelating ligands and, in particular, only when these ligands contain in their molecular structures highly elec tronegative donor atoms (e.g., oxygen). Commonly, the magnetic proper ties, the color, and the light absorption and emission characteristics of the lanthanide complex species differ little or not at all from those of either the hydrated cations or the stripped cations in ionic crystals. Isomerism and slow substitution reactions are uncommon and poorly characterized. Both the ground-state electronic configurations of the lanthanide cations and the radial extension of the 4 / orbitals are important in account ing for differences between the properties of these complex species and those of the d transition-metal ions and for the general properties of the lanthanide species themselves. Among the d transition-metal ions, the d orbitals are in the valence-shell arrangements and are thus both involved in bonding and affected i n their energy distributions by the nature of the ligands present. Shielding of the 4/ orbitals is sufficient, however, that there is little likelihood of electrons i n them participating in bonding through hybridization or other type of orbital interaction. It is probable, therefore, that cation-ligand interactions among the lanthanide complex species are largely electrostatic with a minimum of crystal- or ligand-field


21.

MOELLER

309

Lanthanide Elements

stabilization (22, 34, 36, 40, 4?)- The expected resemblances between these species and the corresponding ones derived from the ions Ca+ , S r , and B a are observed i n practice. Experimental evidences, largely i n terms of magnetic and spectral data, and very rapid substitution reactions supporting this point of view, are multiple (36). B y contrast, although there are some evidences for covalent interaction (22-25), this type of interaction is much less extensive. 2

+2


+ 2

In terms of this model, both cation size and differences i n the sizes of the various cations are important. I n any state of oxidation, a lanthanide ion is comparatively large. Strong electrostatic attractions between such an ion and a ligand must, therefore, be limited. These attractions may be expected to increase in magnitude for a given oxidation state, as the cationic radius decreases, and for a given lanthanide element, as the cationic charge increases. Both of these variations are well established (36). Although several factors appear to mitigate against their formation, many complex species derived from the lanthanide ions have been de scribed (36). Some of these appear to exist only as ion pairs i n solution and do not have sufficient stability to permit them to carry through series of reactions without concomitant dissociation. Others exist only i n the solid state and dissociate or decompose upon dissolution. Still others are of sufficient stability to exist as such, both in solution and as isolable solids. Essentially all of the species described are derived from the tripositive ions, and essentially no information is available relative to complex species i n nonaqueous systems. A broad classification of typical species is given i n Table I (36). Some Topics of Current Investigative and Practical Interest Interest in the lanthanide complex species is centered currently largely in coordination number and stereochemistry, i n thermodynamic stability and its interpretation, i n the effects of the ligand on the properties of the lanthanide ion, and i n practical applications. Coordination

Number and Stereochemistry

Although many of the compositions summarized i n Table I appear to be consistent with an assignment of coordination number 6 to each of the lanthanide ions i n its complex species, an increasingly impressive array of evidence supporting coordination numbers of 7, 8, 9, and even 10 and 12 (36) has appeared recently. Included among the experimental evidences are ionic compositions such as [ L n ( H 0 ) ] , [Ln(H 0)6Cl ]+, [ L n ( D T P A ) ] " , [Ln(NTA) ]~ , [ L n ( H E D T A ) ( I M D A ) ] , and [ L n F ] , each of which suggests operating a coordination number larger than 6. 2

2

2

3

9

+3

2

- 2


2

7

- 3

310

WERNER CENTENNIAL

Table I.

A Classification of Typical Complex Species Oxidation State of Ln

Class

Ion-pair associations (in solution)

1

6

[LnX]+* (X = CI, Br, I, N 0 , SCN, C10 ) [Ce(OH)] *

+3

3

4

+4

+

L n C V z N H a (x = 1 - 8) L n X 6 ap (X = S C N , I, C10 ) Ln(N0 ) -3TBP Ln(C10 ) 4 D M A L n X - z phen (X = CI, S C N , N 0 , C H 0 ; x = 2,3)

+3

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Examples' '

3

4

3

3

4

3

3

3

Chelates (in solution or isolable)

2

3

2

[Eu(EDTA)]-* [Ln(on) ] [Ln(diket) -zH 0] (x = 1 - 3) (BH)[Ln(diket) ] [Ln(EDTA)(H 0) ]" [Ln(NTA) ] - (x = 1,2) [Ln(HEDTA)(IMDA)]~ [Ce(on) ] [Ce(diket) ]

+2 +3

3

3

2

4

2

x

3

3

3a!

2

+4

4

4

Miscellaneous (halo)

+3 +4

M [LnF ], M i[LnF ] M i[LnF ] (Ln = Ce, Pr) M i[LnF ] (Ln = Ce, Pr, Nd, Tb, Dy) (BH) [LnCl ] I

4

6

3

7

3

6

3

2

6

Abbreviations: ap, antipyrine; T B P , tri-(n-butyl)phosphate; D M A , N,Ndimethylacetamide; phen, 1,10-phenanthroline; E D T A , ethylenediamine-AîV ,A ',iV tetraacetate; on, 8-quinolinolate; diket, 1,3-diketonate; B , organic amine; N T A , nitrilotriacetate; H E D T A , iV'-(2-hydroxyethyl)ethylenediamine-iV,i\r,iV -triacetate; I M D A , iminodiacetate. Water molecules often present also but not always indicated. 0

r

r

v

/

6

( D T P A = diethylenetriamine-iV ,iV ,iV ,iV ,iV -pentaacetate; T = tropolonate). Similar evidence is afforded by the syntheses of compounds such as [Ln(diket) • 3 H 0 ] , (BH)[Ln(diket) ], [ H L a ( E D T A ) (H 0) ], M [ L n T ] {37), and many others (17, 32, 36, 42, 4S). Appropriate stoichiometric composition does not in itself positively indicate a coordination number. However, the ions [LnCHÔ^]" " and [ G d ( H 0 ) 6 C l ] have been detected and described quantitatively i n crystals (36). Crystal-structure determinations have established 10-coordination in the molecular species [ H L a ( E D T A ) (H 0) ] and 9-coordination i n the ionic species [ L n ( E D T A ) (H 0) ]~ [Ln = L a , Tb), with the 10-coordinate anionic species persisting for L n = L a - S m and the 9-coordinate species for L n = T b - L u (19, 26, 27). Among these ions, the transition from 10- to 9-coordination appears i n the vicinity of L n = E u , G d as a consequence of r

3

I

r

,

r/

/,

2

4

2

4

1

2

2

+

2

2

4

3


3

4

21.

MOELLER

311

Lanthanide Elements

cationic size being decreased {26). Eight-coordination has been suggested for the 1,3-diketone chelates of the type (BH)[Ln(diket) ] on the basis of spectroscopic data (1, 2) and for the tropolonate ions [LnT ]~ on the basis of proton nuclear magnetic resonance data (37). A n ion of the type [ L n ( H 0 ) ] has trigonal prismatic geometry, with a water molecule opposite each rectangular face (18). In crystals of the salt L a ( S 0 ) - 9 H 0 , some of the L a ions are surrounded icosahedrally by 12 oxygen atoms from sulfate ions, whereas other La+ ions are 9-coordinate (20, 81). The ion [Gd(H 0) Cl ]+ is probably a hybrid of a square antiprism and a triangular dodecahedron, with coordination number 8 (80). In the molecule [ H L a ( E D T A ) (H 0) ], the four carboxylate oxygen atoms, the two ethylenediamine nitrogen atoms, and one of the water oxygen atoms lie very nearly at seven of the eight vertices of a quasi-D d dodecahedron surrounding the La+ ion (26, 27). The mean of the posi tions of the other three water molecules defines the eighth dodecahedral position. The entire arrangement is that of two hemispheres—one con taining the four water molecules and the other the ethylenediaminetetraacetate donor sites. Removing the acidic proton effects a general shrink age which results in a single water molecule being ejected and the formation of the geometrically similar ion [ L a ( E D T A ) ( H 0 ) ] - (26, 27). Eightcoordination in the [Ln(diket) ]~ ions also gives a dodecahedral array of donors about the central Ln+ ion (2). 4

4

2

2

4

3

9

+3

+ 3

2


3

2

6

2

2

4

2

3

2

3

4

3

Geometrical isomerism is potentially possible among many of the lanthanide chelates. The 1,3-diketone chelates, the species derived from the aminepolycarboxylic acids, the tropolonates, and many other complex derivatives are asymmetric and, thus, potentially capable of exhibiting optical isomerism. That the only resolutions reported have been limited to some tris(diketone) compounds (35) may reflect more the tendency of these ionically-bonded species to racemize rapidly in polar environments than in the absence of asymmetry. The existence of more than a single modification of each of several tetrakis(di-keto)europium(III) compounds may reflect asymmetry also (1). Thermodynamic

Stability

Properly speaking, the thermodynamic stability of a complex species is measured by the free-energy change (AG) that occurs in its formation from its components. Both the enthalpy (AH) and the entropy (AS) changes are reflected in the free-energy change and may be significant i n determining its numerical magnitude. Indeed, when complex species are based upon chelating ligands, the entropy change is often the more signifi cant of the two (36). The free-energy change is, of course, directly related to the thermodynamic formation constant (K) of the species in question. Few, if any, thermodynamic formation-constant data are available for the



312

WERNER

CENTENNIAL

lanthanide complex species at a standard state of infinite dilution, but many concentration-based values have been determined experimentally (36, 41), often under conditions of constant ionic strength (ju)—i.e., con stancy of activity coefficients. Typical of some of the more recent data are those listed in Table II. Enthalpy and, ultimately, entropy changes have been obtained for a number of systems in terms of the tem perature dependence of the formation-constant values, but only in a few instances have the enthalpy changes been determined calorimetrically (12-15, 29, 38, 47)' Some typical thermodynamic data are given in Table III. Table II.

Some Formation-Constant Data for 1:1 Complex Species at 25°C. logK

Cation

PDTA(21)«

IB(5)*>

La Ce+

16.42 16.79 17.17 17.54

1.57 1.62 1.80 1.91

2.22 2.37 2.48 2.54

17.97 18.26 18.21 18.64 19.05 19.30 19.61 20.08 20.25 20.56 18.78

2.00 1.98 1.86 1.73 1.65 1.63 1.61 1.61 1.62 1.65 1.64

2.63 2.71 2.71 2.87 2.95 2.98 3.03 3.13 3.18 3.21 2.88

+ 3 3

p +3 r

Nd+ Pm+ Sm+ Eu+ Gd+ Tb+ Dy+ Ho+ Er+ Tm Yb+ Lu

3

—

3

3

3 3

3

3

3

3

+ 3 3

+ 3

Y+3 0 6 c

K NO ~(6)

HIBU6Y

—

z

—

1.63 — —

—

P D T A , propvlenediamine-iV^^Âr'.tetraacetate ( I B , isobutyrate ( = 2.00 M). H I B , a-hydroxyisobutyrate.

M

2.48 2.04 — —

1.13 — — —

0.56 — — —

= 0.1 M 20°C). }

M

Formation-constant or free-energy data indicate that for a given ligand a lanthanide ion gives a generally less stable complex species than does a d transition-metal ion (41). Exact comparisons among the lan thanide ions as a whole are restricted by the limited number of ligands for which data for the entire series are available. Predictions, in terms of the electrostatic concept of bonding, that thermodynamic stability should i n crease with decreasing crystal radius of the L n ion, or with increasing nuclear charge, for a given ligand are i n overall accord with observation for the lighter cations (La+ -Sm+ or E u ) , but not necessarily for the heavier cations (Eu+ -Lu ). For the ions above G d , trends i n stabilities + 3

3

3

+3

3

+ 3

+ 3


21.

MOELLER

Table III.

313

Lanthanide Elements

Enthalpy and Entropy Changes for the Formation of Some Typical 1:1 Complex Species at 25°C. tt

EDTA(29) Ion

AH°

La Ce+

+ 3 3

p +3


r

Nd+ Pm Sm Eu+ Gd+ Tb Dy+ Ho Er+ Tm Yb Lu

3

NTA(88)

Dipic(12, H)

b

AS°

AH

IB(5)

c

AS°

AH°

AS

AH

AS

0

-2.926 -2.938 -3.198 -3.623

59.7 60.8 61.4 61.3

0.320 -0.215 -0.502 -0.803

48.8 48.8 48.9 48.8

-3.125 -3.547 -3.913 -4.012

25.8 26.1 26.1 26.5

3.47 3.33 3.02 2.84

18.8 18.6 18.4 18.3

-3.349 -2.558 -1.730 -1.114 -1.211 -1.356 -1.708 -1.870 -2.310 -2.512 -0.588

64.4 67.6 71.2 75.5 77.3 78.0 78.3 79.1 79.2 79.1 77.5

-1.047 -1.029 -0.626 -0.61 0.350 0.543 0.593 0.585 0.400 0.180 1.027

49.1 49.1 50.7 52.8 54.8 56.1 56.9 57.8 58.0 57.7 56.0

-4.283 -4.073 -3.582 -2.689 -2.169 -1.946 -1.850 -1.834 -1.925 -2.191 -1.438

25.9 26.6 27.8 30.5 32.4 33.2 33.8 34.2 34.0 33.9 33.8

2.66 2.91 3.45 4.38 5.04 5.31 5.49 5.39 5.35 5.35 5.36

18.1 18.8 20.1 22.6 24.5 25.3 25.8 25.5 25.4 25.5 25.5

+ 3

+3 3 3

+ 3

3

+ 3 3

+ 3

+ 3

+ 3

Y+3

° A l l AH values in kcal./mole; all AS values in cal./mole-deg. papers for significance of A H vs. AH°, AS vs. AS°. At20°C. Dipic, dipicolinate.

Consult original

6 c

are qualitatively of three types: increase with increasing nuclear charge, little or no change with increasing nuclear charge, and maximum stability with some cation i n the series (36). Detailed examination of formationconstant data commonly show discontinuities i n stability at the E u or G d ion, which suggest that i n the middle of the series stability is less than would be predicted on the basis of the purely electrostatic model. It is significant also that the Y+ ion, which on the basis of electrostatic inter action alone should position itself with the D y - E r + ions, does so only where trends of the first type are observed (36). For the other two cases, this ion appears with the lighter lanthanide ions, specifically, close to the N d ion. N o completely adequate explanation has yet been offered to account for observed trends i n thermodynamic stability, the gadolinium break, and the migratory behavior of the yttrium ion (36). To the crystal-field, stereochemical, and thermodynamic arguments previously presented (86) may be added both variants and some new approaches. Structural studies in the E D T A series suggest strongly that change i n the coordination num ber of the central Ln+ ion does indeed occur i n the general Gd+ -ion region (26). Choppin and co-workers (6-8, 48) have shown by stability-constant, enthalpy, and entropy data that ions of the type L n X ( X = CI, S C N , N 0 ) are outer-sphere ion pairs, i n which the primary hydration sphere of + 3

+ 3

3

+3

3

+ 3

3

3

+ 2

3


314

WERNER CENTENNIAL

the lanthanide ion is largely retained, and for which the enthalpy change favors and the entropy change opposes their formation. The thermo dynamic stabilities of these species reflect only changes in ionic attraction and vary linearly with properties such as ionic conductance that are pro portional to the radii of the hydrated cations. Carboxylate and monofluoro complex ions, on the other hand, appear to be of the inner-sphere type (5, 14, 88, 48), in which the primary hydration sphere of the L n ion has been broken to greater or lesser degree, and for which enthalpy change opposes and entropy change favors their formation. The stabilities of these species indicate that three hydration regions exist, namely L a - N d , Pm+ -Tb+ , and D y - L u + , for the first and last of which the hydration sphere increases slowly i n size with increasing atomic number and for the second of which the increase is rapid. Variation i n stability thus reflects the effects of opposing enthalpy and entropy contributions. The same type of explanation appears to be reasonable for the aminepolycarboxylate chelates (86, 47). Although most reported enthalpy and entropy values show wide and unsystematic variations, eliminating the complicating effects of changing effective hydration number of the uncomplexed lan thanide ion near the middle of the series has shown that the enthalpy of reaction of the diglycollate or dipicolinate ion with the crystalline ethylsulfates varies nearly monotonically with atomic number (47). Stabiliza tion of the complex species with respect to those of lanthanum, gadolinium, and lutetium is of the order of only a few hundred calories per mole (47). The gadolinium break and the position of yttrium are consequences of fortuitous combinations of effects. Certainly attempts to correlate these variations directly with crystal radii are impractical because the variations in crystal radii may well be smoothed out or altered i n the hydration process.


+ 3

+ 3

3

3

+3

+ 3

3

Bonding Little can be added to or taken from the wealth of information sup porting the concept of electrostatic interaction between ligand and lan thanide ion (86). That thermodynamic stability data do not agree com pletely with the predictions of the simple ionic model can be associated with changes in the sizes of and degrees of penetration into the hydration spheres of the cations without obviating the general concept. Asymmetry, bond-length data, and change in coordination number with increasing atomic number among the ethylenediaminetetraacetate chelates are all i n complete accord with the concept of electrostatic interaction (26). Effects of Complexation on the Properties of Ln

+Z

Ions

As may be expected, complexation has all of the effects attendant upon reducing the concentrations of the lanthanide ions in solution. Further-



21.

MOELLER

Lanthanide Elements

315

more, by shielding the cations themselves from external influences and chemical attack, complexation often enhances the solubilities of the lan thanide species i n non-aqueous solvents and their extractability into such media. Volatility may be enhanced also and even selectively, as is evi denced by the successful gas chromatographic separation of the lanthanide ions as their tris(dipivaloylmethane) chelates (11). The effects upon color and light absorption and upon magnetic properties are strikingly less with the lanthanide ions than with the d transition-metal ions, as a consequence of course of the fact that the electrons that are responsible for these proper ties i n the lanthanide species are not in the valency shell. Of particular current interest are the fluorescence and laser properties of certain chelates of specific lanthanide ions (1,3, 4,9,82,89,44)That a number of the cations have characteristic fluorescence spectra has been well-known for many years. However, present interest and development can be traced to Weissman's observation (49) that certain europium (III) chelates, either as crystals or i n benzene solution, fluoresce v i a a mechanism involving the broad absorption of energy by the aromatic portions of the ligands and reemission of a portion of this energy as narrow and highly char acteristic spectral lines. Subsequent investigation has shown that initial absorption of energy raises the chelate to a vibrational level i n the first excited singlet state (Si). Deactivation v i a internal conversion to lower levels follows this process, and the energy released often appears as molec ular fluorescence or phosphorescence. N o t the least significant pattern for energy release may involve successively an internal conversion to a triplet state, a radiationless transition to a lower-lying lanthanide ion state, and a radiative transition to a still lower ionic state. The last of these processes gives line-like, coherent radiation and is responsible for laser-type behavior. Line spectra result i n particular with chelates of the ions Sm+ , Eu+ , Tb+ , and Dy+ , ions that are close i n configuration to the half-filled 4 / arrangement. The red emission from europium (III) and the green emission from terbium (III) are particularly characteristic. Of the chelating agents studied, certain of the 1,3-diketones (e.g., dibenzoylmethane, benzoylacetone) have shown particular promise, especially in the tetrakis compounds. 3

8

8

3

7

Applications Improvement in fractional separations i n terms of the varying sta bilities of lanthanide complex species is a well-known and highly important area of applied chemistry (34, 36). The complex species are often useful i n analytical determinations. Certain of them possibly can be applied i n constructing amorphous transparent or liquid laser devices.


316

WERNER

CENTENNIAL

Conclusion Were Werner alive today, he would be gratified to know of the develop ments i n the chemistry of the lanthanide complex species that, although not implicit i n his original concepts, have nevertheless done much to supplement and add to an understanding of these concepts.


Literature

Cited

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