D. G. Karroker
Sovonnah River Laboratory E. I. du Pont d e Nernours and Co. Aiken, southCarolina 29801
Coordination of Trivalent Lanthanide Ions
Lanthanum and the succeeding 14 lanthanide elements form the longest continuous series of chemically similar elements in the periodic table. The normal valence of the lanthanide ions is 3+. Laa+ has the electronic configuration of the closed Xe shell, and the succeeding ions in the series successively add 14 electrons to the 4f snb-shell. The lanthanide ions are strongly electropositive and have ~omparat~ively large ionic radii. The shrinkage of the ionic radii as the 4 j sub-sheu is filled is popularly referred to as the "lanthanide contraction," although a similar shrinkage in ionic radii occurs as d sub-shells are filled. The lanthanide series has a 22y0 change in ionic radii, 1.061 A for La3+ to 0.848 A for Lu3+ (1) (Fig. 1).
invoked (2) to explain certain effects of the extremely complicated spectra that result from the f-f electronic transitions in the lanthanide ions. However, energy shifts in the lanthanide spectra between different compounds are only 10-20 cm-I where complex formation wit.h ions of the 3d transition metals results in spectral shifts of the order of 1000 cm-'. Bonding between lanthanide ions and coordinating ligands depends primarily on thc electronegativity of the bonding atom in the ligand. Bond formation follows the order F-, OH-, H20, NOa-, C1-, etc., for monodentate ligands. Complcx formation with hidentate ligands in the presence of water is usually successful only with ligands that form chelate rings through oxygen atoms, as carboxolate anions (RCOI-) and 0-diketonates
The added stilbility of resonating structures allows these ligands to compete successfully with OH- or H 2 0 in the coordination sphere of the lanthanide ion. Ligands bonding through nitrogen or sulfur atoms normally cannot compete with water for a position in the coordination sphere, and their compounds wit,h lanthanides must be synthesized in nonaqueous media. Nearly anhydrous fluorides can be precipitated from aqueous solutions under carefully controlled conditions, but anhydrous chlorides, bromides, etc., must be made by nonaqueous methods, usually gas-solid reactions. A general comparison between the bonding and coFigure 1.
Radii of trivalent lonthmide ions.
Reproduced from reference
( 1 ) with permission of the American Chemical Society.
Table 1. Comparison of 4f-3d Metal lons The influence of the shrinking ion size on coordination number and coordination geometry is a unifying concept in the coordination chemistry of the lanthanide ions. Yttrium, although not a lanthanide, has chemical properties very similar to lanthanum; Y3+ has the electronic configuration of the closed Kr shell. The ionic radius of YS+ is intermediate between Ho3+ and Er3+, and its coordination behavior is normally identical with these ions. Lanthanide lon-Ligond Bonding Characteristics
Unlike the d orbitals of the transition elements, the j orbitals do not contribute significantly to complex formation or to bonding of the lanthanide ions. Covalent contributions to bond formation have been 1 The information mntained in this article was developed during the course of work under Contract AT(07-2)-1 with the US. Atomic Energy Commission.
424 / Journol of Chemical Education
Lanthanide ions
Transition metal ions
Metal orbitals Ionic radii Common
4f 1.06-0.85 A
3d 0.75-0.6
numbers Typical eoordinst,inn
6, 7, 8, 9
4,6 Square planar Tetrahedron
.
Bond direction Bond strengths
Trigonal prism Sotrare ant,inrism
orbital interaction Strong direction bonding Bond strengths determined hfirbital interaction normally in following order CN-, NH,, HzO, nu -..,nIonic, rapid ligand ex- Often covalent; eachange valent camplexw normally exchange slowly
Little preference in bond directions Ligands bond in order i f electronegativity F-, OH-, H a , NO$-, CI-
A
Solution complexes
A
ordination properties of the lanthanide ions and the more widely studied ions of the 3d transition elements is given in Table 1. The bonding of ligands to lanthanide ions is essentially electrostatic, with little, if any, interaction between the 4f orbitals and ligand orbitals, in contrast to the strong interactions between 3d orbitals and ligand orbitals found in the bonding of ligands to transition metal ions. The interaction of 3d orbitals and ligand orbitals leads to strong directional bonding, but there is little directional influence in the electrostatic bonding of the lanthanide ions. Because of their larger size, lanthanide ions generally have higher coordination numbers than transition metal ions. Coordination numbers greater than 6 for transition metals ions only form with difficulty because of the strong ligand repulsion in the coordination spheres. Coordination Geometry
Coordination numbers of the lanthanide ions normally are from 6 to 9, but are often higher with ligands of small bideutate ligands, such as nitrates. Monodentate ligands (F-, H20, C1-, etc.) usually have a maximum coordination of 9 for lanthanides; bidentate ligands normally form chelates that are 6, 7, or 8 coordinate, as Ln&, Ln1G.H20, LnK-, where Ti is a bidentate ligand. Monodentate ligauds surrounding the lanthanide ion form a coordination polyhedron based upon either a trigonal prism or the octahedron. Both polyhedra have 6-coordinate lanthanide ions, but the coordination may be expanded to 7, 8, or 9 by coordination of additional ligands through the square faces of the trigonal prism (3) (Fig. 2) or by "capping" the octahedron (Fig. 3). The trigonal prism and the octahedron can be considered related structures-the top and bottom faces of an octahedron define a trigonal antiprism; rotation of either the top or bottom face by 180" forms a trigonal prism (Fig. 4). Similarly, a 180' rotation of the top face of the trigonal prism forms an octahedron. The bonding atoms of bidentate ligands often form 8-coordinate ~olvhedra of either square antiprismatic " or dodecahedra1 geometry. These structuresire quik similar; the square antiprism has two opposite square faces connected by 8 triangular faces; the dodeca-
hedron has 12 triangular faces. A "bend" along opposing diagonals of the square faces and narrowing of the angles converts the square antiprism into a dodecahedron (Fig. 4). These structures can pass smoothly into each other, depending on the angle of the bend. The description of an intermediate structure as a "distorted square antiprism" or as a "distorted dodecahedron" is a matter of taste, rather than an absolute quantity (4). As might be predicted from Figure 4, the energy -~difference between the two polyhedra is small (5). The assignment of a Cartesian polyhedron to represent the atoms coordinating with the ion should be viewed, in general, as an approximation. The world of a lanthanide ion is spherical-in its coordination sphere the ion seeks a compromise that will achieve spherical shielding for itself and minimize repulsion between the coordinating ligands. The atoms coordinating with an ion are best represented as points on the surface of a sphere; their assignment to corners of a Cartesian polyhedron may be expected to involve a greater or lesser degree of approximation. Lanthanide Coordination in Crystals
Crystal structures for a series of analogous compounds of all Ln3+ions demonstrate the shrinkage in ion size as the 4fsub-shell is filled. As the size of the lanthanide ion decreases, the repulsion between ligands in the coordination sphere increases and becomes large enough to make the structure energetically unstable. At this point, the coordination number of the lanthanide ion decreases, and the crystal structure changes. As an illustration, the lattice parameters of the anhydrous
.
TOP
Side Figure 3.
"Copped" odahedral coordination.
Anti -Prism
Intermediate
Dodecahedron
Trigonol Prismatic Polyhedra
Figure 2. Trigowl Primatic Coordination. Reproduced from reference (36)with permission of the American Chemical Society.
Figure 4. Eight coordinate structures, Y(CH3COCHCOCHsIr.2HD token from reference ( 1 7 d and YICF&OCHCOCFalr-from reference (19) with permission of the American Chemical Society.
Volume 47, N u m b e r 6, June 1970
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425
Table 2. Lattice Parameters of LnCI3 a, A
b, A
,
Molecular vol. da
Hexaeonal. 2 M/cell CNa = 9
TbCI,
Orthorhombie, 4 M/cell CN = 8 3.86 11.71 8.48 Monoclinic, 4 M/eell CN = 6
96.4
CN = coordination number
lanthanide chlorides are given in Table 2. The hexagonal cell structure (UC13 type) has a coordination number of 9 for the lanthanide ion, and is the stable structure for the chlorides of La3+ to Gd3+. The lattice parameters for EuCI3 and GdCb are essentially the same, despite the smaller size of the Gd3+ ion. The energy of repulsion between the ligands is greater in GdC13 than in EuCI3, and becomes great enough to make $-coordination the more stable structure for TbCI3. (An unstable form of DyC13 has the TbCI, structure.) The TbCl, structure-note the molecular volumes in Table 2-still has a strong ligand repulsion. The crystal structure again changes at. DyC13to a structure with a G-coordinate lanthanide ion, the stable form for the anhydrous chlorides of Dy-Lu. The lattice paramet,ers for the monoclinic, 6-coordinate trichlorides of Dp-Lu show that this structure is, in its turn, becoming unstable. If there were two or three additional lanthanide ions, one would anticipate that another change in structure, and decrease in coordination, might o&r. Larger halide ions, such as bromide and iodide, increase the repulsion forces between ligands, and thus, favor a decrease in the coordination number of the lanthanide ion. This decrease is illustrated (Table 3) with the coordination numbers for the anhydrous Table 3. Coordination Numbers of LnX3
lanthanide halides. La3+ has a coordination number of 9 in LaF8, LaCI,, and LaBr3, but 8 in LaIt. The coordination number of Lna+decreases a t a lighter Ln3+ ion in each series, from 9-coordinate to 8-coordinate Ln3+ a t TbC13,SmBr3,and LaL. The crystal structure of anhydrous lanthanide halides is based upon a trigonal prismatic coordination of the lanthanide ion. Staggered arrangements of trigonal prisms (6) in the crystal give the lanthanide ion 6, 8, or 9 halide ions within bonding distance. Non-Cartesian Geometry and Partial Coordination The crystal structures of the lanthanide fluorides illustrate two unusual forms that occur in lanthanide ion coordination, non-Cartesian geometry and partial coordination. The arrangement of the 9 fluoride ions around La3+ in LaF3 is an example of non-Cartesian geometry. In discussing the arrangement of the -fluoride ions in the La3+coordination sphere (Fig. 5), Zalkin, et al. (7) state, "We have failed to find any simple description for the geometry of these neighbors." The
Figure 5. ence
Lonthanide trifluoride structures. A, Reproduced from refer(71 and B, reproduced f m m reference I81 with permission from tho
American Chemical Society.
coordination sphere of the lanthanum ion in LaF3 illustrates that a compromise to obtain maximum shielding and minimum ligand repulsion does not necessarily result in the bonding atoms assuming a geometry that may be described in Cartesian terms. The structure of YF8 (8) represeuts "partial coordination" (Fig. 5). This structure is found for the anhydrous fluorides of Sm3+-Lu3+, and is basically a trigonal prismatic coordination sphere. However, eight fluoride ions are 2.30 A from the Y3+ion, and the ninth fluoride is separated from the Y3+ion by 2.60 A. The ninth fluoride is not equivalent to the other eight, hut is still too close not to influence the coordination sphere of the Y3+ ion. Rather than assign a coordination number of 9 or 8 to the Y3+ion in YF,, the coordination number of '%+"is suggested (3). Polymorphism
F-,1.34 d ; C1-,
1.80 d ; Br;
1.90 A; I;
426 / Journal of Chemical Education
2.23 A.
The small energy differences between structures of high coordination manifest themselves in polymorphism,
the occurrence of two or more structures for a single compound. Polymorphs are often found in lanthanide compounds with small anions, such as fluorides and oxides. The small energy difference necessary to stabilize a structure of lower stability can usually be obtained by controlling the conditions of preparation. The regions of temperature stability for the A, B, and C forms of the lanthanide sesquioxides are shown ic Figure 6 (9). Types A, B, and C have hexagonal,
euhic) has an equal mixture of 6- and 7-coordinate lanthanide ions; and the Type E (Yb, Lu monoclinic) has only 6-coordinate lanthanide ions. The average coordination numbers are 7.5, 6.5, and 6 for the lanthanide ions in Types A, 0 , and E, respectively; the decrease in ionic size parallels the decrease in coordination, as with the sesquioxides.
Chelate Structures
Bidentate ligands, such as p-diketones, restrict the geometry of the lanthanide ion coordination sphere. Only a few isolated lanthanide chelates have had complete structural determination. As with monodentate ligands, the crystal structure normally changes over an entire series of lanthanide complexes. Three common types of chelates are LnK3, LnKa.xHzO(x = 1,2, or 3), 0 0-
I
//
IONIC RADIUS, 1 Figure 6. Regions of temperature stability for lanthanide rerquioxide. Reproduced from reference (91 with permission of the Americm Chemical Society.
monoclinic, and cubic cells, respectively. Lanthanide ions in these structures have complicated coordination spheres. Type A has 7 oxygen atoms coordinated to each lanthanide ion; six oxygen atoms form an octahedron and the seventh coordinates along a threefold axis (10). Type B has "strings" of coordination polyhedra along the [ l , l , l ] planes of the crystal, with a repeating coordination pattern for the lanthanide ion of 7-7-6 in each string. The 7-coordinate ions have six oxygen atoms coordinating on the corners of a trigonal prism, and the seventh atom coordinating through one face; the 6-coordinate ion is coordinated in a distorted octahedron, with a seventh oxygen atom barely outside the coordination sphere (11). Type C oxides have 6-coordinate lanthanide ions, one-fourth in regular octahedral coordination, and three-fourths in a highly distorted octahedra (18). The average coordination of the lanthanide ion is 7 for Type A oxides, 6%/3for Type B oxides, and 6 for Type C oxides; the preference for structure Ain seaquioxidesof the largest lanthanide ions, B for intermediate ions, and C for the smallest ions is a logical consequence of the ionic size. The lanthanide sesquisulfides also have three structures with mixed coordination sites (13). Type A (La through Dy, orthorhombic cell) has lanthanide ions at alternating 7- and 8-coordinate sites, Type D (Dy-Tm,
and M+LnK4- (K=RC-CH=C-R', M = alkali ion, RIN+). The expected coordination polyhedraas yet no structure has been determined-for LnKa compounds is octahedral or trigonal prismatic; for Ho14.Hz0, where R = R' = +, the coordination sphere is a capped octahedron (14), but for Yh(acac)a 00 I I1 H20 (acac = CH3C=CH-CCH3, acetylacetonate), the coordination sphere is a capped trigonal prism (16). Crystal structures of L a ( a c a ~ ) ~ . 2 H ~(26), O Ho(acac)a.3HzO and Y ( a c a ~ ) ~ . 3 H ~(17) O are described as showing a distorted square antiprism for the coordination polyhedron. The description of this structure as a distorted square antiprism, rather than a distorted dodecahedron, has been questioned (4). Eu00
I
//
(tta)3.2Hz0 (tta = CF3C-CH-C-CnSH3, thenoyltrifluoroacetonate) (18) was also found to have a square antiprism coordination polyhedron. The coordination polyhedra for lanthanide ions 8-coordinated acetyiaceto'iatej (19) and kr(tta)r- (30j: It is premature to accept these as typical structures until crystal structure determinations of many more lanthanide chelates are available for comparison. It is noteworthy, however, that the structures found for coordination polyhedra of 8-coordinate lanthanide ion with bidentate ligands are usudly based on square antiprismatic or dodecahedra1 forms rather than trig&l prismatic. The coordination polyhedra formed in lanthanide carbonates or nitrates have a variety of ~ossibilities,because either of these ligands may de mono- or bidentate. Anions of carboxylic acids, such as acetates, also have this Laz(COa)3.8Hz0 (81) has a 10-coordinate lanthanum ion in a coordination polygon made up of water, monodentate carbonate, and bidentate carbonate. Structures for compounds with the anions of carboxylic a c i d ~ a c e t a t e s formates, , etc.-have not been determined, hut infrared and visible spectral evidence indicates that the acetate ion may coordinate as a monodentate, bidentate, or polymeric ligand (83). Volume 47, Number 6, June 1970
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427
Coordination of Lanthanide Ions in Solutions
Aquo Ions. The decreasing size in ions of the lanthanides can be predicted to result in a decrease in the coordination of lanthanide ions in solutions or in aqueous complexes. The assignment of a specific geometry to the coordination sphere of a solute and/or complexed lanthanide ion is impossible; such an assignment would probably be meaningless, in view of the lability of the complexes. The best approximation of the coordination structure is probably a sphere: coordination numbers are probably the only observable quantity which have meaning in a solution environment. Coordination numbers of lanthanide ions in solution cannot be determined directly; evidence to demonstrate coordination changes rests upon indirect evidence, such as the implausibility of abrupt changes in a smoothly varying property in solutions of lanthanide ions or complexes without some cause. The point of view advanced here and by others is that a change in the continuity of a property is often evidence for a change in the coordination number of the lanthanide ion. From a comparison of the partial molal volumes of lanthanide ion in aqueous chloride solutions with partial molal volumes calculated for lanthanide ion in diierent coordinations, Spedding and co-workers (23) have deduced that lanthanide ions La3+ to Nd3+ are 9coordinate, Tba+ to Lu3+ are &coordinate, and the intermediate lanthanide ions Sm3+,Eu3+,and Gd3+are mixtures of both coordinations. A graph of the partial molal volumes versus the crystal radii of the lanthanide ions is shown in Figure 7; the straight lines on the graph were calculated for 9- and 8-coordinate hydrated lanthanide ions.
='O 2.0
I , ,"
,."" -r.
Figure 8. Hypersensitive rpedro of Nda+. o) rolid Nd(BrOda.9HzO; b) 5.35 X 10-2 M Nd3+ in water; c) 5.35 X M Nd" in 11.4 M HCI; dl solid NdCi7.6HeO: Re~mducedfrom ...., -.. . ~el, ,rolid NdrlS0111.8Hr0. .. . reference (24) with permission of the American Chemicol Society. ~
competition between Nd8+ and H + for water of coordination, Nd3+ can only achieve 8-coordination. Ion exchange studies (25) provide convincing evidence that chloride complexing is not significant in chloride solutions. Studies of complex formation of lanthanide ions by pressure-jump (26), temperature-jump (27, 28), and ultrasonic absorption methods (23) also provide evidence for a change in the coordination number of aqua Ln3+ ions with decreasing ionic size. These methods all involve an energy shock to disturb a solution of a lanthanide complex, and measure the relaxation time of the system, which is the time for re-establishment of equilibrium. The relaxation time is on the order of 100 psec, and so can be measured for favorable complexes by the conductivity change of the solution. The mechanism (30) proposed to explain the measured relaxation times considers the step involved in complex formation as
"2
d
Figure 7. Portiol mold volumes of hydrated Lna+ion.. Reproduced from of the Americbn Chemicol Society. ,eference (23) with
A study (24) of the measured spectral effects in the visible absorption bands from hypersensitive transitions of the hydrated Nd3+ ion also provides evidence that the Nd3+ ion is 9-coordinate. A comparison of the shape of the absorption bands of aquo Nda+ with 9coordinate Nd3+ in solid Nd(Br0&.9H20 and S-coordinate Nd3+in NdC13.6H20shows (Fig. 8) a striking similarity between aquo NdS+ and 9-coordinate Nd3+. A coordination change in the aquo Nd3+complex is also demonstrated in these spectral studies; in concentrated HC1 solutions the band shape is altered to a shape resembling that of S-coordinate Nda+. An 11-12 M HCI solution lacks the water necessary to allow each H + ion to achieve its normal 4-coordination. I n the 428
/
Journal of Chemicol Educofion
Equilibrium Between Ion Pair and Complex Figure 9.
Complex formation.
1 ) Diffusion toget,her of the hydrated cation and anion 2 ) Partial lass of solvent to farm an ion pair 3 ) Loss of water from the first coordination sphere of the cation
ods-is considered an excellent verification of the concept of the influence of ionic size on the coordination of the lanthanide ions. Solvent Extraction Complexes
4) Formation of a fully chelrtted species (Fig. 9)
The equilibrium at step (3) is proposed as the slow rate determining step in the sequence; by measuring the relaxation time at varying concentrations, both k , and k,, the rate constants for the forward and reverse reactions, respectively, can be determined graphically. The values of k , are 105-106 greater than k,; k , values are essentially the same for temperature-jump measurements on lanthanide ion-murexidez (27) and for pressure-jump measurements on lanthanide ion-oxalate ($6) complexes. The independence of lc, from the complexing ion agrees with the assumption that the rate-determining step is the loss of a water molecule from the coordination sphere of the lanthanide ion, and thus, depends only on the hydration properties of the lanthanide ion. The values for k, (Fig. 10) are essentially
The extraction of lanthanide nitrates by organic phosphates, such as tri-n-butyl phosphate (TBP), involves the formation of a Ln(NOJ3.3TBP complex and its extraction into an organic phase. The extraction coefficients (31, 32) do not in general, shdw the linear increase with atomic number of the lanthanide ion that would he predicted if the species in the organic phase were identical for all complexes. Normally (Fig. l l ) , the slope of extraction coefficients changes in the region Eu to Dy; a study of the absorption spectra of Nd3+and Er3+ indicates that these ions are probably 8- and 6coordinate, respectively, in Ln(NO&. 3TBP complexes (33). The difference in coordination can be explained
Figure 10. Rote constants for lonthonide hydration. Reproduced from reference 126) with permission of the lourno1 of lnorgonic m d Nuclear Chemirty.
constant for oxalate complexes of the "light" lanthanides (La3+ to Eu3+), decrease for complexes of Gd3+, Tb3+, and DP+, and then become again essentially constant for the remainder of the lanthanide ions. The experimental k, values can all be explained by a decrease in hydration of the lanthanide ion in the region Gd3+to Tb3+. Ions of lower coordination have more energy involved in the bond to each coordinated water molecule. Thus, their reaction rate is decreased. The results of these experiments are interpreted as showing a change in the coordination of the hydrated Ln3+ ions in the region Eu3+to Tba+. The kinetic studies lead to a slightly diierent oonclusion than partial molal volume measurements, which show the change in coordination in the region Sm3+to Gd3+. This difference may be attributed to differences in the systems studied. The gencral agreement between results derived by totally different experimental meth-
Didribution coefficients of Lo8+ between TBP and HNOI from reference (311 with permission of the Journal of lnargonic om' Nudeor Chemirty. Figure 1 I. rolutionr
by proposing that two nitrate ligands in Nd(N0a)a 3TBP are hidentate, and one nitrate is monodeutate; in Er(N0&.3TBP, all nitrates are monodentate. The TBP ligand ((C4H90)3P=0) is always monodentate. The TBP complexes of the light lanthanide nitrates are inferred to have 8-coordinate Ln3+ions, and the TBP complexes of heavy lanthanide ions to have 6-coordinate Ln3+ ions. A transition from 8- to 6-coordinate Ln3+ions is expected among the complexes of the intermediateEu through Dy-lanthanides, coincident with a transition in solvent extraction behavior between the light and heavy lanthanide ions. A change in the coordination of the Lna+ ion-and hence in the structure of the complex-may be responsible for the transition. NMR studies ($4) of the proton and chemical shifts for the closely similar tri-n-amyl phosphate complexes demonstrate a difference in structure Volume
47, Number 6, June 1970 / 429
between complexes of the light and heavy lanthanide nitrates. The "synergistic" extraction of lanthanide ions by a mixture of an acidic chelating agent [di-2-ethylhexylphosphoric acid (HDEHP)] or thenoyltrifluoroacetylacetone (tta) and a neutral organic phosphate (TBP, trioctylphosphate (TOPO), etc.) has been interpreted (55) as indicating that the coordination of the lanthanide ion in the extracted complex changes between light and heavy lanthanides. A mixture of HDEHP and TBP is a stronger extractant than pure HDEHP for Gd3+ and the lighter lanthanides; the heavy lanthanides show no difference in extraction between the mixture and pure HDEHP. From the dependence of the extraction on the concentration of the chelating and coordinating agents, the extracted species for light lanthanides can he deduced as Ln(DEHP)%L,,where L is either H1O or the neutral organic phosphate. If the Ln3+ ions (La-Gd) that form these complexes are assumed to be &coordinate, extraction is enhanced hecause the replacement of water by the neutral phosphate improves the compatibility of the extracted species with the organic phase. The heavy lanthanides Th through Lu, form complexes of the composition Ln(DEHP)a, with the lanthanide ion 6-coordinate. The neutral organic phosphate is unable to enter the coordination sphere, and thus does not affectthe extraction. The evidence for coordination effects on the chemistry of lanthanide ions in solution is generally indirect, and in very few cases is beyond the probability of some degree of error. However, the sum of information on lanthanide ion coordination in solutions provides a high probability that the general model of decreasing coordination number with decreasing size is as definite and as important in solutions as in crystals. Acknowledgments
The author is grateful to D. H. Templeton and Allan Zalkin, of the University of California, Berkeley, and
430
/
Journal of Chemical Education
to Jane P. Bibler, of Augusta College, Augusta, Georgia, for their advice and criticism of the manuscript. Litemture Cited (1) T ~ n r p ~ s ~ D. o nH., , AND DAUBEIP. C. H., J . Am. Chem. Soc., 76,5237
-- -,.
, \l -05.,,
(2) HENRIB, D. E., AIPD CHOPPIN.G.R., J . C h m . Phya., 49,477 (1968). a , L.. AND W m a x ~ C. , N., Quart. Rm. (London). 21. (3) M u m ~ s n ~ m E. I09 (1967). (4) L m p ~ s n S. . J.. AND ROW B. J.,
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