Covalency in f-element bonds

Journal of Alloys and Compounds 344 (2002) 55–59

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Covalency in f-element bonds Gregory R. Choppin* Department of Chemistry, Florida State University, Tallahassee, FL 32306 -4390, USA

Abstract The actinide concept proposed by Seaborg in 1945 involves a close relationship between the trivalent 4f (Ln) and the 5f (An) elements. The trivalent ions of these two families have very similar properties due to strong ionic bonding and similar ionic radii. Their similarity led to the successful formation of element 95–103 over the next 30 years by accurate prediction of the chemistry of the undiscovered elements. In the early 1950s, a small degree of covalency was proposed in the bonding of the lanthanide elements. However, controversy continues today on the degree of covalency in the bonds of the 4f and 5f elements. The interpretation of data in terms of covalency in the M–L bonds is reviewed with emphasis on which orbitals are involved.  2002 Elsevier Science B.V. All rights reserved. Keywords: Covalency; f-elements; Luminescence; Absorption spectra; Thermodynamics; Lanthanides; Actinides

1. Introduction The strong similarity in the solution chemistry of the 4f and the 5f elements played a most important role in the discovery experiments of the transplutonium actinide elements as it allowed necessary predictions of the chemical behavior in separations of the new elements. The accuracy was sufficient to allow prediction of the exact drop in which five atoms of Md would be eluted from a cation ion-exchange column in their separation from the many-fold larger number of atoms of Fm and Es [1]. The strong similarity in properties of the Ln and An elements is due to the strongly ionic nature of the bonds formed by these elements with ligands and the very similar ionic radii of the trivalent 4f and 5f cations. While there is agreement that the metal–ligand bonds of both 4f and 5f elements are predominately electrostatic, there have been discussions in the literature since the early Table 1 Positions (r m ) of maxima of the radial distribution of d and f orbitals Atom Ce (pm) Electronic structure r m , f-orbital r m , d-orbital r m , s-orbital

2

U (pm) 0

[Xe]6s 5d 4f 38 116 207

2

[Rn]5f 3 6d 1 7s 2 56 129 194

*Corresponding author. Fax: 11-904-644-8277. E-mail address: [email protected] (G.R. Choppin).

1950s that some degree of covalency is also present [2]. The strong ionicity in the bonds of these elements is reflected in the absence of significant variation in ligand field effects in their complexes and in the correlation of the trends in the complexation thermodynamics and kinetics for both series of elements with changes in cationic radii. The ionic character of the bonding in the complexes results in a range of coordination numbers (3 to 12) and geometries which are due to steric and electrostatic factors with no evidence of bonding orbital overlap constraints. While the hard acid character of the f-element cations is evidence for the electrostatic nature of their bonding in aqueous solution, it should be recognized that the majority of complexes studied were formed in solution by the replacement of Ln 31 –OH 2 bonds by Ln 31 –OR bonds. The degree of covalency in both types of such Ln–O bonds would be expected to be quite similar and the change in bonding strength reflected in various measurements can be expected to be dominated by the differences in electrostatic and steric characteristics of the complexed and the aquated ions. For both series of f-elements, the f-orbitals are relatively well shielded from interactions with neighboring atoms or ions. However, it should be noted that the crystal field model was invoked in the 1930s to explain lanthanide spectra since there is a dependence of the spectra on the coordination symmetry of the metal ion [3]. The 5f orbitals are more spatially extended as shown in the values from relativistic calculations in Table 1 and may experience more interaction with the fields of neighboring ions or

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G.R. Choppin / Journal of Alloys and Compounds 344 (2002) 55–59

atoms, suggesting that the actinides may have a greater degree of covalency in their bonding if f-orbitals are involved. The evidence is strong for a significant degree of covalency involving 5f and 6d orbitals in the bonding of the actinide atom and the oxygen atoms in the actinyl cations AnO 1 and AnO 21 2 2 . However, the degree of covalency in the bonding of the trivalent and tetravalent 4f and 5f cations remains controversial. In the following sections, evidence of covalency in f-element bonds from spectral, NMR, Mossbauer and thermodynamic studies and utilization of this in separation systems is reviewed.

2. Optical spectra Spectroscopic properties of the f-elements have played a major role in evaluation of covalency in their bonds [4,5]. The shielding of the f orbitals from the effects of the field of surrounding anions in dipolar molecules results in minimal disturbance of the electronic transitions between the energy levels of the f orbitals. In contrast to the broad d–d absorption bands of the transition elements, the f–f absorption bands of the lanthanides in solids and in solution are almost as narrow as for the gaseous ions and are perturbed much less by complexation. Transitions between the levels within the same electronic subshell, DL 5 0, are forbidden, resulting in weak intensities. The intensities of f–f band transitions increase in non-centrosymmetric systems by mixture of a degree of d orbital character with the f states. Centrosymmetric systems can have the intensities enhanced by coupling of the f electronic states with a vibrational state, thereby removing the center of symmetry. The narrowness of the f–f transition bands allows observation of small shifts in the peak wavelength reflecting a shift of the baricenter of the 2S11 LJ level. This has been termed the nephelauxetic effect and may indicate a covalent contribution to the bonding between lanthanide ions and ligands [6]. Such peak shifts have been used to conclude that the metal–oxygen bond in complexes in aqueous solution is weakly covalent [5] and involves hybrid orbitals with 4f contribution. The difference in covalency between Pr–F bonds and Pr–O bonds was estimated to be approximately 2% from the peak shifts [7]. Am(III) complexes of aminopolycarboxylate ligands have been found to have larger spectral shifts of these peaks than those of the analogous lanthanide complexes [8]. This was interpreted as indicating less shielding of the 5f orbitals than of the 4f orbitals and, consequently, a greater degree of covalency in actinide bonding than in lanthanide bonding. Studies in shifts in the wavenumber of the 5 D 0 → 7 F 0 transition in various Eu(III) complexes led to the conclusion that the nephelauxetic effect depends on the covalency of the metal–ligand bond and on the coordination number since the effect is larger for lower coordina-

tion numbers [9]. A more definitive explanation of the nephelauxetic effect and its possible correlation with covalency and bonding remains a challenge to theoreticians and experimentalists. Although most f→f absorption bands of the f-elements have low intensities, certain ones show unusually large sensitivities to ligand effects. These are termed hypersensitive transitions and both symmetry and covalency play roles in this enhanced sensitivity [10,11]. The more basic the ligand donor atom, the more intense is the hypersensitive band and the hypersensitivity is proportional to the nephelauxetic ratio which increases for halide compounds in the order I 21 .Br 21 .Cl 21 .F 21 which is that of increasing soft character of the halides. Since such softness is related to increased covalency character, this has been used as an argument for a relationship between hypersensitivity and covalency. An excellent review of experimental data and theoretical interpretation of hypersensitivity has been given by Gorller-Walrand and Binnemans [9]. A covalency model for hypersensitivity has been proposed in which the transitions gain intensity from charge transfer [12]. The Judd–Ofelt theory was modified to include charge transfer states [13]. More recently, it has been proposed that the covalency in Ln(III) complexes can be measured by the 6s electron density [14]. In summary, while there is still considerable controversy over the model that should be applied to the interpretation of hypersensitivity, there is a number of models and experimental evidence supporting those models that indicates that hypersensitivity due to orbital mixing in the lanthanides plays a significant role and that such orbital mixing reflects a degree of covalency in the bonding.

3. Nuclear magnetic resonance studies Calculation of the magnetic moments of the lanthanide elements requires inclusion of J values, reflecting the inner nature of the f orbitals which must be sufficiently shielded from the field of the surrounding ligands to prevent quenching of the L orbital moment. This provides strong support for a model which does not involve significant participation of the f orbitals in bonding. However, NMR spectroscopy provides evidence for some covalency in the bonding of lanthanide complexes. Paramagnetic and diamagnetic shifts of the 17 O NMR signal in aqueous solutions of lanthanide ions were attributed to electron transfer from the oxygen atom of the hydration water to the lanthanide ions [15]. The dependence of the 17 O shifts on the number of f electrons was interpreted as evidence for very weak covalent interaction between the oxygen 2s (or a linear combination of the 2s12p) orbitals and the lanthanide 6s orbital with the 4f and 5d orbitals playing, at most, a minor role in the bonding [16]. This association of f-element covalency with the 6s (Ln) and 7s (An) orbitals is consistent with the lack of structural geometry in the

G.R. Choppin / Journal of Alloys and Compounds 344 (2002) 55–59

complexes which would be imposed by inclusion of d or f orbitals in bonding. Magnetic susceptibility measurements of polyaminocarboxylate complexes agree with the model of predominately ionic interactions. Additional evidence for the predominately ionic nature of the bonding in f-element complexes is found in the linear correlation of the log of stability constants for complexation as a function of ligand basicity (as measured by the acid constants). The proposal that the 17 O chemical shifts in solutions of lanthanide perchlorates is due to the formation of covalent bonds involving the lanthanide 6s orbital and the oxygen of H 2 O has been supported by studies of proton and 17 O shifts in solutions of various lanthanide compounds [17]. These results indicate that although the bonding must be primarily electrostatic, covalency is present in the bonding between water molecules and the lanthanide ions. The lanthanide ions in aqueous solution induce contact shifts in the 1 H NMR spectra of ligands. Such contact shifts are due to a small fraction of the unpaired electron spin density being delocalized into orbitals of the ligating atom. By contrast the pseudocontact shifts, also observed in the 1 H spectra of ligands bonded to lanthanides, are due to through-space interaction between the electrons and their nuclear magnetic dipoles. Cations inducing the shifts must have anisotropically unpaired electron distribution. Contact shifts are much less prevalent for lanthanide ion complexes than for transition metal complexes because of the smaller degree of orbital overlap, reflecting the strongly electrostatic nature of the lanthanide bonds. This absence of contact shift, however, does not mean that lanthanide–ligand bonds are purely electrostatic but only that the orbitals involved in bonding have little 4f character. In the formation of Eu 31 complexes with alanine at high pH, bidentation occurs in which the Eu 31 ion is coordinated to an oxygen of the carboxylate and a nitrogen of the amine group of the a-carbon [18]. The contact shifts of the a-carbon indicate that there is greater spin delocalization in the lanthanide–nitrogen bonds than in the lanthanide–oxygen bonds which may reflect a higher degree of covalency in the Eu–N bonds than in the Eu–O bonds (as expected for the ‘softer’ N donor). In summary, the data from contact shifts and the NMR spectra of lanthanide complexes are consistent with a degree of covalent bonding between the lanthanide and donor atoms, with covalency increasing for softer donors.

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to the stronger mixing of the 6d and 7s orbitals. For a given valence state, the isomer shift for actinide compounds increased with increasing electronegativity of the ligand when the coordination number and the geometry of the solid crystal remained constant [19]. This correlation was interpreted as reflecting electron transfer from the ligand to the metal through covalent interaction which increased with the oxidation state of the actinide ion. Mossbauer measurements of 129 I in solid NpI 3 , PuI 3 , ThI 4 , UI 4 had rather constant isomer shifts with no significant difference between the tri- and tetravalent compounds. However, large differences in the isomer shifts between these An and the Ln compounds (LaI 3 , GdI 3 , EuI 3 ) [20] were interpreted as evidence of a greater degree of covalency in the bonding of the An elements relative to that of the Ln elements with d and s orbitals of the Ln and An ions involved in the covalency.

5. Thermodynamics of complexation A general characteristic of the data for the thermodynamics of complexation of the f-elements is the strong similarity for the lanthanides and actinides when compared for trivalent ions of the same ionic radii. However, careful inspection of very strong complexes indicate some differences. This is shown in Fig. 1 in which the slope of the correlation of log bAmL versus that of log bNdL has a value of approximately of 1.1 rather than 1.0 although Nd(III) and Am(III) have very similar radii. This slight enhance-

4. Mossbauer studies Mossbauer chemical shifts of solid compounds of lanthanides and actinides show an increase in the range of values as the oxidation state increases. The spread of shift values for complexes of a definite oxidation state is larger for the actinide cations and consistent with covalency due

Fig. 1. Correlation aminocarboxylates).

of

log b101

for

NdL

and

AmL

(L5

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G.R. Choppin / Journal of Alloys and Compounds 344 (2002) 55–59

ment in stability of the actinide complexes may be a reflection of a degree of covalency in these aminopolycarboxylates, presumably due to the difference in the An–N vs. the Ln–N bonds. Stronger complexation of the lanthanide ions has been measured for soft donor ligands. The comparison of the D (sorption coefficient) of actinide and lanthanide elements to cationic exchange resin in solutions of chloride and perchlorate solutions (Fig. 2) [21] and of thiocyanate as eluant with both cation and anion-exchange resins [22] were interpreted as reflecting stronger Am–Cl and Am–N (of SCN 2) interactions. Jorgensen et al. [23] attributed these observations to participation of the 6d and / or 7s (not the 5f) orbitals in the bonding of the actinides. The difficulty (noted earlier) of observing covalent effects when a M–O bond for hydrated water is replaced by a M–O bond of a ligand can be avoided by studies in organic solvents. The complexation of Nd[(CF 3 CO) 2 CH] 3 with a series of oxygen and nitrogen donor bases has been studied in CDCl 3 solvent [24]. The resulting complexation constants were analyzed by E parameters (associated with the electrostatic part of the metal–ligand interaction) and C parameters (associated with the covalent part of the interaction). For Ln–O interaction, a linear fit of log b1 and EB was obtained with a correlation coefficient of 0.86; the correlation between the log b1 and the CB parameters was much poorer (R 2 5 0.35). However, the situation was reversed for the Ln–N bonding with a good fit (R 2 5 0.966) for the log b1 values CB and poor fit (R 2 5 0.34) with the EB values. This pattern of correlation can be interpreted as reflecting an increased covalency in Nd–N base interactions relative to that in the Nd–O base interactions. The interpretation of the E and C parameters in terms of lanthanide and actinide covalency in the bonding with various ligands has been discussed in an article by Carugo and Castellani [25]. The pattern of the enthalpy of

Fig. 3. Differences in enthalpies of 1:1 complexation for Ln(III) ions with pairs of hard donors, F 2 and O 22 and of soft donors, Cl 2 and Br 2 .

complexation for Ln(III) ions with hard donors F and O and soft donors Cl and Br (Fig. 3) indicates a similar pattern, suggesting similar covalent differences [2].

6. Summary In summary, the data are consistent with ionicity as the predominant character of both lanthanide and actinide bonding. However, there is compelling evidence for some small amount of covalency, which is stronger in the actinide bonds. The argument for this extra covalency involving the f orbitals does not seem to have strong experimental support and more likely, the s orbitals play the major role in the covalency of the bonding of the f-element cations. Involvement of s orbitals is also consistent with the large variety of geometric coordination structures observed for both series.

Acknowledgements Preparation of this paper was assisted by a contract from the Division of Chemical Sciences, OBES-USDOE.

References

Fig. 2. Variation of log D ( 5 [M(resin) /M(soln) ]) for Am(III) and Nd(III) in non-complexing HClO 4 solutions and complexing HCl solutions.

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