Open questions on bonding involving lanthanide atoms

In-depth understanding of the bonding characteristics of the lanthanide ions in contemporary lanthanide-based materials is mandatory for tailoring their properties for novel applications.

Here, the authors elaborate on open questions regarding the bonding situation in mainly molecular lanthanide (4f) compounds, where, as compared to their actinide (5f) analogs in which

covalency of the bonds is a common feature, this is still under discussion for the 4f compounds. The bonding properties of the lanthanide 4f elements (Ln = La–Lu), and the electron-poor

actinide 5f elements (An = Th–Cm) exhibit similarities in some regards, but generally, they show fundamental differences. Formally, the oxidation states span a range from +II to +IV for the

lanthanides, whereas they vary from +II to +VIII for actinides. In nature, commonly found states are +III (Ln) or +IV/+VI (An). The general notion is that the lanthanides form predominantly

ionic bonds, at least in their +III oxidation state, whereas the actinides are capable of undergoing covalent bonding, but to a lesser extent than the d-block transition metals. This again

strongly depends on the oxidation state of the actinides, with the most prominent example being the covalent bonds formed in the actinyl ions [An(V)/(VI)O2]1+/2+. However, this traditional

view on the chemical behavior of the elements at the bottom of the periodic table is rapidly changing nowadays. Advances in spectroscopy give indications that lanthanide atoms can also be

involved in covalent bonding interactions. This opens a question about the nature of such bonds, which will also affect further chemical and physical properties like reactivity, bond

stability, and emission as well as magnetic behavior. Advances in Ln(II) and Ln–(inter)metallic coordination chemistry unveil new lanthanide-based materials with unexplored physical and

chemical properties—a treasure chest for the development of novel applications with tailored properties. COVALENCY IN LANTHANIDE BONDING The classical definition for a covalent chemical bond

is the accumulation of electron density between the involved atoms. The covalency of the bond can be described by the mixing coefficient, and it will increase if the orbital overlap is

large or the energy difference between metal and ligand orbitals is minimized (cf. Fig. 1d)1. The interconnection between the orbital overlap and energy-degeneracy-driven covalency, and the

subsequent impact on reactivity or bond stability for both lanthanide and actinide compounds, is not well understood and at the forefront of lanthanide and actinide research. Recently, the

group of Gregory Nocton reviewed Yb, Eu, Tm, and Ce organometallic lanthanide compounds with intermediate oxidation states like for example LnCp3 (Ln = Ce, Eu, or Yb; Cp = cyclopentadienide,

(C5H5)−), [Cp*2Yb(bipy)], [Cp*2Yb(dad)], and [Cp*2Yb(phen)] (Cp* = pentamethylcyclopentadienide, (C5Me5)−; bipy = 2,2'-bipyridine, C10H8N2; dad = 1,4-diazabutadiene; phen =

phenanthroline, C12H8N2)2. These materials exhibit charge transfer from the ligand to the metal, indicative of increased bond covalency as a result of improved energy match between the

ligand and the lanthanide metal. Generally, low ionization energies of the respective lanthanide ion allow for a higher covalency of the compound. Given its low ionization energy, this may

be one reason why most of the recent studies into lanthanide covalency of the trivalent ions are dealing with Ce compounds3,4,5,6,7,8,9. Although the formula of cerocene, [Ce(cot)2] (cot =

cyclooctatetraene dianion, (C8H8)2−, for instance, would suggest a +IV oxidation state, it was demonstrated, e.g., by Ce K-edge X-ray absorption near edge structure (XANES) to accord better

with a +III state3,4. Recently, Stefan G. Minasian, S. Chantal E. Stieber et al. clearly showed that the Ce atom in [Ce(cot)2] forms covalent bonds with the participation of Ce 4f electrons

by C K-edge XANES, Ce M4,5-edge XANES, and Configuration Interaction (CI) computations5. These unusual bonding properties of the Ce atom lead to quantum chemical phenomena rare for molecular

systems. The authors discuss evidence that the overlap-driven covalency is more important for the stabilization of the chemical bond in [U(cot)2] than in [Ce(cot)2], even though the mixing

coefficient is comparable. The latter results from the better energy match of the 4f orbital at a Ce3+ ion as compared to the 5f orbital at a U3+ ion with the ligand orbitals. Lukens et al.

determined the stabilization of the ground state of [Ce(cot)2], which is a result of mixing between the ligands’ orbitals and the 4f orbitals of the Ce atom using the Hubbard molecule

model6. Significant stabilization of Ce(IV) was observed through the tris(piperidinyl)imidophosphorane ligand, [NP(pip)3]−7. Spectroscopic studies (UV-visible, electron paramagnetic

resonance, and Ce L3-edge X-ray absorption spectroscopies), in conjunction with density functional theory studies, of the homoleptic imidophosophorane redox pair [Ce(NP(pip)3)4] and

[(Et2O)KCe(NP(pip)3)4] reveal dominant covalent metal–ligand interactions. The electronic basis of the destabilization of the f orbitals in these complexes was interrogated by Ce L3-edge

XANES and theoretical modeling. In comparison to the actinides, Eric J. Schelter et al. provided experimental evidence for the larger covalent character of 4f05d0 Ce(IV) multiple bonds as

compared to its 5f06d0 Th(IV) actinide congener by comparing a series of Th(IV) and Ce(IV) imido complexes8. This is in line with the recent literature dealing with the comparison of the

covalent character of tetravalent cerium and the tetravalent actinides9,10. Apart from Ce, there are examples for Ln(IV)O2 (Ln = Ce, Pr, and Tb) illustrating by O K-edge XANES and Ln

L3/M4,5-edge XANES that Ln 4f, 5d and O 2p orbitals mix, i.e., increasing bond covalency, as a result of good energy match between Ln and ligand orbitals11. In addition, in a series of

[Ln(III)Cl6]3– complexes (Ln = Ce, Nd, Sm, Eu, and Gd) and [Ce(IV)Cl6]2–, Cl K-edge XANES spectroscopy and density functional theory (DTF)/time-dependent DFT (TD-DFT) computations revealed

the participation of Ln 5d orbitals in the bonding for all molecules, whereas also Ce 4f was mixed with ligand orbitals for [Ce(IV)Cl6]2– 12. In all cases where an increase of mixing

coefficient/intermediate valence state is observed, the Ln 5d orbitals play a substantial role. The many examples in the literature show that actually the Ln 5d orbitals are rarely

completely empty as the tabulated ground state electronic configurations for most of the lanthanide elements state. Since they also have larger spatial extension compared to the 4f orbitals,

is it possible that orbital overlap between metal and ligand valence orbitals, not only energy match, also play a role in the covalency of the Ln–ligand bonds? Recently, we demonstrated

that the An M4,5-edge high energy resolution XANES (HR-XANES) and resonant inelastic X-ray scattering (RIXS) spectroscopy techniques can be used to distinguish between the classical notion

of overlap-driven covalency and energy-degeneracy-driven covalency for actinyl compounds (cf. Fig. 1c, d and e)13. It would be beneficial to develop and apply similar approaches for Ln-based

compounds. A well-established way to probe the bond covalency, and thus the overall bonding properties, is to evaluate the level of mixing of metal and ligand valence orbitals (the mixing

coefficient in Fig. 1d), through ligand K-edge XANES spectra5,11,12. The orbital overlap of metal and ligand orbitals and the energy degeneracy can be explored by valence band X-ray emission

spectroscopy selective to Ln 5d or 4f electronic density in the valence band. Specifically, probing the occupied Ln 4f orbital is of interest to answer the open question if lanthanide atoms

can form covalent bonds using their 4f orbitals similarly to actinide atoms, for which the usage of 5f orbitals has been documented. These studies have recently become possible due to

experimental advances at the European Synchrotron Radiation Facility (ESRF), allowing energy resolution of 30 meV (valence band (VB)-HR-RIXS at the Ln M4,5-edges)14. UNUSUAL LN(II)-BASED AND

LN–(INTER)METALLIC CLUSTER-BASED MATERIALS Unusual Ln(II)-based molecular and Ln–(inter)metallic cluster-based compounds have largely unexplored bonding properties, and it is unclear

whether and how commonly the lanthanide atoms in those materials can form covalent bonds. Ln(II) materials outside of those based on Sm(II), Eu(II), and Yb(II) are rare, but the group of

William J. Evans synthesized, e.g., [K(2.2.2-cryptand)][Ln(II)(C5H4SiMe3)3] (2.2.2-cryptand = 4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8]hexacosane, C18N2H36O6) across almost the entire

Ln series (excluding Pm)15. The groups of Enrique Batista, Stosh A. Kozimor and Ping Yang characterized the Ln(II) oxidation states using classical Ln L3-edge XANES spectroscopy and DFT as

well as complete active-space second-order perturbation theory (CASPT2) computations15. This conventional experimental method is powerful; however, in case the additional electron is

transferred to the Ln 5d instead of the 4f orbitals or activation of Ln(II)/Ln(III) 4fn5d0 to Ln(II)/Ln(III) 4fn-15d1 takes place, the more advanced Ln L2,3-edge HR-XANES spectroscopy would

be much more instructive. We and Kristina O. Kvashnina, Pieter Glatzel et al. showed that the method resolves the transitions to 4f and 5d states for Ln(III) materials and thus allows their

separate evaluation upon bond variations16 (cf. Fig. 1a, b)17,18. Figure 1a, b shows that the pre-edge probes the energy position of the 4f orbitals and gains intensity when Ln 4f are mixed

with 5d orbitals in Yb2O3. This example for a solid-state compound demonstrates the potential of the spectroscopic technique for probing the bonding properties of Ln molecular materials. The

Ln/An L3-edge HR-XANES spectrum measures the energy shift between 4f/5f and 5d/6d orbitals, and this spectroscopic tool is available for Ln (4f, 5d) and An (5f, 6d)17,19. From the Ln 4f

with 5d mixing and the energy shift, we can learn to what extent electronic density is transferred from the 4f to the Ln 5d orbitals, and we can use the findings to evaluate these orbitals’

involvement in the Ln–ligand bonding. Similar questions regarding the involvement of lanthanide orbitals in covalent bonding arise when studying intermetalloid cluster compounds, in which

Lnn+ ions are embedded in a shell of (semi)metal atoms20,21, in comparison with An-centered analogues22,23. We raise the question—can we develop spectroscopic tools to measure also the two

parameters to which is proportional the mixing coefficient _λ__ML_, energy match (EM − EL) and orbital overlap (SML), in order to study their interconnection with bond stability and

reactivity also for the lanthanide elements (Fig. 1d)? OUTLOOK The recent studies mentioned above showcase both the possibility and also the necessity to dig more deeply into the nature of

the lanthanide–element bond. The paradigm of being mostly ionic has dominated the discussion and also the design of new lanthanide compounds and Ln-based materials, but a new and modern view

of the bonding characteristic will allow for a rapid and sustainable change. Further developments of analytical techniques have allowed for new and fresh views into lanthanide compounds,

and these have set the course for future work. It has been discussed that HR-XANES and RIXS are powerful spectroscopic tools for the investigation of bonding character. This way, the open

question about the nature of the bonds involving Ln atoms may be answered. We envisage that once the bonding is much better understood, the design of compounds that exhibit new physical and

chemical properties for applications like activation of small molecules, materials with unusual reactivity, luminescent materials, quantum materials, or single molecular magnets with high

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INFORMATION AUTHORS AND AFFILIATIONS * Institute for Nuclear Waste Disposal (INE), Karlsruhe Institute of Technology, P.O. 3640, D-76021, Karlsruhe, Germany T. Vitova * Institute for

Inorganic Chemistry, Karlsruhe Institute of Technology, P.O. 3640, D-76021, Karlsruhe, Germany P. W. Roesky * Fachbereich Chemie and Wissenschaftliches Zentrum für Materialwissenschaften,

Philipps-Universität Marburg, D-35043, Marburg, Germany S. Dehnen Authors * T. Vitova View author publications You can also search for this author inPubMed Google Scholar * P. W. Roesky View

author publications You can also search for this author inPubMed Google Scholar * S. Dehnen View author publications You can also search for this author inPubMed Google Scholar

CONTRIBUTIONS T.V., P.W.R., and S.D. contributed equally to the manuscript. CORRESPONDING AUTHORS Correspondence to T. Vitova, P. W. Roesky or S. Dehnen. ETHICS DECLARATIONS COMPETING

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THIS ARTICLE CITE THIS ARTICLE Vitova, T., Roesky, P.W. & Dehnen, S. Open questions on bonding involving lanthanide atoms. _Commun Chem_ 5, 12 (2022).

https://doi.org/10.1038/s42004-022-00630-6 Download citation * Received: 08 December 2021 * Accepted: 14 January 2022 * Published: 02 February 2022 * DOI:

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