Open questions on bonding involving lanthanide atoms

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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


blocking temperature will emerge, which represents both a great challenge and a fantastic opportunity. REFERENCES * Neidig, M. L., Clark, D. L. & Martin, R. L. Covalency in f-element


complexes. _Coord. Chem. Rev._ 257, 394–406 (2013). Article  CAS  Google Scholar  * Tricoire, M., Mahieu, N., Simler, T. & Nocton, G. Intermediate valence states in lanthanide compounds.


_Chem. Eur. J._ 27, 6860–6879 (2021). Article  CAS  Google Scholar  * Dolg, M. & Fulde, P. Relativistic and electron-correlation effects in the ground states of lanthanocenes and


actinocenes. _Chem. Eur. J._ 4, 200–204 (1998). Article  CAS  Google Scholar  * Edelstein, N. M. et al. The oxidation state of Ce in the sandwich molecule cerocene. _J. Am. Chem. Soc._ 118,


13115–13116 (1996). Article  CAS  Google Scholar  * Smiles, D. E. et al. The duality of electron localization and covalency in lanthanide and actinide metallocenes. _Chem. Sci._ 11,


2796–2809 (2020). Article  Google Scholar  * Lukens, W. W., Booth, C. H. & Walter, M. D. Experimental evaluation of the stabilization of the COT orbitals by 4f orbitals in COT2Ce using a


Hubbard model. _Dalton Trans._ 50, 2530–2535 (2021). Article  CAS  Google Scholar  * Rice, N. T. et al. Homoleptic imidophosphorane stabilization of tetravalent cerium. _Inorg. Chem._ 58,


5289–5304 (2019). Article  CAS  Google Scholar  * Cheisson, T. et al. Multiple bonding in lanthanides and actinides: direct comparison of covalency in thorium(IV)- and cerium(IV)-imido


complexes. _J. Am. Chem. Soc._ 141, 9185–9190 (2019). Article  Google Scholar  * Kloditz, R. et al. Series of tetravalent actinide amidinates: structure determination and bonding analysis.


_Inorg. Chem._ 59, 15670–15680 (2020). Article  CAS  Google Scholar  * Gregson, M. et al. Emergence of comparable covalency in isostructural cerium(iv)– and uranium(iv)–carbon multiple


bonds. _Chem. Sci._ 7, 3286–3297 (2016). Article  CAS  Google Scholar  * Minasian, S. G. et al. Quantitative evidence for lanthanide-oxygen orbital mixing in CeO2, PrO2, and TbO2. _J. Am.


Chem. Soc._ 139, 18052–18064 (2017). Article  CAS  Google Scholar  * Löble, M. W. et al. Covalency in lanthanides. An X-ray absorption spectroscopy and density functional theory study of


LnCl6x– (x = 3, 2). _J. Am. Chem. Soc._ 137, 2506–2523 (2015). Article  Google Scholar  * Vitova, T. et al. The role of the 5f valence orbitals of early actinides in chemical bonding. _Nat.


Commun._ 8, 1–9 (2017). Article  Google Scholar  * Amorese, A. et al. Crystal electric field in CeRh2Si2 studied with high-resolution resonant inelastic soft x-ray scattering. _Phys. Rev.


B_. 97, 245130 (2018). Article  CAS  Google Scholar  * Fieser, M. E. et al. Evaluating the electronic structure of formal Ln(II) ions in Ln(II)(C5H4SiMe3)(3)(1-) using XANES spectroscopy and


DFT calculations. _Chem. Sci._ 8, 6076–6091 (2017). Article  CAS  Google Scholar  * Kvashnina, K. O., Butorin, S. M. & Glatzel, P. Direct study of the f-electron configuration in


lanthanide systems. _J. Anal. Atom. Spec._ 26, 1265–1272 (2011). Article  CAS  Google Scholar  * Vitova, T. et al. Actinide and lanthanide speciation with high-energy resolution X-ray


techniques. _J. Phys. Conf. Ser._ 430, 012117 (2013). Article  CAS  Google Scholar  * Pruessmann, T. et al. Opportunities and challenges of applying advanced X-ray spectroscopy to actinide


and lanthanide N-donor ligand systems. _J. Synchrotron Radiat._ 29, 53–66 (2022). Article  CAS  Google Scholar  * Vitova, T. et al. High energy resolution x-ray absorption spectroscopy study


of uranium in varying valence states. _Phys. Rev. B_ 82, 235118 (2010). Article  Google Scholar  * McGrady, J. E., Weigend, F. & Dehnen, S. Electronic structure and bonding in


endohedral Zintl clusters. _Chem. Soc. Rev._ 51, 628–649 (2022). Article  CAS  Google Scholar  * Wilson, R. J., Lichtenberger, N., Weinert, B. & Dehnen, S. Intermetalloid and


heterometallic clusters combining p-block (Semi)metals with d- or f-block metals. _Chem. Rev._ 119, 8506–8554 (2019). Article  CAS  Google Scholar  * Lichtenberger, N. et al. Main Group


Metal–Actinide Magnetic Coupling and Structural Response Upon U4+ Inclusion Into Bi, Tl/Bi, or Pb/Bi Cages. _J. Am. Chem. Soc._ 138, 9033–9036 (2016). Article  CAS  Google Scholar  *


Eulenstein, A. R. et al. Substantial π-aromaticity of the anionic heavy-metal cluster [Th@Bi12]4−. _Nat. Chem_. 13, 149–155 (2021). Article  CAS  Google Scholar  Download references AUTHOR


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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