Open questions on bonding involving lanthanide atoms

Abstract

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)O 2 ] 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 bondingThe 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 LnCp 3 (Ln = Ce, Eu, or Yb; Cp = cyclopentadienide, (C 5 H 5 ) − ), [Cp* 2 Yb(bipy)], [Cp* 2 Yb(dad)], and [Cp* 2 Yb(phen)] (Cp* = pentamethylcyclopentadienide, (C 5 Me 5 ) − ; bipy = 2,2'-bipyridine, C 10 H 8 N 2 ; dad = 1,4-diazabutadiene; phen = phenanthroline, C 12 H 8 N 2 ) 2 . These materi...

Overlooked Effects of La-4f Orbitals in Endohedral Metallofullerenes

“…Recently, the covalency in the lanthanide bonding and the non-negligible contribution of 4f orbitals in the bonding have attracted more attention. − Different from the traditional view of lanthanides mainly forming ionic bonds, advanced spectral measurements indicate that they also can engage in covalent bonding interactions. The covalency of bonds is enhanced by the large orbital overlap and small energy difference between the metal and ligand orbitals.…”

Cluster-Geometry-Associated Metal–Metal Bonding in Trimetallic Carbide Clusterfullerenes

“…The spin density is mainly shared by the three Er atoms with negligible contribution from the acetylide unit C 2 (Figure b). The well-examined Sc 3 C 2 @C 80 shows that the spin density is mainly localized on the C 2 unit, and its electronic structure is best described as (Sc 3 ) 9+ (C 2 ) 3– @C 80 . − The α -SOMO of Er 3 C 2 @C 80 evidently exhibits the localization of a single electron between the three Er ions (Figure c), presenting Er–Er–Er bonding with the main composition of the s and p orbitals of Er (Figure S4 shows other canonical molecular orbitals of Er 3 C 2 @C 80 , and α -SOMO – 4 also has the Er–Er–Er bonding feature). Additionally, the color-filled contour plot of electron localization functions (ELFs) further confirms the localization of the single electron between Er ions (Figure S5a).…”

“…The electrochemical property of Er 3 C 2 @I h (7)-C 80 , however, astonishingly varies from that of Sc 3 C 2 @I h (7)-C 80 . Er 3 C 2 @ I h (7)-C 80 exhibits the reversible oxidation step and first reversible reduction step at +0.21 and −0.89 V, respectively, which are shifted in the anodic and cathodic directions by 390 and 240 mV, respectively, in comparison with that of Sc 3 C 2 @ I h (7)-C 80 (Table 1). This indicates a considerable difference of the frontier MO energies for these two molecules, which is highly associated with their electronic configurations and thus the bonding nature of the inner clusters discussed above.…”

“…Metal–metal bonding has been increasingly utilized to make advances in numerous areas, such as molecular magnets, − catalyses, and molecular electronics . Of particular interest with the 4 f elements, the lanthanide–lanthanide (Ln–Ln) bond features different bonding natures from the typical d -block transition metals; thus, it is of crucial importance for the understanding of lanthanide coordination chemistry. , Since the Coulombic repulsion between lanthanide centers is much stronger than their bonding interactions, the Ln–Ln bond is still challenging to be realized in the established coordination compounds …”

Incorporating Comonomers into Polymeric Phosphate Ligands Can Tune the Affinity and Capacity for Rare Earth Element, La

“…11 For example, La 3+ has an absolute chemical hardness of 15.4 eV, while that of Ac 3+ , the corresponding actinide, is 14.4 eV. 12 In a recent publication, Vitova et al 10 identified open questions in the current understanding of the covalent bonding of lanthanides such as the involvement of 5d electron orbitals and the nature of orbital overlap between lanthanide ions and the complexing ligand. 12 While the community lacks an orbital-level understanding of ligand-ion complexation of lanthanides, there are many studies that investigate the effect of ligand chemistry on the thermodynamics and kinetics of lanthanide-ion complexation.…”