Lanthanides: Divalent Solid Halides

Abstract

Divalent rare earth metals (Sc, Y, and the lanthanides) are known in halides almost throughout the whole series. Two classes may be distinguished: RX 2 = (R 2+ )(X − ) 2 type dihalides, in which R 2+ has the electronic configuration [Xe]6s 0 5d 0 4f n . Their structural behavior parallels that of the heavier alkaline‐earth metal halides; for example, EuBr 2 and SrBr 2 are isostructural. All of these class I compounds are insulators. In the second class, R 2+ has the electronic configuration [Xe]6s 0 5d 1 4f n −1 ; the occupation of a 5d state leads to (semi)metallic behavior. Their crystal structures resemble those of intermetallic phases, for example, LaI 2 crystallizes with the CuTi 2 type of structure. This class of dihalides RX 2 is often written as (R 3+ )(e − )(X − ) 2 . Except for the binary dihalides, there are a number of ternary halides, for example, CsTmCl 3 and the mixed‐valent K 5 Dy 3 I 12 (class I), as well as BaLaI 4 (class II). Oxide‐halides with divalent lanthanides are also known, for example, {OSm 4 }I 6 or the mixed‐valent Eu 2 O 2 Br.

Tetramethylcyclopentadienyl Ligands Allow Isolation of Ln(II) Ions across the Lanthanide Series in [K(2.2.2-cryptand)][(C5Me4H)3Ln] Complexes

“…This small change from Ln­(III) to Ln­(II) is consistent with that observed in going from 4f n to 4f n 5d 1 electron configurations with the new ions in the [K­(crypt)]­[Cp′ 3 Ln], 3-Ln , series . In contrast, 4f n to 4f n +1 reductions with the traditional ions, Eu­(II), Yb­(II), Sm­(II), and Tm­(II), give bond distance changes of 0.1–0.2 Å. − The small change in distances between the Nd­(III) and Nd­(II) complexes suggests that the added electron occupies the d z 2 orbital.…”

“…In recent years, the range of oxidation states available to the rare-earth metals in crystallographically characterizable molecular complexes available for reactivity in solution has greatly expanded. , Up until 2001, it was thought that only six lanthanides could form crystallographically characterizable molecular complexes of Ln­(II) ions in solution: Eu, Yb, Sm, Tm, Dy, and Nd. − These complexes could be made by reduction of 4f n Ln­(III) precursors and formed Ln­(II) ions with 4f n +1 electron configurations as expected. However, it is now known that yttrium and all of the lanthanides (except Pm which was not studied due to its radioactivity) can form isolable molecular complexes of Ln­(II) ions if reductions are done in the proper coordination environment. − Specifically, reduction of tris­(cyclopentadienyl) complexes with silyl-substituted ligands C 5 H 3 (SiMe 3 ) 2 (Cp″) , and C 5 H 4 SiMe 3 (Cp′) − provided access to Ln­(II) ions across the series, as shown in eq . , This was also extended to the actinides, Th, U, , Pu, and Np .…”

Molecular and Electronic Structure, and Hydrolytic Reactivity of a Samarium(II) Crown Ether Complex

“…The electronic structure of classical Ln II compounds that are typically thought of as adopting 4 f n+1 configurations is readily explained through the stabilization imparted by self-exchange that maximizes in half- and fully filled orbitals as exemplified by Eu II (4 f 7 ) and Yb II (4 f 14 ). In contrast, other divalent lanthanides such as Nd II and Dy II have been observed in both 4 f n +1 and 4 f n 5 d 1 configurations where calculated differences in energies between the configurations are small . Thus, the particular configuration adopted is currently challenging to predict either experimentally or theoretically.…”

“…Despite the unusual and potentially useful properties of encapsulated divalent lanthanide complexes, structurally characterized examples of cryptand and crown-ether complexes other than those containing Eu II remain scarce owing to their substantially increased reactivity with water and oxygen . Oddly, Sm II compounds have been known since the early 20th century, and are in fact historic enough to have once been denoted by samarous (or samaric 3+), and yet few structurally characterized examples exist outside of binary halides . Among the few well-characterized examples with Sm II are [Sm­(2.2.2-cryptand)] 2+ that was recently obtained via an unanticipated displacement of K + from [K­(2.2.2-cryptand)] + , Sm­(18-crown-6)­(ClO 4 ) 2 , where Sm II is bound by both the crown ether and chelating perchlorate anions, and the sandwich complex, [Sm­(15-crown-5) 2 ]­[ClO 4 ] 2 , where the perchlorate anions are outer sphere …”

In-Plane Cation Ordering and Sodium Displacements in Layered Honeycomb Oxides with Tetravalent Lanthanides: Na2LnO3 (Ln = Ce, Pr, and Tb)

“…Lanthanides in the formal 2+ oxidation state (in halides, oxyhalides, etc.) exhibit unique magnetic properties with insulating, semimetallic, or metallic behavior. − Tetravalent lanthanides find application in several energy-related technologies including as catalysts for water splitting, automotive exhausts, and steam reforming and as solid-state (SS) electrolytes in fuel cells because of their reversible redox properties. − However, most of these applications employ the use of binary oxides, LnO 2 (Ln = Ce, Pr, and Tb) . As a general rule, ternary oxides stabilize tetravalent lanthanides to a greater extent than binary oxides. , The primary distinctions of these compounds from the transition-metal analogues is their relative redox instability and the increased ionic radii of the B site cation.…”