Lanthanides: Divalent Organometallic Chemistry

Abstract This article focuses on the chemistry of Ln 2+ (Ln = lanthanide) complexes that exhibit at least one Ln–C bond. We include the group 3 elements Sc, Y, and La for completeness due to their chemical similarity, though strictly they are not part of the lanthanide series. The vast majority of complexes discussed contain one or more of the classical divalent lanthanides (Sm, Eu, or Yb) because the divalent organometallic chemistry of these elements has been known for nearly 50 years. Ln 2+ organometallic chemistry for the other lanthanides is underdeveloped since, as a result of their larger standard reduction potentials, they are more difficult to access. Therefore, this article gives a historical perspective, together with state‐of‐the‐art results from the last 20 years to show the current vibrancy of this research field. As the ligand environment is the most vital factor for the stabilization of the lanthanide +2 oxidation state, sections are divided according to the supporting ligand: substituted cyclopentadienyls (CpR, C5R5; R = H, alkyl, silyl), phospholyls (PC4R5; R = H, alkyl, silyl), alkyls, aryls, and alkynides. Common synthetic routes are outlined throughout, together with a discussion of how the stability and reactivity profiles of Ln 2+ organometallic complexes change depending on the supporting ligand environment. Divalent organolanthanide chemistry is dominated by substituted Cp ligands, thus these are discussed first. The early development of classical Ln 2+ organometallic chemistry using the Cp* (C5Me5) ligand is outlined, followed by a discussion of contemporary non‐classical divalent lanthanide CpR chemistry, which has utilized bulky alkyl and silyl substituents to stabilize Ln 2+ ions for all lanthanides save Pm. Phospholyls are covered next as they have similar electronic properties to CpR. Phospholyl ligands with bulky alkyl and silyl substituents stabilize Ln 2+ ions analogously to CpR ligands with similar steric properties, hence divalent Sm, Eu, Yb, and Tm phospholyl complexes are presented. Alkyl complexes are the next most common family of classical Ln 2+ complexes. These vary in size and complexity from monodentate silyl‐ and alkyl‐stabilized examples to multidentate bis(iminophopshorano)methanide ligands. Classical divalent lanthanide aryl complexes are less numerous than alkyls, but synthetic routes to these complexes are presented herein as these are also useful starting materials. Finally, classical divalent lanthanide alkynide chemistry, the least developed of all ligand systems discussed in this article, is covered for completeness.