The discovery of ferrocene nearly 70 years ago marked the genesis of metallocene chemistry; although the ferrocenium cation was discovered soon afterwards, a derivatized ferrocenium dication was only isolated in 2016 and the monoanion of ferrocene has only been observed in low temperature electrochemical studies. Here we report the isolation of a derivatized ferrocene anion in the solid state as part of an isostructural family of 3 d metallocenates which consist of anionic complexes of a metal centre (manganese, iron or cobalt) sandwiched between two bulkyCp ttt ligands (where Cp ttt is {1,2,4-C 5 H 2 t Bu 3 }). These air- and thermally-sensitive complexes rapidly decompose above -30 °C, however, we were able to characterise all metallocenates by a wide range of physical techniques and ab initio calculations. These data have allowed us to map the electronic structures of this metallocenate family, including an unexpected high-spin S = 3/2 ground state for the 19e - derivatized ferrocene anion.

Low coordinate metal complexes can exhibit superlative physicochemical properties, but this chemistry is challenging for the lanthanides (Ln) due to their tendency to maximize electrostatic contacts in predominantly ionic bonding regimes. Although a handful of Ln 2+ complexes with only two monodentate ligands have been isolated, examples in the most common +3 oxidation state have remained elusive due to the greater electrostatic forces of Ln 3+ ions. Here, we report bent Ln 3+ complexes with two bis(silyl)amide ligands; in the solid state the Yb 3+ analogue exhibits a crystal field similar to its three coordinate precursor rather than that expected for an axial system. This unanticipated finding is in opposition to the predicted electronic structure for two-coordinate systems, indicating that geometries can be more important than the Ln ion identity for dictating the magnetic ground states of low coordinate complexes; this is crucial transferable information for the construction of systems with enhanced magnetic properties.

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.

Alkali metal amides are vital reagents in synthetic chemistry and the bis(silyl)amide {N(SiMe3)2} (N′′) is one of the most widely-utilized examples. Given that N′′ has provided landmark complexes, we have investigated synthetic routes to lithium and sodium bis(silyl)amides with increased steric bulk to analyse the effects of R-group substitution on structural features. To perform this study, the bulky bis(silyl)amines {HN(SitBuMe2)(SiMe3)}, {HN(SiiPr3)(SiMe3)}, {HN(SitBuMe2)2}, {HN(SiiPr3)(SitBuMe2)} and {HN(SiiPr3)2} (1) were prepared by literature procedures as colourless oils; on one occasion crystals of 1 were obtained. These were treated separately with nBuLi to afford the respective lithium bis(silyl)amides [Li{μ-N(SitBuMe2)(SiMe3)}]2 (2), [Li{μ-N(SiiPr3)(SiMe3)}]2 (3), [Li{N(SitBuMe2)2}{μ-N(SitBuMe2)2}Li(THF)] (4), [Li{N(SiiPr3)(SitBuMe2)}(DME)] (6) and [Li{N(SiiPr3)2}(THF)] (7) following workup and recrystallization. On one occasion during the synthesis of 4 several crystals of the ‘ate’ complex [Li2{μ-N(SitBuMe2)2}(μ-nBu)]2 (5) formed and a trace amount of [Li{N(SiiPr3)2}(THF)2] (8) was identified during the recrystallization of 7. The reaction of {HN(SitBuMe2)2} with NaH in the presence of 2 mol % of NaOtBu gave crystals of [Na{μ-N(SitBuMe2)2}(THF)]2 (9-THF), whilst [Na{N(SiiPr3)2}(C7H8)] (10) was prepared by deprotonation of 1 with nBuNa. The solid-state structures of 1–10 were determined by single crystal X-ray crystallography, whilst 2–4, 7, 9 and 10 were additionally characterized by NMR and FTIR spectroscopy and elemental microanalysis.