Although the vast majority of lanthanide (Ln) chemistry is in the +3 oxidation state, Ln(II) complexes are accessible and can exhibit unique physicochemical properties. Classically, divalent Ln solution chemistry was limited to the classical Ln(II) ions Sm, Eu and Yb, but the advent of synthetic routes to Ln(II) diiodide starting materials for Tm, Nd and Dy around the turn of the 21 st century spurred new investigations into non-classical Ln(II) chemistry. Substituted cyclopentadienyl ligands (Cp R ) dominate organometallic Ln(II) chemistry, and have proven to be especially adept in stabilizing non-classical Ln(II) complexes; tris-Cp R ligand fields lower the energy of valence ndz 2 orbitals to allow their occupation upon reduction of Ln(III) precursors. In 1997, Lappert and co-workers studied the reduction of [La(Cp′′)3] (Cp′′ = C5H3(SiMe3)2-1,3) with a K mirror in DME at low temperatures by EPR spectroscopy; a La(II) intermediate was assigned based on an eight-line hyperfine pattern arising from 99.95% abundant La (I = 7/2). The first structurally authenticated molecular La(II) complex was reported by Lappert and co-workers in 2008 to form via the low temperature reactions of [La(Cp′′)3] with a K mirror and 18-crown-6 in Et2O, or 2.2.2-cryptand in THF. Additionally, Lappert attempted to prepare [La(Cp tt )3] -(Cp tt = C5H3 t Bu2-1,3) from the reduction of [La(Cp tt )3] with an excess of K and 18-crown-6; however, only a dinuclear La(III) inverted arene complex, [K(18-crown-6)(η 2 -C6H6)2][{La(Cp tt )2}2(μ-C6H6)], was obtained in benzene, whilst [La(Cp tt )2(μ-OMe)]2 was isolated in DME. This reductive strategy has been honed over the last two decades by Evans and co-workers to provide pseudo-trigonal planar Ln(II) anions, [Ln(Cp′)3] -(Cp′ = C5H4SiMe3), for the whole Ln series apart from radioactive Pm; this methodology was extended to allow structural authentication of the An(II) anions [An(Cp′′)3] - (An = Th, U, Np, Pu). More recently, neutral axial M(II) Cp R complexes [M(C5 i Pr5)2] (M = Y, La, Ce-Pr and Sm-Lu; ) have also been prepared.