Synthesis and reactivity of lanthanide(II) hydrides

The field of molecular lanthanide(II) hydride chemistry has emerged as a distinct and rapidly developing subfield within f-block organometallic chemistry, offering access to reactivity patterns that diverge significantly from those of the more established lanthanide(III) hydrides. This review provides a comprehensive survey of all structurally characterised divalent hydride complexes reported to date, with particular emphasis on their synthesis, stabilising ligand frameworks, and reactivity. Fundamental concepts governing lanthanide chemistry, including ionic radii effects, steric saturation, and Schlenk-type redistribution, are discussed in the context of the unique challenges associated with stabilising Ln(II)–H bonds. Two principal synthetic strategies, hydrogenolysis of Ln(II) alkyl precursors and hydride transfer to Ln(II) amido complexes, are critically evaluated. The reactivity of lanthanide(II) hydrides is examined across reduction, σ-bond metathesis, and insertion reactions, highlighting the emerging catalytic applications and mechanistic divergences from Ln(III) analogues. While the field has historically been dominated by ytterbium(II) systems, recent breakthroughs have enabled the isolation of europium(II) and samarium(II) hydrides, significantly expanding the scope of accessible Ln(II) chemistry. These advances underscore the importance of ligand design and steric saturation in stabilising highly reducing metal centres. Looking forward, the review identifies the isolation of hydrides based on the “non-classically stable” lanthanide(II) ions (Tm, Dy and Nd) as a critical unmet challenge and argues that overcoming limitations associated with precursor instability, homoleptic redistribution, and solvent activation will be essential for unlocking new modes of small-molecule activation and catalysis within the f-block. • Structure and synthesis of molecular Ln(II) hydrides. • Reactivity patterns and catalysis. • Novel reaction pathways.