Beyond the norm: Exploring the unusual oxidation states of lanthanides (+1, +II, +IV and +V) and their potential applications

• Comprehensive analysis of lanthanides in rare oxidation states (+I, +II, +IV, +V). • Advanced techniques (X-ray absorption near edge structure, electron paramagnetic resonance, and spectroscopy) unravel lanthanide complex chemistry. • Stabilization mechanisms explored through ligand effects, geometry, and electronic properties. • Geometric trends in lanthanide compounds highlight challenges and opportunities. • Expansive potential applications in medicine, catalysis, magnetism, opto-electronic devices, noble gas separation processes, and metal–organic chemical vapour deposition. Lanthanides (Lns), typically exhibiting a stable +III oxidation state, are attracting growing attention due to their ability to form less common oxidation states, including +I, +II, +IV, and +V. This review examines the preparation, characterization, and potential applications of lanthanide compounds in these unusual oxidation states. Experimental methods discussed include advanced techniques such as infrared spectroscopy, ultraviolet–visible spectroscopy, X-ray absorption near edge structure, and electron paramagnetic resonance, which facilitate the in-depth analysis of the physicochemical properties of these compounds. Stabilization mechanisms, including ligand effects, electronic interactions, and geometric factors, are also explored. Notably, the stabilization of Ln(I)-based clusters is attributed to B 8 2– ligand interactions, symmetry transitions ( C s → C 7v ), and electronic delocalization effects. Lanthanide(II) complexes, synthesized via salt metathesis, reductive pathways, and transmetalation, are stabilized by ligands such as N-heterocyclic carbenes, cyclopentadienyl, and silylamide derivatives, with reductions using potassium graphite enabling their isolation in solvents like toluene and THF. These complexes exhibit diverse geometries, including bent metallocenes and distorted octahedral structures, with 18-crown-6 facilitating unique coordination environments, making them valuable for catalysis, quantum computing, bio-imaging, and OLED applications. Ln(IV) species, such as Tb(IV) and Pr(IV) complexes, exhibit enhanced covalency through electron rich siloxide and phosphinimine ligands, with ligand-field stabilization energies rationalized via density functional theory calculations. The +V oxidation state, observed in PrO 2 + and (η 2 -O 2 )PrO 2 , has been confirmed through gas-phase ionization experiments, revealing Pr N and Pr O bonds. These findings expand the understanding of lanthanide chemistry and open avenues for innovative applications in catalysis, medicine, materials science, and environmental monitoring.