The homoleptic complexes of cerium with the tris­(piperidinyl)­imidophosphorane ligand, [NP­(pip)3]−, present the most negative Ce3+/4+ redox couple known (<−2.64 V vs Fc/Fc+). This dramatic stabilization of the cerium tetravalent oxidation state [>4.0 V shift from the Ce3+/4+ couple in 1 M HClO4(aq)] is established through reactivity studies. Spectroscopic studies (UV–vis, electron paramagnetic resonance, and Ce L3-edge X-ray absorption spectroscopy), in conjunction with density functional theory studies, reveal the dominant covalent metal–ligand interactions underlying the observed redox chemistry and the dependence of the redox potential on the binding of potassium in the inner coordination sphere.

The thermochemistry, descriptive chemistry, spectroscopy, and physical properties of the tetravalent lanthanides (Pr, Nd, Tb and Dy) in extended phases, gas phase, solution, and as isolable molecular complexes are presented.

A redox pair of Ce4+ and Ce3+ complexes, isotypic to previously prepared Tb4+ and Tb3+ complexes, respectively, is reported to establish a comparative physical and spectroscopic analysis of tetravalent lanthanide ions.