%AGould, Colin%AMcClain, K.%AReta, Daniel%AKragskow, Jon%AMarchiori, David%ALachman, Ella%AChoi, Eun-Sang%AAnalytis, James%ABritt, R.%AChilton, Nicholas%AHarvey, Benjamin%ALong, Jeffrey%BJournal Name: Science; Journal Volume: 375; Journal Issue: 6577 %D2022%I %JJournal Name: Science; Journal Volume: 375; Journal Issue: 6577 %K %MOSTI ID: 10322634 %PMedium: X %TUltrahard magnetism from mixed-valence dilanthanide complexes with metal-metal bonding %XMetal-metal bonding interactions can engender outstanding magnetic properties in bulk materials and molecules, and examples abound for the transition metals. Extending this paradigm to the lanthanides, herein we report mixed-valence dilanthanide complexes (Cp iPr5 ) 2 Ln 2 I 3 (Ln is Gd, Tb, or Dy; Cp i Pr5 , pentaisopropylcyclopentadienyl), which feature a singly occupied lanthanide-lanthanide σ-bonding orbital of 5 d z 2 parentage, as determined by structural, spectroscopic, and computational analyses. Valence delocalization, wherein the d electron is equally shared by the two lanthanide centers, imparts strong parallel alignment of the σ-bonding and f electrons on both lanthanides according to Hund’s rules. The combination of a well-isolated high-spin ground state and large magnetic anisotropy in (Cp iPr5 ) 2 Dy 2 I 3 gives rise to an enormous coercive magnetic field with a lower bound of 14 tesla at temperatures as high as 60 kelvin. %0Journal Article