Ultrahard magnetism from mixed-valence dilanthanide complexes with metal-metal bonding

Metal-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)₂Ln₂I₃(Ln is Gd, Tb, or Dy; Cp^iPr5, pentaisopropylcyclopentadienyl), which feature a singly occupied lanthanide-lanthanide σ-bonding orbital of 5d_z2parentage, 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)₂Dy₂I₃gives rise to an enormous coercive magnetic field with a lower bound of 14 tesla at temperatures as high as 60 kelvin.