Gould, Colin A.McClain, K. RandallReta, DanielKragskow, Jon G.C.Marchiori, David A.Lachman, EllaChoi, Eun SangAnalytis, James G.Britt, R. DavidChilton, Nicholas F.Harvey, Benjamin G.Long, Jeffrey R.2025-05-302025-05-300036-8075Bibtex:gould_ultrahard_2022PubMed:35025637http://www.scopus.com/inward/record.url?scp=85122925199&partnerID=8YFLogxKhttps://hdl.handle.net/1885/733754693Metal-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 (CpiPr5)2Ln2I3(Ln is Gd, Tb, or Dy; CpiPr5, pentaisopropylcyclopentadienyl), which feature a singly occupied lanthanide-lanthanide σ-bonding orbital of 5dz2parentage, 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 (CpiPr5)2Dy2I3gives rise to an enormous coercive magnetic field with a lower bound of 14 tesla at temperatures as high as 60 kelvin.This work was funded by NSF grants CHE-1800252 and CHE-2102603 (C.A.G. and J.R.L.) and 1905397 (E.L. and J.G.A.); the Naval Air Warfare Center Weapons Division (NAWCWD) NISE-219 program (K.R.M. and B.G.H.); ERC grant 2019-STG-851504 (N.F.C.); and Royal Society fellowship URF191320 (N.F.C.). N.F.C. also thanks the University of Manchester and the Computational Shared Facility at the University of Manchester for support. J.G.A. and E.L. acknowledge support from the Gordon and Betty Moore Foundation's EPiQS Initiative through grant GBMF9067. A portion of this work was performed at the National High Magnetic Field Laboratory, which is supported by NSF cooperative agreement no. DMR-1644779 and the state of Florida.5enPublisher Copyright: © 2022 American Association for the Advancement of Science. All rights reserved.2022-01-1310.1126/science.abl547010.1126/science.abl547085122925199