Kwon, HyunchulMcClain, K. RandallStaab, Jakob K.Smith, Patrick W.Harvey, Benjamin G.Erodici, Matthew P.Teat, Simon J.Harris, T. DavidMinasian, StefanChilton, Nicholas F.Long, Jeffrey R.2026-02-252026-02-250002-7863Bibtex:kwon_triangular_2026WOS:001684055700001https://hdl.handle.net/1885/733806563Mixed-valence complexes featuring lanthanide-lanthanide bonding have recently been shown to act as single-molecule magnets with unprecedented operating temperatures and magnetic coercivities. Here, we present the synthesis and detailed examination of the electronic structure, bonding, and magnetic properties of mixed-valence trinuclear clusters (C5 i Pr5)3Ln3H3I2 (Ln = Tb, Dy, Ho, Er, and Tm). Near-infrared and X-ray absorption spectra, together with computational results, confirm these clusters possess a three-center, one-electron sigma bond. This metal-metal bonding leads to strong intermetal exchange coupling, resulting in magnetic behaviors that starkly contrast with typical multinuclear lanthanide complexes. Notably, structural, spectroscopic, and computational studies of the thulium cluster reveal valence delocalization through a bonding orbital of 5d-parentage between the three Tm centers. This observation represents the first example of a nontraditional electronic structure for thulium involving 5d rather than 4f orbitals. Magnetic analysis reveals a complex interplay between single-ion magnetic anisotropy and ferromagnetic exchange, governing the overall magnetic anisotropy of these clusters. Magnetic susceptibility measurements for Ln = Tb-Er indicate thermally well-isolated high-moment ground states arising from strong magnetic coupling, although the maximum values are lower than those expected for complete parallel alignment of the sigma and 4f electrons. Computational analyses suggest that collinear alignment of the local anisotropy axes results in out-of-plane anisotropy for Ln = Er and Tm, whereas noncollinear alignment induces in-plane anisotropy for Ln = Tb, Dy, leading to distinct magnetic relaxation properties. Together, the results highlight the diverse magnetic behaviors that can be realized through lanthanide-lanthanide bonding and outline a synthetic path forward toward maximizing the magnetic anisotropy in f-element clusters.The characterization of all compounds using x-ray diffraction, spectroscopy, and magnetometry was supported by NSF grant CHE-2350466. The synthesis of all compounds was supported by the Naval Air Warfare Center Weapons Division ILIR program. Computational analyses were supported by ERC grant STG-851504, Royal Society URF191320, and Leverhulme Trust RPG-2023-025. This research used resources of the Advanced Light Source, which is a DOE Office of Science User Facility under contract no. DE-AC02-05CH11231. We thank the University of Manchester for access to the Computational Shared Facility and the ILJU Academy and Culture Foundation for supporting H.K. through an Overseas Ph. D. Scholarship. P.W.S. and S.G.M. were supported at LBNL by the Director, Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences (CSGB), U.S. Department of Energy (DOE), Quantum Information Science program under contract no. DE-AC02-05CH11231. The Stanford Synchrotron Radiation Lightsource is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under contract no. DE-AC02-76SF00515. Danh Ngo and Adrian Huang are acknowledged for helpful discussions, and Meg Takezawa for experimental assistance with Raman spectroscopy.11en©2026 The authorsComplexesExchangeIonsSingle-molecule magnetsTransition2026-02-0610.1021/jacs.5c19340105030528094