McClain, K. RandallKwon, HyunchulWhyatt, YasminErodici, Matthew P.Mattioni, AndreaMeihaus, Katie R.Harvey, Benjamin G.Chilton, Nicholas F.Long, Jeffrey R.2026-02-272026-02-270002-7863Bibtex:mcclain_mixed-valence_2025https://hdl.handle.net/1885/733806713Mixed-valence lanthanide complexes of the type (CpiPr5)2Ln2I3 (CpiPr5 = pentaisopropylcyclopentadienyl; Ln = Gd, Tb, Dy, Ho, Er) featuring Ln–Ln bonding interactions can exhibit strongly coupled high-moment ground states with a large axial magnetic anisotropy. Here, we report the synthesis, structures, and magnetic properties of the aryloxido-bridged mixed-valence dilanthanide complexes (CpiPr5)2Ln2(OArtt)3 (1-Ln, Ln = Gd, Dy; –OArtt = 3,5-bis(tert-butyl)phenoxide anion). The solid-state structures of the two complexes are distinct, with 1-Dy exhibiting a nearly symmetric structure and a short Dy–Dy bond of 3.265(1) Å, suggesting valence delocalization, while 1-Gd has an asymmetric structure with each Gd atom in a distinct coordination environment, indicative of valence localization. Static magnetic susceptibility data confirm that 1-Gd is valence localized, with only very weak antiferromagnetic exchange between the GdII and GdIII centers at low temperatures. In contrast, magnetic susceptibility data for 1-Dy reveal strong magnetic coupling to give a large angular momentum ground state with magnetic blocking below 40 K. Solid-state Raman spectra for 1-Dy are indicative of Dy–Dy bonding that persists up to ambient temperatures. Computational analyses suggest that the bonding interaction in 1-Dy becomes highly polarized in the presence of slight structural asymmetry and that this effect becomes more pronounced at higher temperatures, as supported by variable-temperature single-crystal X-ray diffraction data. Detailed exploration of the vibronic coupling is consistent with vibronic-driven valence localization at elevated temperatures in 1-Dy.This research was supported by NSF grant CHE‑2350466. Additional support was provided by the Naval Air Warfare Center Weapons Division NISE program (K.R.M. and B.G.H.), ERC grant STG‑851504 (Y.W., A.M. and N.F.C.), Royal Society URF191320 (N.F.C.), and Leverhulme Trust RPG‑2023‑025 (N.F.C.). We thank the University of Manchester for access to the Computational Shared Facility, the ILJU Academy and Culture Foundation for support of H.K. through an Overseas Ph.D. Scholarship, and Dr. T. David Harris for discussions and editorial assistance.11en©2025 American Chemical Society2025-10-0110.1021/jacs.5c05894105018662180