S 1 Supporting Information: A Binuclear Fe(III)-Dy(III) Single Molecule Magnet. Quantum Effects and Models. Marilena Ferbinteanu, ∗, †,◊ Takashi Kajiwara, †,§ Kwang-Yong Choi, ‡ Hiroyuki Nojiri, ‡ Akio Nakamoto, # Norimichi Kojima, # Fanica Cimpoesu, *, †,∇ Yuichi Fujimura, † Shinya Takaishi † and Masahiro Yamashita †,§ † Department of Chemistry, Graduate School of Science, Tohoku University, Aramaki, Aoba-ku, Sendai 980-8578, Japan ‡ Institute for Materials Research, Tohoku University, Katahira 2-1-1, Sendai 980-8577, Japan # Department of Basic Science,Graduate School of Arts and Sciences, The University of Tokyo, Komaba, Meguro-ku, Tokyo 113-8654, Japan § Japan Science and Technology Agency (JST), Japan ◊ Permanent address: University of Bucharest, Faculty of Chemistry, Department of Inorganic Chemistry, Dumbrava Rosie 23, Bucharest 70254, Romania ∇ Permanent address: Institute of Physical Chemistry, Splaiul Independentei 202, Bucharest 77208, Romania E-mail: marilena@agnus.chem.tohoku.ac.jp , marrirena@yahoo.com Magnetic measurements Variable-temperature magnetic susceptibility measurements were made using a SQUID magnetometer MPMS 5S (Quantum Design) at 0.5T. Diamagnetic correction was determined from Pascal's constants. The comparison of the Fe III LS -Dy III binuclear (1), [Fe III LS (bpca)(µ- bpca)Dy(NO 3 ) 4 ]⋅ 4.5CH 3 NO 2 ⋅H 2 O, with a related Fe II LS -Dy III complex, having similar local d-f structure and incorporating the diamangnetic Fe II LS shows that the general pattern in the χT vs T curve is mostly due to the Dy III ion itself. The reference Fe II LS complex shows the similar bpca bridging d-f geometry, but differs in the coordination of the lanthanide ion. Namely, an extended chain is formed on the expense of replacing two chelating NO 3 - groups from 1 with another bridging bpca ligand and one