Importance of Out-of-State Spin–Orbit Coupling for Slow Magnetic Relaxation in Mononuclear FeII Complexes

Two mononuclear high-spin FeII complexes with trigonal planar ([FeII(N(TMS)2)2(PCy3)] (1) and distorted tetrahedral ([FeII(N(TMS)2)2(depe)] (2) geometries are reported (TMS = SiMe3, Cy = cyclohexyl, depe = 1,2-bis(diethylphosphino)ethane). The magnetic properties of 1 and 2 reveal the profound effect of out-of-state spin–orbit coupling (SOC) on slow magnetic relaxation. Complex 1 exhibits slow relaxation of the magnetization under an applied optimal dc field of 600 Oe due to the presence of low-lying electronic excited states that mix with the ground electronic state. This mixing re-introduces orbital angular momentum into the electronic ground state via SOC, and 1 thus behaves as a field-induced single-molecule magnet. In complex 2, the lowest-energy excited states have higher energy due to the ligand field of the distorted tetrahedral geometry. This higher energy gap minimizes out-of-state SOC mixing and zero-field splitting, thus precluding slow relaxation of the magnetization for 2.