Spin Crossover <scp>OFF</scp>/<scp>ON</scp> Triggered by Ligand Chemical Doping in an Fe(<scp>III</scp>) Solid Solution^†

Comprehensive Summary Spin crossover (SCO), characterized by distinct high‐spin (HS) and low‐spin (LS) states, has potential applications in memory, electronic, and electroluminescent devices. The OFF/ON switching of SCO is crucial for obtaining bistable magnetic properties. However, there are few strategies for achieving this switching. Herein, based on a ligand chemical doping strategy, we report an Fe(III) solid solution that can be prepared using a ligand chemical doping strategy, enabling not only the OFF/ON switching of SCO but also the fine‐tuning of the spin transition temperature ( T c ) within a 45 K range near room temperature. The experimental results show that when the polar ligand doping ratio reaches 20%, SCO behavior is triggered, and the crystal phase transforms significantly, becoming loose and flexible. Furthermore, T c can be continuously regulated as the ligand‐doping ratio increases. Density functional theory (DFT) calculations reveal that solid packing‐induced molecular distortion blocks SCO, whereas loosely flexible packing triggers SCO via fluorinated ligand chemical doping.