Quantifying Magnetic Anisotropy of Series of Five‐Coordinate Co^II Ions: Experimental and Theoretical Insights

Abstract Stabilizing large easy‐axis type magnetic anisotropy in molecular complexes is a challenging task, yet it is crucial for the development of information storage devices and applications in molecular spintronics. Achieving this requires a deep understanding of electronic structure and the relationships between structure and properties to develop magneto‐structural correlations that are currently unexplored in the literature. Herein, a series of five‐coordinate distorted square pyramidal Co^II complexes [Co(L)(X₂)].CHCl₃ (where X = Cl (1), Br (2), or I (3)) is reported, all exhibiting easy‐axis magnetic anicotropy. The size of the zero field splitting axial parameter (D) is quantitatively determined (1 = −72; 2 = −67 and 3 = −25 cm⁻¹) using a cantilever torque magnetometry which is further firmly supported by magnetic susceptibility, and EPR measurements. The study of the magnetization relaxation dynamics reveals field‐induced slow relaxation of magnetization due to the predominant Raman relaxation process. Theoretical calculations on 1–3 and optimized model complexes of 1 reveal insights into the electronic structure and highlight the impact of steric and electronic effects on modulating the D values. Overall, the studies reported pave the way for designing a new generation of Co^II complexes with enhanced axiality and a lower rhombicity.