Magnetostructural Correlation in Trigonal Bipyramidal Fe(III) Complexes: Tuning Spin-State Stability and Magnetic Anisotropy via Second Coordination Sphere Substitution

The spin-dependent properties of the transition-metal complexes are strongly influenced by modifications of the first coordination sphere. However, the role of the second coordination sphere in governing these phenomena has remained relatively underexplored. In this study, the ground spin state stability and magnetic anisotropy of 33 trigonal bipyramidal (TBP) iron(III) complexes are examined, starting from the reference complex (PMe3)2FeCl3, using a combination of density functional theory (DFT) and multiconfigurational methods. The spin–orbit coupling is evaluated a posteriori. The complexes were modeled by systematically substituting the second-coordination sphere, i.e., replacing the methyl groups of the phosphine ligands with various alkyl, alkoxy, and acyl groups of increasing bulkiness. Magnetostructural correlations are employed to investigate the impact of these substitutions on the ground spin state and magnetic anisotropy. The effects of structural parameters, such as the axial angle deviation, Tolman cone angle, equatorial deviation parameter, and continuous shape measurement, are explored on the orbital ordering, the energetics of scalar-relativistic and spin–orbit states, zero-field splitting parameters, g-tensors, and the effective magnetic anisotropy barriers. Our investigation reveals that the magnetic anisotropy of these complexes can be systematically enhanced by tuning the second coordination sphere. This can be achieved through a balanced combination of ligand substitutions that account for steric effects and electron-donating or electron-withdrawing properties, offering important guidelines for designing efficient single-ion magnets.