Investigation of Magnetic Properties in Exchange Coupled Radical-Nickelocene Complexes by Adopting a Generalized Spin Hamiltonian

Magnetic interaction between an isotropic spin center and a strongly anisotropic partner presents a unique challenge in molecular magnetism, as the interplay between local anisotropy and exchange pathways often defies conventional Heisenberg exchange. In this work, we address this complexity by investigating a series of nitronyl-nitroxide radicals coupled to nickelocene through polyene spacers of increasing length. Both density functional theory and wave function based multireference approaches are adopted to investigate the nature of isotropic exchange in these systems. To resolve the limitations of standard spin models, we employ a mapping approach based on an effective Hamiltonian derived from CASSCF + NEVPT2 spin–orbit eigenstates. By treating the system’s low-lying electronic structure through this extended framework, we explicitly account for the significant mixing between the ground and first excited spin states which is found to be responsible for an anomalous increase in the axial zero-field splitting (D). We find that increasing the model space to include antiferromagnetic states recovers the full zero-field splitting while enabling finite anisotropic exchange contributions to be extracted. By establishing a systematic distance dependence for these interactions, this work provides a rigorous framework for navigating the nontrivial coupling mechanisms in complex organic–inorganic spin architectures.