Disentangling anisotropy and exchange in Co(NCX)₂ (X = S, Se) chains via THz-EPR spectroscopy

Two isostructural cobalt(II) chain compounds, [Co(NCX)₂(py)₂]_n (X = S, Se) based on μ-1,3-bridging selenocyanate (1) and thiocyanate (2) ligands, were investigated to elucidate the effect of chalcogen substitution on magnetic anisotropy and exchange interactions. Both compounds form linear chains of octahedrally coordinated cobalt(II) ions with identical topology and similar magnetic exchange but different anisotropy. Heat capacity measurements reveal slightly stronger intrachain interactions and higher ordering temperatures for the selenocyanate derivative, while demonstrating a pronounced sensitivity of the critical temperature to sample handling. Frequency-domain Fourier-transform THz-EPR spectroscopy provides direct access to low-energy magnetic excitations, allowing the determination of intrachain excitation energies and effective g_z values without reliance on a specific model. The selenocyanate compound exhibits both a larger chain excitation gap and enhanced axial anisotropy. Temperature-dependent THz-EPR further yields estimates of weak interchain coupling that is essential for long-range magnetic ordering. Ab initio CASSCF/CASPT2/RASSI-SO calculations reproduce the experimental trends and reveal that the enhanced anisotropy in 1 originates primarily from the softer donor character of selenium, leading to increased covalency and enhanced spin-orbit coupling at the cobalt center, while axial ligand orientation dominates the single-ion anisotropy. Together, these results demonstrate how subtle ligand substitution modulates anisotropy and exchange in ferromagnetic cobalt(II)-based Ising chains.