Rationalizing Spin-Crossover Properties of Substituted Fe (II) Complexes

We investigate spin-state transitions in a series of 24 [Fe^II(bpp^X)₂]²⁺ spin-crossover (SCO) complexes using density functional theory (DFT). The TPSSh/def2-TZVP approach demonstrates reasonable accuracy in predicting spin-state energetics compared to other functionals, though significant deviations persist in transition temperature (T_1/2) estimates. Temperature-dependent and quasi-harmonic corrections for low-frequency vibrational contributions to enthalpic and entropic terms yielded only marginal improvements. To improve T_1/2 prediction accuracy, we develop electronic descriptors based on effective fragment orbitals (EFOs) and their occupations, quantifying ligand σ-donation and π-acceptor characteristics that govern ligand field strength. Additionally, we introduce a resonance descriptor (R) derived solely from the effective atomic orbitals (eff-AOs) of isolated ligands. Our analysis reveals that electron-donating groups (EDGs) enhance π-electron density in the ligands while simultaneously reducing both σ-donor and π-acceptor capabilities, ultimately lowering the T_1/2 value. These descriptors perform reasonably well also for a set of 12 [Fe^II(pybox^X)₂]²⁺ SCO complexes. This new methodology provides a computationally efficient framework for modulating spin-state properties in transition metal complexes, enabling rational design of SCO materials.