Theoretical Study of Spin Crossover in 30 Iron Complexes

Spin crossover was studied in 30 iron complexes using density functional theory to quantify the direction and magnitude of dispersion, relativistic effects, zero-point energies, and vibrational entropy. Remarkably consistent entropy−enthalpy compensation was identified. Zero-point energies favor high-spin by 9 kJ/mol on average; dispersion and relativistic effects both favor low-spin by 9 kJ/mol on average. These drivers dominate the thermodynamics (but not the transition nature) of SCO and should be considered in rational design of new spin crossover systems.