• Iron;
  • Density functional calculations;
  • Proton reduction;
  • Spin crossover;
  • Hydrogenases


A biomimetic mononuclear iron(III) model complex was investigated in detail by density functional theory (DFT) calculations. Structural and energetic criteria were employed to confirm the S = 3/2 intermediate state to be the ground state. The ground state was verified by using both pure and hybrid functionals with different amounts of exact Hartree–Fock exchange. A comprehensive study of the influence of the functional as well as thermodynamic corrections to the energetic ordering of spin states was performed. A modified B3LYP functional with 10 % Hartree–Fock exchange was able to reproduce the structural properties in excellent agreement with the experimental data. The thermodynamics of two possible spin-crossover transitions [intermediate-spin to high-spin (IS–HS) and low-spin to high-spin (LS–HS)] were investigated. A torsional profile obtained by rotation of the axial ligand revealed a spin-dependent preference of the ligand orientation. The structure solved by X-ray crystallography corresponds to the global energetic minimum of the complex in the S = 3/2 and 5/2 states but not in the S = 1/2 state. This study demonstrates that the spin multiplicity affects not only the structural properties but may also influence the chemical reactivity of this transition metal complex in general.