An investigation of the unusual ⁵⁷Fe Mössbauer quadrupole splittings and isomer shifts in 2 and 3-coordinate Fe(II) complexes

The ⁵⁷Fe Mössbauer quadrupole splittings (ΔE_Q) and isomer shifts (δ_Fe) in 3-coordinate high-spin Fe(II) complexes are unusually small, and previous attempts to reproduce their ΔE_Q values have been unsuccessful. We show here that, by using large structural models and basis sets, both ΔE_Q and δ_Fe values can be quite accurately predicted by using density functional theory. Four systems were investigated: the three 3-coordinate species [LFeX]⁰ (L = β-diketiminate; X = Cl^-, CH₃^-) and [Fe(SC₆H₂-2,4,6-tBu₃)₃]^-, in addition to an uncommon 2-coordinate high-spin ferrous thiolate, [Fe(SC₆H₃-2,6-mes₂)₂] (mes = mesityl = 2,4,6-Me₃C₆H₂). Both Gaussian-type-orbital and Slater-type-orbital basis sets were investigated, and both yielded ΔE_Q and δ_Fe values in good accord with experiment. There were no improvements in these property predictions when (approximate) relativistic effects were included in the calculations. An MO analysis provided a detailed picture of the origin of the small ΔE_Q values seen in the 3-coordinate complexes. These results extend the scope of DFT/Mössbauer investigations beyond the 4-6-coordinate systems described previously to 2- and 3-coordinate systems, which should open the way to using these parameters in structure refinement, especially in large systems, such as proteins.