Orbital ground state reversal in the high-spin complex Fe(II) (N, N′-dicyclohexylthiourea)6(ClO4)2

Mössbauer spectroscopy of iron (II)(N, N′‐Dicyclohexylthiourea)₆ (ClO₄)₂ shows the existence of two isomers. At low temperatures (T < 200°K) a form A with quadrupole splitting ΔE = 3.31 mm/sec is stable. At and above room temperature a form B with ΔE = 1.32 mm/sec is stable. For 200 °K < T < 300°K both forms are observed with the B/A ratio increasing with increasing temperature. The values of ΔE are typical of Fe²⁺ with an orbital singlet and an orbital doublet ground state for A and B, respectively. The isomer shift and magnetic susceptibility results indicate that both forms of the molecule contain high‐spin ferrous ions. Infrared spectra show that in both forms in the iron is bound to the sulphur atoms of each of the six dicyclohexylthiourea ligands. Mössbauer spectra in external magnetic fields show that the sign of the principal component of the electric field gradient is negative for form A and positive for form B, and that the asymmetry parameter of the electric field gradient tensor is negligible in both forms. The only consistent interpretation of all the results outlined above is in terms of a trigonally distorted octahedral coordination of the Fe²⁺ for both isomers. In form A, the distortion is such that the prolately deformed ${{\mathrm{(t}}_2}_g^0)$ orbital is lowest while in form B the oblately deformed degenerate pair ${{\mathrm{(t}}_2}_g^{+})$ and ${t_2}_g^{-}$ are lowest. The foregoing suggest a low symmetry ligand field component (Δ) of variable sign, going from a compression (Δ > 0) along the trigonal axis at low T to an elongation (Δ < 0) at higher temperatures. The transition is discussed in terms of perturbations of the coordination sphere possibly induced by temperature dependent changes in the hydrogen bonding to the perchlorate anions.