Orbital ground state reversal in the highhyphen;spin complex Fe(II) (N, Nprime;hyphen;dicyclohexylthiourea)6(ClO4)2

摘要

Mouml;ssbauer spectroscopy of iron (II)(N, Nprime;hyphen;Dicyclohexylthiourea)6(ClO4)2shows the existence of two isomers. At low temperatures (T 200deg;K) a formAwith quadrupole splitting Dgr;Eequals; 3.31 mmsol;sec is stable. At and above room temperature a formBwith Dgr;Eequals; 1.32 mmsol;sec is stable. For 200 deg;K T 300deg;K both forms are observed with theBsol;Aratio increasing with increasing temperature. The values of Dgr;Eare typical of Fe2plus;with an orbital singlet and an orbital doublet ground state forAandB, respectively. The isomer shift and magnetic susceptibility results indicate that both forms of the molecule contain highhyphen;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. Mouml;ssbauer spectra in external magnetic fields show that the sign of the principal component of the electric field gradient is negative for formAand positive for formB, 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 Fe2plus;for both isomers. In formA, the distortion is such that the prolately deformedlpar;t20grpar;orbital is lowest while in formBthe oblately deformed degenerate pairlpar;t2plus;grpar;andt2minus;gare lowest. The foregoing suggest a low symmetry ligand field component (Dgr;) of variable sign, going from a compression (Dgr; 0) along the trigonal axis at lowTto an elongation (Dgr; 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.