5T2—1A1 Transitions in six-co-ordinate iron(II) complexes of 2,2′-bi-2-thiazoline and 2,2′-bi-4,5-dihydrothiazine ligands

Abstract

A series of iron(II) complexes of the type [FeL₂X₂] is described [L = a bidentate α-di-imine, 2,2′-bi-4,5-dihydro-thiazine(btz) or 2,2′-bi-2-thiazoline (bt) or an alkyl-substituted derivative (bts, btn, and btne); X = NCS^– or NCSe^–]. Infrared and electronic spectra together with electrical-conductance measurements characterize the complexes as having neutral (probably cis) six-co-ordinate structures in which both the organic ligands and the pseudohalide ions are bound via the nitrogen atoms. On the basis of magnetic- susceptibility measurements and of Mossbauer spectra the complexes fall into three classes. The complex [Fe(btz)₂(NCSe)₂] is low spin (¹A₁ ground state)between 90 and 300 K. Complexes of the ligands btn and btne which bear alkyl substituents adjacent to the donor atoms are high spin (⁵T₂ ground state) in this temperature range. The remaining four complexes exhibit a ⁵T₂-¹A₁ spin transition between 90 and 300 K. The transition is very abrupt in some cases and hysteresis effects are also observed. The electronic spectra of the corresponding nickel (II) complexes indicate a correlation between the apparent Dq(Ni) values and the ground state of the iron(II) complexes.