Electron spin resonance spectroscopy of some dianiono(1,4,8,11-tetra-azacyclotetradecane)-iron(III) and -nickel(III) salts

Abstract

The complexes trans-[NiX₂L][ClO₄][X = Cl or Br; L = 1,4,8,11-tetra-azacyclotetradecane (cyclam)] are shown to dissociate in solution according to the equilibrium [NiX₂L]⁺⇌[NiXL]²⁺+ X^–⇌[NiL]³⁺+ 2 X^–. The e.s.r. spectrum of each species has been recorded and analysed. The unpaired electron is in an orbital which is mainly 3d_z² with significant delocalisation on to a p orbital on each halogen. The e.s.r. spectra of trans-[FeCl₂L][ClO₄] and trans-[Fe(NCS)₂L][NCS] have g ca. 3.2, 2.2, and 1.1, and show that the ground state is ²T₂(t_2g⁵), but the spectrum of trans-[FeBr₂L][ClO₄] exhibits additional lines at g_∥= 2 and g_⊥= 6, showing the presence of a high-spin–low-spin equilibrium with ca. 71% low spin at 293 K.