Optical and magnetic properties of iron(II)–nitrosyl complexes in model compounds

Abstract

The iron(II) nitrosyl complexes [FeL(NO)](L = ethylenediamine-, ethylenedioxydiethylenedinitrilo- or 1,6-diaminohexane-N,N,N′,N′-tetraacetate) have been studied by absorption, low-temperature magnetic circular dichroism (MCD) and EPR spectroscopies. All three complexes exhibit similar spectral properties. The EPR spectra can be understood in terms of an axial site with a S= electronic ground state. Analysis of the optical spectra has been carried out following extended-Hückel molecular orbital calculations on [Fe(H₂O)₅(NO)]²⁺ assuming C_4ν symmetry for the iron(II) ion with the Fe–NO bond providing the major axis of distortion. Thus it has been possible to identify and assign the MCD transitions within the Fe–NO fragment of the complexes. This analysis provides a basis for understanding and interpreting the magneto-optical spectra of the nitrosyl complexes of iron(II) sites in non-haem iron proteins.