Abstract
Some complexes of iron(II) with quadridentate Schiff bases e. g. NN'-ethylenebis(salicylideneimine) and several of its ring-substituted derivatives, and NN'-o-phenylenebis(salicylideneimine) have been prepared and shown from magnetic and Mossbauer investigations to be high-spin. These ligands take up planar configurations around bivalent metals, but the complexes are believed to have pentaco-ordinate structures. The complexes form apparently pentaco-ordinate mononitrosyls by reaction with nitric oxide. The mononitrosyls are high-spin, with room temperature moments corresponding to three unpaired electrons. On cooling to 80°K, nitrosyl NN'-ethylenebis(salicylideneiminato)iron and nitrosyl NN'-o-phenylenebis(salicylideneiminato)iron change sharply to low-spin states at approximately 175°K and between 160-180°K respectively with a corresponding decrease in the N-O stretching frequency. The Mossbauer parameters change dramatically over the temperature range. Various reasons for spin-pairing, and the assignment of oxidation states to the iron and nitric oxide are discussed. Low temperature magnetic and infrared studies show that some change of the 5-nitro substituted nitrosyl to low-spin species has occurred at 80°K. The Mossbauer parameters remain constant from room to liquid nitrogen temperatures. Another series of compounds, the tris(monothio-B-diketonato)iron(III) complexes, have been shown to provide further examples of spin-state isomerism. The complexes are predominantly high-spin at room temperature, with moments slightly reduced from the spin-only value. On cooling to 80°K, the complexes exhibit spin-pairing, the degree of which varies from compound to compound. Tris(pent-3-ene-4-thiono-2-onato)iron(III) changes abruptly from a high-to a low-spin state at approximately 150K. Both spin-isomers, for each complex, are observed in the Mossbauer spectra. The Mossbauer parameters for high-and low-spin iron(III) and the spectra are in qualitative agreement with the magnetic data. Determinations of magnetic moments by an n.m.r. method show that the complexes are predominantly high-spin in solution. A complex of iron(II) with the bidentate Schiff base ligand, salicylaldimine, and the adducts formed by bis(salicylaldehydato)iron(II) with monodentate nitrogen-donor ligands have also been investigated.