Spin-crossover and charge-transfer phenomena in iron-based polynuclear complexes

The research presented in this dissertation focuses on the investigation of a series of iron-based polynuclear complexes, including dinuclear triple-stranded helicates and heteropentanuclear cyanide-bridged complexes. These complexes undergo spin crossover or charge-transfer coupled spin transitions under the influence of external factors. Emphasis is placed on the description of the Mossbauer spectroscopy results, which are complemented by density functional theory calculations and corroborated by a combination of X-ray crystallography and magnetic susceptibility data.;Information about the spin crossover behavior of two FeII dinuclear complexes with triple-helical structures was obtained based on variable temperature, variable field Mossbauer spectroscopy studies. Together with density functional theory calculations, these studies provided insight into the electronic structure of the iron-based dinuclear complexes.;The spin-crossover and charge-transfer properties of a series of heteropentanuclear, cyanide-bridged complexes are discussed next. Complexes with {FeII 3FeIII2} and {Fe II3CoIII2} cores undergo thermally-driven high spin to low spin transitions at the FeII centers upon cooling, whereas the related cluster that contains a {FeII 3CrIII2} core contains exclusively low spin FeII ions. In the latter pentanuclear cluster, these diamagnetic FeII sites are within bridges that mediate weak ferromagnetic interactions between remote CrIII ions. A high-field Mossbauer study provided the first direct experimental evidence of uncompensated spin density at the FeII diamagnetic sites. The dissertation also describes the Mossbauer spectroscopy investigation of an isostructural cluster with a {Co3Fe2} core for which a thermally-induced, coupled charge transfer/spin transition between CoIII LS-NC-FeIILS and CoII HS-NC-FeIIILS states was observed. The properties of this cluster are sensitive to both temperature and solvent content.;The studies presented in this dissertation offer a detailed description of both electronic and magnetic properties of iron-based polynuclear complexes with spin crossover and/or charge transfer properties.