High-spin Co(II) in monomeric and exchange coupled oligomeric structures: Magnetic and magnetic circular dichroism investigations

The Co(II) ion is a good spectroscopic probe to study the electronic structure of the active centers of metalloenzymes. Especially interesting are magnetic properties of the high-spin Co(II) ion because of the complexity resulting from an unquenched orbital angular momentum. The objectives of this contribution are high-spin Co(II) monomers, dinners and linear trimers in various coordination. Magnetic data. reported in literature, for the high-spin Co(II) complexes are reviewed. The investigation of some selected high-spin Co(II) monomers, both in distorted tetrahedral and octahedral coordination, and several distorted octahedral coordinated dimers and linear trimers is presented. Special attention is paid to the internally exchange coupled Co(II) dimers and linear trimers. The complete experimental data set, consisting of the temperature dependent magnetic susceptibility, the magnetic field dependent magnetization and the variable temperature - variable field - magnetic circular dichroism (VTVH-MCD) is the powerful basis for extracting important electronic parameters such as zero field splitting (ZFS) and exchange coupling constant values. The data obtained for the exchange constant and the ZFS parameters are correlated to the environment of the Co(II) ion and the bridging fragments. Theoretical analysis is performed on the base of different models and under which conditions the simple spin-only Hamiltonian can be applied or when the orbital angular momentum must be included into consideration. The applicability of ZFS parameter as an indicator for the kind of coordination is discussed. The magnetic, magnetization and MCD investigations accompanied by theoretical calculations complement each other. Using the three independent experimental techniques, the parameters extracted for each individual method can be crosschecked. (C) 2008 Elsevier B.V. All rights reserved.