Spin Switching in Crystals Containing Tetranuclear Fe2Co2 Clusters as Structural Units: Interplay of Intra- and Intercluster Interactions

A microscopic model has been elaborated for the description of charge transfer-induced spin transitions in crystals containing tetranuclear Fe2Co2 clusters as structural units. The model takes into account the energy spectrum of each Fe2Co2 cluster, formed by the states arising from its initial configuration, two low-spin FeII and two low-spin CoIII, final configuration two low-spin FeIII, and two high-spin CoII, as well as the states that originate from four intermediate configurations of the type of low-spin FeII, low-spin CoIII, low-spin FeIII, and high-spin CoII. Two different types of cooperative interactions are accounted for in the model, namely, the electron–deformational coupling arising as a result of the observed elongation of the cobalt-nitrogen bonds under the low-spin CoIII→ high-spin CoII transition and the interaction via the field of phonons that originates from the coupling of the Co-ions with the full symmetric displacements of the nearest ligand surrounding, which are modulated by crystalline vibrations. The role of cooperative interactions is discussed in detail. Different types of spin transitions are predicted, including the gradual and abrupt ones as well as those manifesting hysteretic behavior. Within the framework of the developed approach, a qualitative and quantitative explanation of the experimental data on the {[(Tp*)Fe(CN)3]2[Co(bpyMe)2]2}(OTf)2·2DMF·H2O compound recently reported oniere is given.