Abstract
Iron-sulfur clusters are mixed-valence systems exhibiting both localized and delocalized valence states. We discuss here spin-coupling models for two types of oxidized [3Fe-4S] clusters with localized Fe3+ valence states; a Heisenberg Hamiltonian with isotropic antiferromagnetic exchange fits the data well. Reduced [3Fe-4S] clusters, on the other hand, contain a trapped Fe3+ site and a delocalized Fe3+-Fe2+ pair. The pair has spin S12=9/2 (formally ferromagnetic coupling) and is antiferromagnetically coupled to the Fe3+ S3=5/2 spin to yield a system spin S=2. We discuss also recent results for [4Fe-4S] clusters such as [3Fe-4S]→[4Fe-4S] conversions, incorporation of other metals into the iron-sulfur core, and the observation of novel spin states.
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