Cubane-type [Co.sub.4][S.sub.4] clusters: synthesis, redox series, and magnetic ground states

The carbene cluster [[Fe.sub.4][S.sub.4][([Pr.sup.i.sub.2]NHC[Me.sub.2]).sub.4]] as an accurate structural and electronic analogue of the fully reduced cluster of the iron protein of Azotobacter vinelandii nitrogenase, including a common S = 4 ground state and the issues related to the existence and magnetism of other [[[M.sub.4][S.sub.4][L.sub.4]].sup.2] clusters are demonstrated. The octanuclear clusters which exhibited rhomb-bridged noncubane (RBNC) structure and the clusters with the [Fe.sub.8][S.sub.8] core that possessed edge-bridged double-cubane (EBDC) stereochemistry are two structural solutions for the [M.sub.8][S.sub.8] core in the form of topological isomers whose stability might depend on valence electron count.