Lawrence F. Dahl investigated the structure of an iron carbonyl tetramer by diffraction. Four bonded iron atoms form a tetrahedron, while four triply bridging carbonyl carbon atoms form an interpenetrating tetrahedron, connecting an unusual molecular geometry with metal–metal bonding.[ref]
Structural Characterization of the Cyclopentadienyliron Carbonyl Tetramer, Fe4(eta5-C5H5)4(CO)4. A Cubane-Type Structure Containing a Completely Bonding Tetrametal Cluster Unit by Melvin A. Neuman and others. 1972, pp.3383–3388: original diffraction structure and tetrametal cluster unit
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