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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]

Melvin A. Neuman helped characterize an iron carbonyl tetramer containing a bonded four-metal metal cluster. The structure consists of interpenetrating tetrahedra of iron atoms and bridging carbonyl carbon atoms, giving a concrete example of how diffraction reveals metal–metal connectivity.^[ref]

Neutral counting assigns five electrons to cyclopentadienyl ligand and two to CO. Thus each $\mathrm{FeCpCO}$ fragment has $8+5+2=15$ valence electrons and contributes three skeletal electrons; each $\mathrm{CoCpCO}$ fragment has sixteen and contributes four:For A the six-pair four-vertex count is the nido count derived from a five-vertex trigonal bipyramid. Removing an axial vertex leaves a tetrahedron: the three equatorial vertices plus the remaining axial vertex. For B the corresponding arachno fragment is obtained by removing both axial vertices from that same parent, leaving a three-metal triangle. Small clusters require specifying the parent and removed vertices; mechanically calling every nido fragment a square pyramid is wrong.

The Fe tetramer is indeed known to have a tetrahedral $\mathrm{F}{\mathrm{e}}_4$ core, with four face-capping CO groups. The carbon atoms of those carbonyls and the iron atoms form interpenetrating tetrahedra, giving a cubane-like Fe–C connectivity. Bridging rather than terminal CO changes the distribution of local donation, not the total electron count above.^[ref]

Trinh-Toan helped characterize an iron carbonyl tetramer by diffraction. Its four metal atoms form a bonded tetrahedron, interpenetrated by a second tetrahedron of triply bridging carbonyl carbon atoms. The result is a concrete structural example of a metal metal cluster.^[ref]