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John E. McGrady studies how electronic structure controls the chemical bonds and magnetism of inorganic metal clusters. In an endohedral metal cluster, a metal atom sits inside a cage; calculating the interaction between that atom and the cage can distinguish a nearly independent enclosed metal from a structure with strongly shared electron density. His group uses density functional theory and multiconfigurational methods to examine these possibilities.^[ref]

Homepage: mcgrady.chem.ox.ac.uk/

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]

A metal cluster contains several connected metal centers. A triangular three-metal cluster can support three edge metal-metal bonds; early-metal clusters often have stable counts below the 18-electron rule.

A suitable structure is a triangular Ti${}_3$ metal cluster with one 1,2-Bis(dimethylphosphino)ethane chelate and one terminal chloride at each titanium, and one bridging ligand $\mu$-Cl spanning each edge. Include a direct Ti–Ti single metal-metal bond along each edge. The last panel distinguishes these direct bonds from the chloride bridges.

The six chlorides have total formal charge $-6$, so equivalent centers are Ti(II), $d^2$. At each titanium the two phosphorus donors supply four electrons, the terminal chloride supplies two, and the two incident chloride bridges each donate a pair in ionic electron counting. Two incident Ti–Ti single bonds supply another two electrons:Each $d^2$ center can contribute one electron to each of its two edge bonds, making three bonds from six metal electrons overall. This is a 14-electron early-metal cluster, not an 18-electron complex. Forcing the 18-electron rule here would invent excessive bond orders. chloride bridges donate to more than one center, so summing local counts is not the same as counting distinct electrons of the whole cluster.

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]