A molecular ion or neutral molecule known as a metal cluster compound contains three or more metals and exhibits strong metal-metal interactions. Fe2(CO)9 and Fe3(CO)12 were easily isolated as a result of the formation of metal carbonyl clusters such as Ni(CO)4 and Fe(CO)5. Rundle and Dahl discovered that Mn2(CO)10 included a "unsupported" Mn-Mn link, proving that metals may form bonds with one another in molecules. The synthesis of extremely large clusters from the platinum metals, including [Rh13(CO)24H3]2, was proven by Paolo Chini in the 1970s. Single-crystal X-ray diffraction has been useful in this field of cluster chemistry. Early metals with low valentities frequently have clusters of their halides that have substantial M-M bonding. Contrast this with the higher halides of these metals and almost all of the late transition metal halides, where metal-halide bonding is abundant. Later, nitrogenase was shown to contain a distinctive MoFe7S9 active site. Ferredoxin was first shown to contain Fe4S4 clusters in the 1970s. While some of the Fe-S clusters have a catalytic function, most of them act as redox cofactors. There are several different Fe-S clusters with CO ligands that have been discovered in the field of bioinorganic chemistry. Metal atom clusters called elementoid clusters have more direct element-element contacts than element-ligand contacts and are stabilised by ligands. Al77, Ga84, and Pd145 ligand stabilised cores are examples of structurally characterised clusters.