The development of metal carbonyl clusters such as Ni(CO) and Fe(CO) led quickly to the isolation of Fe(CO) and Fe(CO).  Rundle and Dahl discovered that Mn(CO) featured an "unsupported" Mn-Mn bond, thereby verifying the ability of metals to bond to one another in molecules. In the 1970s, Paolo Chini demonstrated that very large clusters could be prepared from the platinum metals, one example being [Rh(CO)H].  This area of cluster chemistry has benefited from single-crystal X-ray diffraction.
Many metal carbonyl clusters contain ligands aside from CO. For example, the CO ligand can be replaced with myriad alternatives such as phosphines, isocyanides, alkenes, hydride, etc.  Some carbonyl clusters contain two or more metals.  Others contain carbon vertices.  One example is the methylidyne-tricobalt cluster [Co(CH)(CO)]. The above-mentioned cluster serves as an example of an overall zero-charged (neutral) cluster. In addition, cationic (positively charged) rather than neutral organometallic trimolybdenum  or tritungsten clusters are also known. The first representative of these ionic organometallic clusters is [Mo(CCH)(OCCH)(HO)].