The metalmetal interaction in policarbonyl metal clusters remains one of the most challenging and controversial issues in metalorganic chemistry, being at heart of a generalized understanding of chemical bonding and of specific applications of these molecules. In this work, the interacting quantum atoms (IQA) approach is used to study the metalmetal interaction in dimetal polycarbonyl dimers, analyzing bridged (Co-2(CO)(8))), semibridged ([FeCo(CO)(8)](-)) and unbridged (Co-2(CO)(8), [Fe-2(CO)(8)](2-)) clusters. In all systems, a delocalized covalent bond is found to occur, involving the metals and the carbonyls, but the global stability of the dimers mainly originates from the Coulombic attraction between the metals and the oxygens.