Owing their large radii, transition metals generally form clusters that are larger than main group elements. One consequence of their increased size, these clusters often contain atoms at their centers.  A prominent example is [FeC(CO)]. In such cases, the rules of electron counting assume that the interstitial atom contributes all valence electrons to cluster bonding.  In this way, [FeC(CO)] is equivalent to [Fe(CO)] or [Fe(CO)].