The nature of the bonding interactions in individual isomeric structures of the above carbonyls was studied using the analysis of domain averaged Fermi holes (DAFH). The main focus was directed on the confrontation of the picture of the bonding resulting from this analysis with the predictions of empirical 18-electron rule. This rule assumes, namely, the presence of direct metal-metal bond(s) for both carbonyls, but the detailed insights provided by the DAFH analysis show that the straightforward association of metal-metal bond with the favorable electron count only is too simplistic, and provided the actual structure of individual isomeric species is not taken into account, the predictions of this rule may fail. This is, e.g., the case of the C-2 nu isomer of the carbonylate anion [Fe-2(CO)(8)](2-) where the DAFH analysis denies the existence of direct metal-metal bond similarly as in the case the isoelectronic CO2(CO)(8). Similar discrepancies between the predictions of the 18-electron rule and DAFH analysis were found also in the case of the C-2 nu isomer of the neutral Fe-2(CO)(8) carbonyl, where the DAFH analysis detects the presence of a single bent Fe-Fe bond rather than the double bond anticipated by the 18-electron rule.