Triangular clusters [{MFe(CO)(4)}(3)](3-) (M = Cu, 4; Ag, 5; Au, 6) were selectively obtained by heating Fe(CO)(4)(MIMes)(2) (M = Cu, 1; Ag, 2; Au, 3; IMes = C3N2H2(C6H2Me3)(2)). 1-3 were synthesized by reacting Na-2[Fe(CO)(4)]center dot 2thf with 2 equiv of M(IMes)Cl. As previously described, the direct reactions of Na-2[Fe-(CO)(4)]center dot 2thf with one equivalent of M(I) salts resulted in the triangular cluster [{CuFe(CO)(4)}(3)](3-) for Cu, whereas the square clusters [{MFe(CO)(4)}(4)](4-) were formed for Ag and Au. Thus, depending on the synthetic protocol adopted, both the triangular [{MFe(CO)(4)}(3)](3-) and square [{MFe(CO)(4)}(4)](4-) polymerization isomers can be selectively obtained, at least for Ag and Au. Polymerization isomerism, that is two compounds having the same elemental compositions but different molecular weights, was investigated in [{MFe(CO)(4)}(n)](n-) (n = 3, 4; M = Cu, Ag, Au) by means of structural and theoretical methods and the role of metallophilic interactions was computationally studied by means of the atoms-in-molecules (AIM) approach.