The binuclear cyclopentadienyliron carbonyls Cp2Fe2(CO)(n) (n = 4, 3, 2, 1; Cp = eta(5)-C5H5) have been studied by density functional theory (DFT) using the B3LYP and BP86 methods. The trans- and cis-Cp2Fe2(CO)(2)(mu-CO)(2) isomers of Cp2Fe2(CO)(4) known experimentally are predicted by DFT methods to be genuine minima with no significant imaginary vibrational frequencies. The energies of these two Cp2Fe2(CO)(2)(mu-CO)(2) structures are very similar, consistent with the experimental observation of an equilibrium between these isomers in solution. An intermediate between the interconversion of the trans- and cis-Cp2Fe2(CO)(2)(mu-CO)(2) dibridged isomers of Cp2Fe2(CO)(4) can be the trans unbridged isomer of Cp2Fe2(CO)(4) calculated to be 2.3 kcal/mol (B3LYP) or 9.1 kcal/mol (BP86) above the global minimum trans-Cp2Fe2(CO)(2)(mu-CO)(2). For the unsaturated Cp2Fe2(CO)(3), the known triplet isomer Cp2Fe2(mu-CO)(3) with an Fe=Fe double bond similar to the O=O double bond in O-2 is found to be the global minimum. The lowest-energy structure for the even more unsaturated Cp2Fe2(CO)(2) is a dibridged structure Cp2Fe2(mu-CO)(2), with a short Fe-Fe distance suggestive of the Fe equivalent to Fe triple bond required to give both Fe atoms the favored 18-electron configuration. Singlet and triplet unbridged structures for Cp2Fe2(CO)(2) were also found but at energies considerably higher (20-50 kcal/mol) than that of the global minimum Cp2Fe2(mu-CO)(2). The lowest-energy structure for Cp2Fe2(CO) is the triplet unsymmetrically bridged structure Cp2Fe2(mu-CO), with a short Fe-Fe distance (similar to 2.1 angstrom) suggestive of the sigma + 2 pi + (2)/(2)delta Fe-(4)-Fe quadruple bond required to give both Fe atoms the favored 18-electron rare gas configuration.