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 O2 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[triple bond]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 (approximately 2.1 A) suggestive of the sigma + 2pi + (2/2)delta Fe[quadruple bond]Fe quadruple bond required to give both Fe atoms the favored 18-electron rare gas configuration.