Unsaturation in Binuclear Cyclopentadienyliron Carbonyls

The binuclear cyclopentadienyliron carbonyls Cp₂Fe₂(CO)_n (n = 4, 3, 2, 1; Cp = η⁵-C₅H₅) have been studied by density functional theory (DFT) using the B3LYP and BP86 methods. The trans- and cis-Cp₂Fe₂(CO)₂(μ-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(μ-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(μ-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(μ-CO)₂. For the unsaturated Cp₂Fe₂(CO)₃, the known triplet isomer Cp₂Fe₂(μ-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(μ-CO)₂, with a short Fe−Fe distance suggestive of the Fe⋮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(μ-CO)₂. The lowest-energy structure for Cp₂Fe₂(CO) is the triplet unsymmetrically bridged structure Cp₂Fe₂(μ-CO), with a short Fe−Fe distance (∼2.1 Å) suggestive of the σ + 2π + ²/₂δ FeFe quadruple bond required to give both Fe atoms the favored 18-electron rare gas configuration.