Binuclear Pentalene Iron Carbonyl Complexes

The bicyclic hydrocarbon pentalene, although unstable in the free state, forms a stable iron carbonyl derivative cis-(eta(5),eta(5)-C8H6)Fe-2(CO)(4)(mu-CO). In this connection, the series of binuclear pentalene iron carbonyl derivatives C8H6Fe2(CO)(n) (n = 7, 6, 5, 4) have been investigated by density functional theory. The lowest energy C8H6Fe2(CO)(6) structure is predicted to be trans-(eta(5),eta(5)-C8H6)Fe-2(CO)(6) without an iron-iron bond. However, a cis-(eta(5),eta(1)-C8H6)Fe-2(CO)(6) structure with an uncomplexed pentalene C=C double bond and a formal Fe-Fe bond of length 2.646 angstrom (BP86) lies only ca. 3 kcal/mol above this global minimum. The CO dissociation energy of C8H6Fe2(CO)(6) to give C8H6Fe2(CO)(5) is predicted to be only ca. 12 kcal/mol, explaining the formation of C8H6Fe2(CO)(5) rather than C8H6Fe2(CO)(6) in reactions of dihydropentalene with iron carbonyls. In addition, the experimentally known cis-(eta(5),eta(5)-C8H6)Fe-2(CO)(4)(mu-CO) structure is found to be the lowest energy C8H6Fe2(CO)(5) structure by more than 25 kcal/mol (BP86). For the tetracarbonyl two triplet and two singlet cis-C8H6Fe2(CO)(4) structures lie within 7 kcal/mol of each other (BP86) with the triplet structures being of slightly lower energies. The singlet C8H6Fe2(CO)(4) structure is predicted to be thermodynamically unfavorable with respect to disproportionation into C8H6Fe2(CO)(5) + C8H6Fe2(CO)(3).