Binuclear Pentalene Iron Carbonyl Complexes

Abstract

AbstractThe bicyclic hydrocarbon pentalene, although unstable in the free state, forms a stable iron carbonyl derivative cis‐(η5,η5‐C8H6)Fe2(CO)4(μ‐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‐(η5,η5‐C8H6)Fe2(CO)6 without an iron–iron bond. However, a cis‐(η5,η1‐C8H6)Fe2(CO)6 structure with an uncomplexed pentalene C=C double bond and a formal Fe–Fe bond of length 2.646 Å (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‐(η5,η5‐C8H6)Fe2(CO)4(μ‐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.