Binuclear Pentalene Iron Carbonyl Complexes

The bicyclic hydrocarbon pentalene, although unstable in the free state, forms a stable iron carbonyl derivative cis ‐(η 5 ,η 5 ‐C 8 H 6 )Fe 2 (CO) 4 (μ‐CO). In this connection, the series of binuclear pentalene iron carbonyl derivatives C 8 H 6 Fe 2 (CO) n ( n = 7, 6, 5, 4) have been investigated by density functional theory. The lowest energy C 8 H 6 Fe 2 (CO) 6 structure is predicted to be trans ‐(η 5 ,η 5 ‐C 8 H 6 )Fe 2 (CO) 6 without an iron–iron bond. However, a cis ‐(η 5 ,η 1 ‐C 8 H 6 )Fe 2 (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 C 8 H 6 Fe 2 (CO) 6 to give C 8 H 6 Fe 2 (CO) 5 is predicted to be only ca. 12 kcal/mol, explaining the formation of C 8 H 6 Fe 2 (CO) 5 rather than C 8 H 6 Fe 2 (CO) 6 in reactions of dihydropentalene with iron carbonyls. In addition, the experimentally known cis ‐(η 5 ,η 5 ‐C 8 H 6 )Fe 2 (CO) 4 (μ‐CO) structure is found to be the lowest energy C 8 H 6 Fe 2 (CO) 5 structure by more than 25 kcal/mol (BP86). For the tetracarbonyl two triplet and two singlet cis ‐C 8 H 6 Fe 2 (CO) 4 structures lie within 7 kcal/mol of each other (BP86) with the triplet structures being of slightly lower energies. The singlet C 8 H 6 Fe 2 (CO) 4 structure is predicted to be thermodynamically unfavorable with respect to disproportionation into C 8 H 6 Fe 2 (CO) 5 + C 8 H 6 Fe 2 (CO) 3 .