Highly Unsaturated Binuclear Butadiene Iron Carbonyls: Quintet Spin States Perpendicular Structures Agostic Hydrogen Atoms and Iron-Iron Multiple Bonds

摘要

The highly unsaturated binuclear butadiene iron carbonyls (C4H6)2Fe2(CO)n (n = 2, 1) have been examined using density functional theory. For (C4H6)2Fe2(CO)n (n = 2, 1), both coaxial and perpendicular structures are found. The global minima of (C4H6)2Fe2(CO)n (n = 2, 1) are the perpendicular structures >2Q-1 and >1Q-1, respectively, with 17- and 15-electron configurations for the iron atoms leading to quintet spin states. The Fe=Fe distance of 2.361 Å (M06-L) in the (C4H₆)₂Fe₂(CO)₂ structure >2Q-1 suggests a formal double bond. The Fe≡Fe bond distance in the (C₄H₆)₂Fe₂(CO) structure >1Q-1 is even shorter at 2.273 Å (M06-L), suggesting a triple bond. Higher energy (C₄H₆)₂Fe₂(CO)_n (n = 2, 1) structures include structures in which a bridging butadiene ligand is bonded to one of the iron atoms as a tetrahapto ligand and to the other iron atom through two agostic hydrogen atoms from the end CH₂ groups. Singlet (C₄H₆)₂Fe₂(CO) structures with formal Fe–Fe quadruple bonds of lengths ∼2.05 Å were also found but at very high energies (∼47 kcal/mol) relative to the global minimum.