Unsaturation in binuclear heterometallic carbonyls: the cyclopentadienyliron manganese carbonyl CpFeMn(CO)n system as a hybrid of the Cp2Fe2(CO)n and Mn2(CO)n systems

Abstract

The experimentally known CpFeMn(CO)₇ system as well as the related unsaturated CpFeMn(CO)_n systems (n = 6, 5) have been investigated by density functional theory. For CpFeMn(CO)₇ unbridged and doubly bridged structures with a heteronuclear Fe–Mn single bond lie within ∼5 kcal mol⁻¹ of each other with the experimentally known unbridged structure being the lower energy structure. The three lowest-energy unsaturated CpFeMn(CO)₆ structures, lying within ∼1 kcal mol⁻¹ of each other, have very diverse structural features. The lowest-energy CpFeMn(CO)₆ structure contains an unusual three-center two-electron C–H–Mn bond involving an agostic hydrogen of the Cp ring. Another such structure is a triply bridged triplet CpFe(μ-CO)₃Mn(CO)₃ closely related to the experimentally known Cp₂Fe₂(μ-CO)₃ having a central FeMn double bond containing the two unpaired electrons of the triplet spin state. The third low-energy CpFeMn(CO)₆ structure is a CpFe(CO)Mn(CO)₄(η²-μ-CO) structure with a four-electron donor bridging η²-μ-CO group similar to that found in the lowest energy Mn₂(CO)₉ structure. The lowest-energy structure of the even more unsaturated CpFeMn(CO)₅ has a short FeMn distance of only ∼2.2 Å suggesting a formal triple bond. Higher energy CpFeMn(CO)₅ structures have a four-electron donor η²-μ-CO group in addition to a formal FeMn double bond. The CpFeMn(CO)₅ structure in which the oxygen atom of the η²-μ-CO group is bonded to manganese lies ∼9 kcal mol⁻¹ in energy below the isomeric structure in which the oxygen of the η²-μ-CO group is bonded to iron.