Unsaturation in binuclear heterometallic carbonyls: The cyclopentadienyliron manganese carbonyl CpFeMn(CO)_:Nsystem as a hybrid of the Cp₂Fe₂(CO)_nand Mn₂(CO)_nsystems

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