Binuclear heterometallic bonding between a first row transition metal and a second row transition metal: The cyclopentadienyliron molybdenum carbonyls Cp2FeMo(CO)n (n = 6, 5, 4, 3, 2).

The geometries and energetics of Cp 2 FeMo(CO) n (n = 6, 5, 4, 3, 2; Cp = η5-C 5 H 5), including the experimentally known pentacarbonyl Cp 2 FeMo(CO) 5 , have been examined by density functional theory. The lowest energy Cp 2 FeMo(CO) 5 isomer is the experimentally known unbridged structure but an doubly bridged isomer lies within ∼2 kcal/mol of energy. The tricarbonyl Cp 2 FeMo(CO) 3 has an energetically favorable triply bridged structure with a formal Fe≡Mo triple bond analogous to the experimental valence isoelectronic Cp* 2 M 2 (µ-CO) 3 (M = Mn, Re; Cp* = η5-Me 5 C 5) structures. • The lowest energy Cp 2 FeMo(CO) 5 structure is the experimentally known unbridged structure. • A doubly bridged Cp 2 FeMo(CO) 3 (µ-CO) 2 is of comparable energy to its unbridged isomer. • The low-energy doubly bridged Cp 2 FeMo(CO) 2 (µ-CO) 2 structures are disfavored relative to disproportionation. • The highly favored lowest energy Cp 2 FeMo(CO) 3 structure is a triply bridged structure with an Fe≡Mo triple bond. • The lowest energy Cp 2 FeMo(µ-CO) 2 are doubly bridged singlet and triplet structures with Fe-Mo quadruple and triple bonds, respectively. The geometries and energetics of the heterometallic binuclear cyclopentadienyl iron-molybdenum carbonyls Cp 2 FeMo(CO) n (n = 6, 5, 4, 3, 2; Cp = η5-C 5 H 5), including the experimentally known pentacarbonyl Cp 2 FeMo(CO) 5 have been examined by density functional theory. The lowest energy structure for the pentacarbonyl Cp 2 FeMo(CO) 5 by the mPWPW19 method corresponds to the unbridged Cp 2 FeMo(CO) 5 isomer related to the experimental Cp 2 Mo 2 (CO) 6 structure with exclusively terminal carbonyl groups. However, the doubly bridged structure, which is related to the experimental trans - and cis -Cp 2 Fe 2 (CO) 2 (µ-CO) 2 structures of the homonuclear diiron derivative, lies only 1.4 kcal/mol in energy above the unbridged structure. The relative energies of the unbridged and doubly bridged Cp 2 FeMo(CO) 5 are reversed using the BP86 method with the doubly bridged structure lying 2.0 kcal/mol below the unbridged structure. The closeness in energy of these two structures suggests a fluxional system for Cp 2 FeMo(CO) 5. The low-energy structures for the tetracarbonyl Cp 2 FeMo(CO) 4 are doubly bridged structures with formal Fe=Mo double bonds in accord with the 18-electron rule. However, Cp 2 FeMo(CO) 4 is predicted not to be viable since its disproportionation into Cp 2 FeMo(CO) 5 + Cp 2 FeMo(CO) 3 is found to be highly exothermic. The tricarbonyl Cp 2 FeMo(CO) 3 has an energetically favorable triply bridged structure with a formal Fe≡Mo triple bond analogous to the experimental valence isoelectronic Cp* 2 M 2 (µ-CO) 3 (M = Mn, Re; Cp* = η5-Me 5 C 5) structures. The low-energy structures of the dicarbonyl Cp 2 FeMo(CO) 2 are doubly bridged structures which can be either singlets with an Fe Mo quadruple bond or triplets with an Fe≡Mo triple bond. The low-energy structures of the hexacarbonyl Cp 2 FeMo(CO) 6 can be interpreted as consisting of [CpFe(CO) 3 ]+ cations linked to [CpMo(CO) 3 ]− anions through one of the iron-bonded carbonyl groups with long ∼4 Å Fe⋯Mo distances indicating the absence of an iron-molybdenum bond. [ABSTRACT FROM AUTHOR]