Dynamic Behavior of the Flyover Bridge Complexes M₂(CO)₅L {μ-[C(R)=C(R′)]₂CO} (M = Ru, Fe; L = CO, PPh₃, AsPh₃, SbPh₃; R, R′ = Me, Ph)

The variable-temperature ¹H and ¹³C NMR studies of a series of isomeric “flyover bridge” complexes M₂(CO)₅L{μ′)₂CO) (M = Ru, Fe; L = CO, PPh₃, AsPh₃, SbPh₃; R, R′ = Me, Ph) show that a fluxional process of the dienone ligand, leading to the exchange of σ- and π-bonds at the two metal centers is occurring in these compounds on the NMR time scale. The activation energy for this process in the entire series is essentially invariant (ΔG* = (70–76) ± 5 kJ/mol) with the nature of M and L and is apparently independent of localized carbonyl exchange at each M(CO)₃ unit.