Energetics of Variable Hapticity of Carbocyclic Rings in Cyclopentadienylmetal Carbonyl Systems of the Second Row Transition Metals C₅H₅M(CO)_nC_mH_m (M = Ru, Tc, Mo, Nb) Including Mechanistic Studies of Carbonyl Dissociation

Decarbonylation of the experimentally known CpRu­(CO)₂(η¹-C₅H₅), CpMo­(CO)₂(η³-C₇H₇), and CpNb­(CO)₂(η⁴-C₈H₈) (Cp = η⁵-C₅H₅), each with uncomplexed 1,3-butadiene units in the C_nH_n ring, as well as the related CpTc­(CO)₂(η²-C₆H₆), to give the corresponding carbonyl-free derivatives CpM­(ηⁿ-C_nH_n) derivatives has been studied by density functional theory. For ruthenium, technetium, and molybdenum the coordinated C_nH_n ring of the intermediate monocarbonyl CpM­(CO)­(η^n–2-C_nH_n) contains an uncomplexed CC double bond and each decarbonylation step proceeds with a significant energy barrier represented by a higher energy transition state. However, decarbonylation of CpNb­(CO)₂(η⁴-C₈H₈) to the monocarbonyl proceeds without an energy barrier, preserving the tetrahapto coordination of the C₈H₈ ring to give CpNb­(CO)­(η⁴-C₈H₈) in which the niobium atom has only a 16-electron configuration. All of the monocarbonyl derivatives CpM­(CO)­(C_nH_n) are predicted to be strongly energetically disfavored with respect to disproportionation to give CpM­(CO)₂(C_nH_n) + CpM­(C_nH_n). This allows us to understand the failure to date to synthesize any of the monocarbonyl derivatives.