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

Decarbonylation of the experimentally known CpRu­(CO)21-C5H5), CpMo­(CO)23-C7H7), and CpNb­(CO)24-C8H8) (Cp = η5-C5H5), each with uncomplexed 1,3-butadiene units in the CnHn ring, as well as the related CpTc­(CO)22-C6H6), to give the corresponding carbonyl-free derivatives CpM­(ηn-CnHn) derivatives has been studied by density functional theory. For ruthenium, technetium, and molybdenum the coordinated CnHn ring of the intermediate monocarbonyl CpM­(CO)­(ηn–2-CnHn) 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)24-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)­(CnHn) are predicted to be strongly energetically disfavored with respect to disproportionation to give CpM­(CO)2(CnHn) + CpM­(CnHn). This allows us to understand the failure to date to synthesize any of the monocarbonyl derivatives.