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)(2)(eta(1)-C5H5), CpMo(CO)(2)(eta(3)-C7H7), and CpNb(CO)(2)(eta(4)-C8H8) (Cp = eta(5)-C5H5), each with uncomplexed 1,3-butadiene units in the CnHn ring, as well as the related CpTc(CO)(2)(eta(2)-C6H6), to give the corresponding carbonyl-free derivatives CpM(eta(n)-CnHn) derivatives has been studied by density functional theory. For ruthenium, technetium, and molybdenum the coordinated CnHn ring of the intermediate monocarbonyl CpM(CO)(eta(n-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)(2)(eta(4)-C8H8) to the monocarbonyl proceeds without an energy barrier, preserving the tetrahapto coordination of the C8H8 ring to give CpNb(CO)(eta(4)-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.