Metallocene versus Metallabenzene Isomers of Nickel, Palladium, and Platinum

The relative energies of singlet and triplet metallocene, metallabenzene, and metallacyclopentadiene C10H10M isomers (M = Ni, Pd, Pt) have been examined using density functional theory. For the C10H10Ni system, the experimentally known triplet nickelocene (η5-Cp)2Ni is the lowest energy isomer by ∼17 kcal/mol with respect to singlet nickelocene. For the C10H10Pd system, the triplet and singlet palladocene structures have similar energies within ∼2 kcal/mol. However, the singlet palladocene has a “slipped ring” (η3-Cp)2Pd structure with two trihapto Cp rings. The C10H10Pt system is different since the platinabenzene CpPtC5H5 isomer is the lowest energy structure. This is in accord with the synthesis of stable substituted CpPtC5H3R2 platinabenzenes by Haley and co-workers [Haley, M. M.; et al. Organometallics 2004, 23, 1174]. However, the slipped ring singlet platinocene (η3-Cp)2Pt to the isomeric platinocene lies only ∼2 kcal/mol above the platinabenzene global minimum, so the energy barrier for conversion of the platinabenzene must be substantial. The following general observations can be made regarding the relative stabilities of isomeric C10H10M (M = Ni, Pd, Pt) structures: (1) Triplet structures become less favorable energetically than isomeric singlet structures in the sequence Ni 3-Cp rather than pentahapto η5-Cp rings leading ultimately to 16- rather than 18-electron metal configurations become increasingly favorable energetically in the sequence Ni 5-Cp)­MC5H5 structures with pentahapto Cp rings are always more favorable energetically than isomeric metallacyclopentadiene (η6-C6H6)­MC4H4 structures with hexahapto benzene rings.