Metallocene versus Metallabenzene Isomers of Nickel, Palladium, and Platinum

The relative energies of singlet and triplet metallocene, metallabenzene, and metallacyclopentadiene C 10 H 10 M isomers (M = Ni, Pd, Pt) have been examined using density functional theory. For the C 10 H 10 Ni system, the experimentally known triplet nickelocene (η 5 -Cp) 2 Ni is the lowest energy isomer by ∼17 kcal/mol with respect to singlet nickelocene. For the C 10 H 10 Pd system, the triplet and singlet palladocene structures have similar energies within ∼2 kcal/mol. However, the singlet palladocene has a “slipped ring” (η 3 -Cp) 2 Pd structure with two trihapto Cp rings. The C 10 H 10 Pt system is different since the platinabenzene CpPtC 5 H 5 isomer is the lowest energy structure. This is in accord with the synthesis of stable substituted CpPtC 5 H 3 R 2 platinabenzenes by Haley and co-workers [ Haley, M. M.; et al. Organometallics 2004, 23, 1174]. However, the slipped ring singlet platinocene (η 3 -Cp) 2 Pt 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 C 10 H 10 M (M = Ni, Pd, Pt) structures: (1) Triplet structures become less favorable energetically than isomeric singlet structures in the sequence Ni < Pd < Pt. (2) Slipped metallocene structures with trihapto η 3 -Cp rather than pentahapto η 5 -Cp rings leading ultimately to 16- rather than 18-electron metal configurations become increasingly favorable energetically in the sequence Ni < Pd < Pt. (3) Metallabenzene (η 5 -Cp)MC 5 H 5 structures with pentahapto Cp rings are always more favorable energetically than isomeric metallacyclopentadiene (η 6 -C 6 H 6 )MC 4 H 4 structures with hexahapto benzene rings.