Trends within a triad: comparison between sigma-alkyl complexes of nickel, palladium and platinum with respect to association of ethylene, migratory insertion and beta-hydride elimination. A theoretical study

Density functional B3LYP calculations have been performed for (diimine)(sigma-methyl)metal(1+) complexes (M = Ni-II, Pd-II or Pt-II), associating ethylene to afford (diimine)(eta(2)-ethylene)(sigma-methyl)metal(1+). All three metals co-ordinate ethylene strongly in the expected order Pt (41.5) > Pd (29.8) > Ni (27.2 kcal mol(-1)). The co-ordination energies for the corresponding sigma-ethyl complexes of Pd and Ni are substantially lower, Pd (16.3) and Ni (10.0 kcal mol(-1)). This is due to loss of a beta-agostic interaction, which in the palladium case is estimated to represent around 10 kcal mol(-1) and in the case of Ni to around 12 to 14 kcal mol(-1). The insertion barriers for the cationic sigma-alkyl eta(2)-alkene complexes are in the order Pt (25.5) > Pd (sigma-methyl, 16.4; sigma-ethyl, 18.0) > Ni (sigma-methyl, 10.4; sigma-ethyl, 11.3 kcal mol(-1)). The insertion step is exothermic for Ni and Pd but slightly endothermic for Pt. For three-co-ordinated (diimine)(sigma-propyl)metal(1+) complexes, beta-hydride elimination is exothermic for Pt (-6.9) and endothermic for Ni (+11.0 kcal mol(-1)). The rather low endothermicity to beta-hydride elimination of Pd (4.8 kcal mol(-1)) is consistent with (diimine)(sigma-methyl)palladium(1+) being a polymerization catalyst promoting branched polyethylene. The termination for a Pd-catalysed polymerization of ethylene is discussed, and a direct beta-hydride elimination from a four-co-ordinated (sigma-alkyl)(diimine)(eta(2)-ethylene)palladium(1+) is excluded due to a barrier of 24.3 kcal mol(-1). In all, the calculations agree remarkably well with known energetics and recognized tendencies.