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

Abstract

Density functional B3LYP calculations have been performed for (diimine)(σ-methyl)metal(1+) complexes (M = Ni^II, Pd^II or Pt^II), associating ethylene to afford (diimine)(η²-ethylene)(σ-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 σ-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 β-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 σ-alkyl η²-alkene complexes are in the order Pt (25.5) > Pd (σ-methyl, 16.4; σ-ethyl, 18.0) > Ni (σ-methyl, 10.4; σ-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)(σ-propyl)metal(1+) complexes, β-hydride elimination is exothermic for Pt (–6.9) and endothermic for Ni (+11.0 kcal mol^–1). The rather low endothermicity to β-hydride elimination of Pd (4.8 kcal mol^–1) is consistent with (diimine)(σ-methyl)palladium(1+) being a polymerization catalyst promoting branched polyethylene. The termination for a Pd-catalysed polymerization of ethylene is discussed, and a direct β-hydride elimination from a four-co-ordinated (σ-alkyl)(diimine)(η²-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.

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