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 = NiII, PdII or PtII), 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.