Abstract
High pressure 1H NMR spectroscopy on a mixture of [Pd(2-methallyl)Cl] 2 (dimer) and Pd(2-methallyl)Cl(PPh 3) (monomer) in CDCl 3 at 22°C gives Δ V ‡ = 0 ± 2 cm 3/mol. At this temperature syn- anti exchange of the allyl hydrogens in Pd(2-methallyl)Cl(PPh 3) occurs, catalyzed by the complex [Pd(2-methallyl)Cl] 2. To explain this Δ V ‡ value we propose an overall cancelling of opposite volume contributions from the initial partial bridge cleavage of dimer and its subsequent association with monomer. At 56° C Δ V ‡ is 11 ± 2 cm 3/mol. At this temperature, 2-methallyl exchange between Pd(2-methallyl)Cl(PPh 3) and [Pd(2-methallyl)Cl] 2 occurs by a process involving dimer cleavage followed by PPh 3 transfer. We propose the bridge cleavage of dimer to Pd(2-methallyl)Cl is the dominant step causing the overall positive Δ V ‡ change.
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