A 100,000-Fold Increase in C-H Bond Acidity Gives Palladium a Key Advantage in C(sp³)-H Activation Compared to Nickel

Carbon-hydrogen bond activation is a pillar of synthetic chemistry. While it is generally accepted that Pd is more facile than Ni in C-H activation catalysis, there are no experimental platforms available to directly compare the magnitude of C-H bond weakening between Ni and Pd prior to bond scission. This work presents the first direct measurements of C(sp³)-H bond acidity (pK_a) and bond dissociation free energy (BDFE) for a species containing a ligated alkane-palladium interaction (R₂CH₂···Pd), also known as an agostic interaction. Through standard-state equilibrium measurements and advanced computational modeling, we show that Pd acidifies C(sp³)-H bonds 100,000 times more than Ni (5 pK_a units), indicating that acidification is a key factor making Pd a privileged metal in C(sp³)-H functionalization reactions. Energy decomposition analysis (EDA) calculations show that this is primarily due to a greater electrophilicity of the palladium containing fragment, as forward charge transfer (ΔE_CTf) from the agostic methylene moiety into [Pd] is significantly increased. More broadly, these valuable findings help unravel fundamental performance differences between Earth-abundant and precious metals, potentially guiding future ligand design efforts for catalysis.