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 (pKa) 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 pKa 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 (ΔECTf) 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.