A Million-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( sp ³ )-H bond acidity (p K _a ) and bond dissociation free energy (BDFE) for a species containing a ligated alkane-palladium interaction (R ₂ CH ₂ ---Pd), also known as an agostic interaction. Through standard-state equilibrium measurements and advanced computational modeling, we show that Pd acidifies C( sp ³ )-H bonds a million times more than Ni (6 p K _a units), indicating that acidification is a key factor making Pd a privileged metal in C( sp ³ )-H functionalization reactions. More broadly, these valuable findings help unravel fundamental performance differences between Earth-abundant and precious metals, potentially guiding future ligand design efforts for catalysis.