A 100,000-Fold Increase in C-H Bond Acidity Gives Palladium a Key Advantage in C(sp)-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)-H bond acidity (p) and bond dissociation free energy (BDFE) for a species containing a ligated alkane-palladium interaction (RCH···Pd), also known as an agostic interaction. Through standard-state equilibrium measurements and advanced computational modeling, we show that Pd acidifies C(sp)-H bonds 100,000 times more than Ni (5 p units), indicating that acidification is a key factor making Pd a privileged metal in C(sp)-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 (Δ) 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.