Coordination-Induced Weakening of a C(sp³)-H Bond: Homolytic and Heterolytic Bond Strength of a CH-Ni Agostic Interaction

The scission of a C(sp³)-H bond to form a new metal-alkyl bond is a fundamental step in coordination chemistry and catalysis. However, the extent of C-H bond weakening when this moiety interacts with a transition metal is poorly understood and quantifying this phenomenon could provide insights into designing more efficient C-H functionalization catalysts. We present a nickel complex with a robust adamantyl reporter ligand that enables the measurement of C-H acidity (pK_a) and bond dissociation free energy (BDFE) for a C(sp³)-H agostic interaction, showing a decrease in pK_aby dozens of orders of magnitude and BDFE decrease of about 30 kcal/mol upon coordination. X-ray crystallographic data is provided for all molecules, including a distorted square planar Ni^IIImetalloradical and "doubly agostic" Ni^II(κ²-CH₂) complex.