Coordination-Induced Weakening of a C(sp 3 )-H Bond: Homolytic and Heterolytic Bond Strength of a CH-Ni Agostic Interaction.

The scission of a C(sp 3 )-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 (p K a ) and bond dissociation free energy (BDFE) for a C(sp 3 )-H agostic interaction, showing a decrease in p K a by 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 III metalloradical and "doubly agostic" Ni II (κ 2 -CH 2 ) complex.