An inhibitory role of 1,5-cyclooctadiene (COD) in nickel-catalyzed,C-H functionalization processes as identified and studied. The bound COD participates in C-H activation by capturing the hydride, leading to a stable off-cycle pi-allyl complex that greatly diminished overall catalytic efficiency. Computational studies elucidated the origin of the effect and enabled identification of a 1,5-hexadiene-derived pre-catalyst that avoids the off-cycle intermediate and provides catalytic efficiencies that are superior to those of catalysts derived from Ni(COD)(2).