We report a mechanistic study of C(sp3) -H bond activation mediated by nickel. A cyclometalated Ni(II) ureate [(PEt3)Ni(κ3 -C,N,N -(CH2)N(Cy)(CO)N((N) -quinolin -8-yl))] was synthesized and isolated from a urea precursor, (Me)(Cy)N(CO)N(H)(quinolin -8-yl), via C(sp3) -H activation. We investigated the effects of solvents and base additives on the rate of C -H activation. Kinetic isotope effect experiments showed that C -H activation is rate determining. Through deuterium label ling and protonation studies, we also showed that C -H activation can be re versible. We extended this reac tion to a range of ureas with primary and secondary C(sp3) -H bonds, which activate readily to for m analogous nickelated products . Finally, we showed that carboxylate additives assist with both ligand dissociation and initial N-H bond activation, consistent with a concerted metalation deprotonation mechanism.