Protonation of C–M bonds and its microscopic reverse, metalation of C–H bonds, are fundamental steps in a variety of metal-catalyzed processes. As such, studies on protonation of C–M bonds can shed light on C–H activation. We present here studies on the rate of protodemetalation (PDM) of a suite of arylnickel(II) complex-es with various acids that provide evidence for a concerted, cyclic transition state for the PDM of C–Ni bonds and demonstrate that five, six, and seven-membered transition states are particularly favorable. Our data show that while the rate of protodemetalation of arylnickel(II) complexes scales with acidity for many acids, several are faster than predicted by pKa. For example, while acetic acid and acetohydroxamic acid are much less acid-ic than HCl, they both protodemetalate arylnickel(II) complexes significantly faster than HCl. Our data also shows how in the case of acetohydroxamic acid, a seven-membered cyclic transition state (CH3C(O)NHOH) can be more favorable than a six-membered transition state (CH3C(O)NHOH). Similarly, five-membered transition states, such as for pyrazole, are highly favorable as well. Comparison of transition-state polarization (from den-sity functional theory) compares these new nickel transition states to better-studied precious metal systems and demonstrates how the base can change the polarization of the transition state giving rise to opposing electron-ic preferences. Collectively, these studies suggest several new avenues for study in C–H activation as well as approaches to accelerate or slow protodemetalation in nickel catalysis.