pi-Allylchloro(NHC)nickel(II) complexes were synthesized and their reactions with O-2 were studied. Ligand steric effects were found to determine the difference between rapid oxidation of the allyl group to produce bis-mu-hydroxonickel complexes and no observable reaction. The ability of the metal-NHC bond to rotate correlates with the ability of the complex to react with O-2. In the limiting cases, conformationally restricted complexes are stable to O-2 and complexes with rapid Ni-NHC bond rotation react rapidly with O-2. Complexes with intermediate conformational flexibility were found to exhibit lesser reactivity with O-2. On the basis of the observed inertness of complexes with saddle-shaped ligands to O-2, we propose the adoption of a nonplanar geometry upon reaction with O-2 to be required. The issue of conformational flexibility versus rigidity is expected to directly impact the catalytic behavior of metal-NHC complexes.