The complex Ni-II(1, 8, -di-2-propenyl-1, 4,8,11-tetraazacyclotetradecane)(2+), (NiL1)2+, was synthesized. X-ray crystallography demonstrates that the complex obtained is the trans-III isomer. The allylic substituents shift the redox couples (NiL1)(3+/2+) and (NiL1)(2+/+) anodically relative to the corresponding couples for Ni-II(1,4,8,11-tetraazacyclotetradecane)(2+), (NiL2)(2+), as expected. Surprisingly, the lifetime of (NiL1)(+) in neutral aqueous solutions is shorter than that of (NiL2)(+). Pulse radiolysis experiments reveal that the allylic substituents are reduced by the central Ni(l) ion. The first step in this reduction is a general acid catalyzed process. The results suggest that this step involves schematically the reaction Ni(l)-NCH2CH=CH2+ + H+ --> Ni(III)-NCH2CH2CH22+. The latter transient decomposes slowly with a half-life time of several minutes. Preliminary results support the suggestion that (NiL2)(+), or other Ni(l)L complexes of this family, might reduce many alkenes present in the solution.