A first ab initio density functional B3LYP/LACVP theoretical rationalization concerning the determination of the active sites of nickel(II) tetraazadinaphtho[14]annulene (NiN4CH3) and its protonated derivative (NiN(4)CH(3)H(2)trans,trans(2+)), when they are submitted to a reduction or an oxidation process, is presented. Condensed Fukui functions (f(k)(+) and f(k)(+)) using the approximations given by frozen core (fc) and finite difference (fd) are applied to determine the reductor and oxidant sites. The obtained results of f(k)(+) and f(k)(-) for the nonreduced NiN4CH3 and reduced NiN4CH3- species, respectively, predict the nickel atom and the four azomethynic carbon atoms as the most probable active sites, in agreement with experimental results using the pulse radiolysis technique. However, in the protonated structure a different result is found. The nickel atom appears as the unique active site, a reductor site in NiN(4)CH(3)H(2)trans,trans(2+) and an oxidant site in NiN(4)CH(3)H(2)trans, trans(+). We also found that both fc and fd predict the same trend, demonstrating that they are completely valid to be used in the subject of metal complexes and verifying the validity of the partitioning scheme of natural bond orbital used.