Ab initio and density functional theory (DFT) methods have been applied to study the structures and kinetic stabilities of the possible products of the reactions of mononuclear nickel with (N-2)(x) (x = 1-4). Energy analyses show that end-on bound Ni(N-2)(x) (x = 1-4) complexes are preferred to side-on and N-4 bound ones. Several decomposition and isomerization pathways for Ni(N-2)(x) (x = 2-4) were investigated at the B3LYP/6-31G* level of theory. The present study suggests that besides the four experimentally assigned complexes (NiN2 (C-infinity v), Ni(N-2)(2) (D-infinity h), Ni(N-3)(2) (D-3h), and Ni(N-2)(4) (T-d)), another two complexes (Ni(N-2)(4) (C-2v) and Ni(N-2)(4) (D-4d)) are likely to be kinetically stable, while other complexes may be kinetically unstable with barrier heights of less than 30 kcal/mol. The present study also suggests that side-on bound N-2 ligand is ready to transform into the end-on bound one, while N-4 ligand is hard to transform into side-on or end-on bound N-2 ligand.