‘Ab initio’ calculations have been performed on the systems Ni(PH₃)₂L, with L = N₂, N₂CH₂, or N₂H₂, to investigate the nature and the energetics of the interaction between nickel and dinitrogen or related molecules. The results indicate that the bonding between the fragment Ni(PH₃)₂ and the unsaturated molecule can be explained by the Chatt–Dewar–Duncanson model, although π back donation is much more relevant than σ donation. Dinitrogen prefers to bind to the fragment Ni(PH₃)₂ in a side-on fashion rather than in an end-on one, since the former mode implies a more pronounced π back donation than the latter. The most efficient electron transfer from the metal to the dinitrogen, therefore, is achieved by side-on bonding and this point is particularly relevant to nitrogen fixation. Also diazomethane prefers to bind in a side-on mode because of the increased π back donation. This molecule gives rise to a stronger bond than dinitrogen with the fragment Ni(PH₃)₂ since it is a better π-acceptor ligand. The species N₂H₂ prefers to bind in a η²-di-imide form rather than in a σ-hydrazido(2–) one and this may account for the hydrogenation of side-on bonded dinitrogen, and provides some information on the problem of nitrogen reduction.