Dinitrogen ruthenium complexes were theoretically studied by means of DFT technique. Several isomers of RuH 2 (N 2 )(PH 3 ) 2 , RuH 2 (N 2 ) 2 (PH 3 ) 2 and RuH 2 (H 2 )(N 2 )(PH 3 ) 2 were studied. Calculations of relative energies, geometrical parameters, vibrational frequencies and natural orbital bond analysis were performed. It is shown that the most stable isomer for each series is characterized by a trans position of the phosphines with the dinitrogen ligand trans to one hydride. As usually observed, the dinitrogen moiety adopts an end-on bonding mode and is weakly elongated from free N 2 (generally not >1%). As shown by NBO analysis, such a bonding mode involves σ-donation of about 0.2 electron from the lone pair orbital of the ruthenium-bound nitrogen toward ruthenium and back-donation of roughly 0.2 electron from the 4d occupied orbital of ruthenium to the two π g * dinitrogen orbitals.