Ab initio RHF and CI calculations have been carried out on the [H3VN2VH3(mu-Na)](-), [H3VN2VH3](-), [H3VN2VH3], and [H3VN2VH3](2-) complexes and related fragments in order to study the energetics and the mechanism of dinuclear dinitrogen activation by vanadium(II) and anadium(III). Analogous calculations have been performed also on the [H3VN2], [H3VN2](-), and [H3VN2Na] complexes to compare mononuclear and dinuclear activation. Various energy gradient optimizations have been performed on the complexes and the various fragments, followed by MRCI calculations on the stationary structures. The results indicate that in the dinuclear complexes dinitrogen is poorly to strongly activated through mechanisms which depend on the presence of the alkali atom, the net charge, and the spin multiplicity of the considered state, while in the mononuclear complexes dinitrogen is always poorly activated.