According to quantum chemical calculations, nitrido chlorides constituted from d(6)transition metals form ring structures which are best described as weak donor-acceptor complexes between the lone pair at nitrogen and the empty d orbitals at the transition metal. The stabilities of the ring structures increase in the order Cr < Mo < W. The transition metal Cr forms only endothermic ring systems, and these in turn decompose into monomers. A comparison of several homologous oligomers of NMoCl3 shows maximum stability for the tetramer, in agreement with the experimental findings. This particular stability of the tetramer is due to a perfect alignment of the nitrogens between two metal centers. The vibrational analyses reveal that the character of the monomers is nearly conserved in the ring systems. An analysis of the electron densities on the basis of Ender's method of atoms in molecules as well as of the natural bond orbitals (NBO) substantiates the weak ring stabilities.