A computational and conceptual density-functional study has been performed on various [3 + 1] complexes of both Re(V) and Tc(V). The fully optimized complexes chloro(3-thiapentane-1,5-dithiolato)oxorhenium(V) and chloro(3-thiapentane-1,5-dithiolato)oxotechnetium(V) show geometries that compare favorably with the X-ray data. These structures were used as a starting point to investigate the relative stability of Tc(V) and Re(V) complexes with various ligands containing combinations of N, O, and S as chelating atoms and to evaluate the stabilizing/destabilizing influence of these N, O, and S combinations. For both Tc and Re complexes, the S content (number and position of S atoms) together with the presence of an oxygen as the central chelating atom turns out to be decisive in the stability of the tridentate complexes, the latter factor being strongly destabilizing and the former stabilizing. The stabilization sequences for both Tc and Re are shown to be identical in the gas phase and in aqueous solutions treated in a polarizable continuum model. The Re(V) complexes are found to be more stable than their Tc(V) analogues. All of the results are successfully interpreted in terms of the hard and soft acids and bases principle, applied at the local level. For this purpose, a softness value for Tc is obtained by interpolating softness trends in neighboring elements of rows 5 and 6 in the periodic table.