To achieve a net-neutral coordination unit in radiopharmaceuticals with a fac-M(CO)(3+) core (M = Tc, Re), facially coordinated monoanionic tridentate ligands are needed. New neutral fac-Re(CO)(3)L complexes were obtained by treating fac-[Re(CO)(3)(H2O)(3)](+) with unsymmetrical tridentate NNN donor ligands (LH) based primarily on a diethylenetriamine (dien) moiety with an aromatic group linked to a terminal nitrogen through a sulfonamide. LHs contain 2,4,6-trimethylbenzenesulfonyl (tmbSO(2)) and 5-(dimethylamino)naphthalene-1-sulfonyl (DNS) groups. X-ray crystallographic and NMR analyses confirm that in both the solid and the solution states all L- in fac-Re(CO)(3)L complexes are bound in a tridentate fashion with one donor being nitrogen from a deprotonated sulfonamiclo group. Another fundamental property that is important in radiopharmaceuticals is shape, which in turn depends on ring pucker. For L- = tmbSO(2)-dien-, tmbSO(2)-Ar-Medien(-), and tmbSO(2)-NN-Me(2)dien(-), the two chelate rings have a different pucker chirality, as is commonly found for a broad range of metal complexes. However, for fac-Re(CO)ONS-clien), both chelate rings have the same pucker chirality because the suffonamido ring has an unusual pucker for the absolute configuration at Re; a finding that is attributable to intramolecular and intermolecular hydrogen bonds from the sulfonamido oxygens to the NH2 groups. Averaging of tmb NMR signals, even at -90 degrees C for Re(CO)(3)(tmbSO(2)-N,N-Me(2)dien), indicates rapid dynamic motion in the complexes with this group. However, examination of the structures suggests that free rotation about the S-C(tmb) bond is not possible but that concerted coupled rotations about the N-S and the S-C bonds can explain the NMR data.