The reaction of [NEt 4 ] 2 [Re(CO) 3 Br 3 ] with equimolar amount of a tridentate NSO ligand in methanol leads to the formation of neutral tricarbonyl rhenium(I) complexes of the general formula Re(CO) 3 (NSO), where the NSO ligand is o-C 5 H 4 N-CH 2 CH 2 -S-CH 2 CH(NHCOCH 3 )COOH (L 1 H), complex 1 or o-C 5 H 4 N-CH 2 CH 2 -S-C(CH 3 ) 2 CH(NHCOCH 3 )COOH (L 2 H), complex 2. Both complexes have been characterized by elemental analysis and spectroscopic methods, while complex 2 has also been characterized by X-rays analysis. At technetium-99m level, the corresponding fac-[ 99m Tc(CO) 3 (NSO)] complexes 3 and 4, were obtained in high yield by reacting ligands L 1 H or L 2 H with the fac-[ 99m Tc(CO) 3 (H 2 O) 3 ] + precursor in water. Their structure was established by chromatographic comparison to the prototype rhenium complex using high-performance liquid chromatographic techniques.