Nitrosylation reactions are rare in the context of low valent Re(I)- and Tc(I)-tricarbonyl complexes so far. We herein describe a method for the conversion of a “M(CO) 3 -moiety” (M=Re, Tc) into a dicarbonyl-nitrosyl moiety “M(CO) 2 NO”, the synthesis of important precursor complexes and intermediates and possible applications for this new kind of Re- and Tc-chemistry.The behavior of the complex [ReCl 3 (CO) 2 (NO)] − in water was studied in detail and compared to that of [ReCl 3 (CO) 3 ] 2− . Contrary to the conversion of [ReCl 3 (CO) 3 ] 2− to the mixed aquo-carbonyl complex [Re(OH 2 ) 3 (CO) 3 ] + in water, one chloride remains initially bound to the metal center in the dicarbonyl-nitrosyl complex, making [ReCl(OH 2 ) 2 (CO) 2 (NO)] + the main species for further reactions. In this context, we isolated and characterized the complex [Re(μ 3 -O)(CO) 2 (NO)] 4 . Examples of complexes with different bi- and tridentate ligands based on ReCl 3 (CO) 2 (NO)] − are discussed.For the development of potential new radiopharmaceuticals we also adapted the nitrosylation technique to the n.c.a. level with 99m Tc. [ 99m Tc(OH 2 ) 3 (CO) 3 ] + served as starting material to form a 99m Tc(CO) 2 (NO)-core. Labelling reactions with ligands such as iminodiacetic acid (IDA), nitrilotriacetic acid (NTA) and diethylenetriamine pentaacetic acid (DTPA) were performed, resulting in the complexes [ 99m Tc(IDA)(CO) 2 (NO)], [ 99m Tc(NTA)(CO) 2 (NO)] and [ 99m Tc(DTPA)(CO) 2 (NO)]. In this way, the “nitrosyl-approach” adds a new and challenging synthetic tool to the already established organometallic chemistry of Re- and Tc-tricarbonyl complexes.