Complexation of ^{99, 99m} Tc(CO) ₃ ⁺ organometallic fragment with 2 + 1 chelating systems was studied. HPLC analysis showed that the dithiocarbamate complexes of ^99m Tc are stronger than their xanthate, acetylacetonate, and γ- methoxycarbonylmethylacetylacetonate analogs. To block the third vacancy in the technetium coordination sphere, we studied thiols, thioethers, primary and secondary amines, hydroxyl ion, imidazole, phosphines, and isocyanides. As determined by ^99m Tc NMR spectroscopy, the mixed-ligand complexes Tc(CO)₃(DTC)L (DTC is dithiocarbamate and L is isocyanide, imidazole, and phosphine) are the most stable to the histidine challenge reaction. HPLC analysis showed that, among these 2 + 1 systems, only the DTC-isonitrile system forms a single complex with ^99m Tc(CO) ₃ ⁺ . The minimal dithiocarbamate-isocyanide concentration providing complete binding of ^99m Tc(CO) ₃ ⁺ is 10^-4 M.