Technetium (99Tc) mobility in the environment increases when 99Tc(IV) is combined with certain organic ligands under reducing environment. Therefore, it is important to detect and characterize 99Tc-organic ligand complex species to understand 99Tc mobility in various environmental conditions. Rhenium (Re) was used as a 99Tc surrogate with ethylenediaminetetracetic acid (EDTA) to produce Re–EDTA complexes, with perrhenate (ReO4−) reduced in the presence of EDTA with tin ion (Sn2+) in an anoxic glove box. Liquid–liquid and vapor–liquid diffusion methods were applied to isolate the Re complexes from the reaction mixture, and they were characterized using ultraviolet–visible, Raman, Fourier-transform infrared spectroscopy, and matrix-assisted laser desorption/ionization-time of flight mass spectroscopy. Bi-metallic base complex [(Sn)(EDTA)(ReO)(OH)(OH2)] was discovered, which transformed to a charge transfer (CT) complex, [CT complex1]=[(OH)2(H2O)2Re2(μ-O2)(EDTA)2·Sn2)] and [CT complex2]=[Re2(μ-O2)(EDTA)2Na4] over a period of 1 month. These CT complexes and a base complex were detected by matrix-assisted laser desorption/ionization-time of flight in black and yellow products, respectively. Oxidation states of +5 and +4 were assigned to Re in [CT complex1] and [CT complex2], respectively, based on Warner’s structure model. These CT complexes were neither broken apart to form ReO2(s) nor reversely transformed to form a base complex during a dissolution experiment, even in oxic conditions. These results suggest that the CT complexes of Re(IV/V) are highly stable and environmentally persistent with mobility higher than immobile 99Tc(IV) oxide.