The preparation of new tricarbonyl(cyclopentadienyl)rhenium and tricarbonyl(cyclopentadienyl)technetium derivatives is described. The approach used was applied to a cold target model, 17alpha-ethynylestradiol substituted at the 17alpha position by a ketotricarbonyl(cyclopentadienyl)rhenium group. The novel organometallic reaction applied here, when extended to potential radiopharmaceuticals, consists of a cyclopentadienyl-ligand transfer between ketoferrocenes and fac-Tc-99m(H2O)(3)(CO)(3)](+) in a water/DMSO 1:1 mixture. This reaction lead to the synthesis of a new estradiol derivative, labeled with Tc-99m. The synthesis of another product was performed by using (TcCl3)-Tc-99(CO)(3)](2-) as the Tc precursor and its X-ray crystal structure was determined. In each case high yields were obtained. (C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004.