The synthesis of piano-stool type complexes [(Cp-R)Tc-99m(CO)(3)] from water and, if possible, directly from [(TcO4)-Tc-99m](-) is a persisting challenge. Such complexes are vary convenient for labeling biomolecules since they are small in size and "innocent" with respect to noncovalent interactions in biological systems. We found that [(Cp-R)Tc-99m(CO)(3)] can be prepared directly from the dimerized Diels-Alder precursors and from [(TcO4)-Tc-99m](-). Since the concentration of the respective monomers in the reaction solutions was undetectable, the formation of [(Cp-R)Tc-99m(CO)(3)] must be based on a metal-mediated retro Diels-Alder reaction with concerted Cp coordination. The authenticity of the formed Tc-99m complexes was confirmed by comparing their HPLC retention times with those of the structurally characterized rhenium surrogates. Besides the principle access to piano-stool complexes for radiopharmaceutical studies, a metal-mediated retro Diels-Alder reaction under concomitant formation of the corresponding piano-stool complex has, to our knowledge, not been observed before and might apply to other metal cations as well.