The synthesis of piano-stool type complexes [(Cp-R)99mTc(CO)3] from water and, if possible, directly from [99mTcO4]- is a persisting challenge. Such complexes are very 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)99mTc(CO)3] can be prepared directly from the dimerized Diels−Alder precursors and from [99mTcO4]-. Since the concentration of the respective monomers in the reaction solutions was undetectable, the formation of [(Cp-R)99mTc(CO)3] must be based on a metal-mediated retro Diels−Alder reaction with concerted Cp coordination. The authenticity of the formed 99mTc 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.