The chemical and biological evaluation of organometallic probes for nuclear medical applications in comparison to their purely organic analogs, or the study of the metal-mediated retro-Diels-Alder behaviour of derivatized cyclopentadienyl dimers is an overarching description of this thesis. Focussing on the synthesis of 99mTc- labelled radiopharmaceuticals and the corresponding cold rhenium-compounds, a variety of organic and inorganic cyclopentadienyl derivatives have been synthesized. Arylsulfonamides, -sulfamides and -sulfamates for the targeting of human carbonic anhydrase IX, hydroxamic acids acting as HDAC-inhibitors, organometallic antibiotics or catechol amine derivatives have been studied. Organometallic complexes with bioactive ligands are nowadays interesting for both, the noninvasive imaging of biological features and the therapeutic treatment of diseases. While several transition metals across the periodic table are frequently used for therapy, 99mTc is the most prominent nuclide in nuclear medical diagnostics. It would be desirable, in a theragnostics sense (therapy and diagnostics), to have identical compounds for combined therapy and non-invasive diagnosis. Among transition-metals, rhenium and technetium belong to the same triad. Therefore it is possible to use identical compounds for combined therapy and imaging. While Re- based compounds can be used for therapy, the homologous compounds with 99mTc can serve as imaging agents for Single Photon Emission Computed Tomography (SPECT). [(CpR)M(CO)3]-type compounds are considered as very stable complexes under various conditions and can synthetically be treated like aromatic organic molecules. Re-compounds are therefore usually synthesized starting from [(Cp)Re(CO)3] and using standard organic synthesis procedures, whereas for the corresponding 99mTc- complexes, the uncoordinated HCp-ligand is prepared prior to complexation, either as a monomer or in its dimeric form.