The preparation of substitution-inert 99Tc metal-fragments: Promising candidates for the design of new 99mTc radiopharmaceuticals

99Tc mixed-ligand complexes displaying characteristic substitution-inert metal-fragments are emerging as alternative platforms in the design of potential 99mTc radiopharmaceuticals. Starting from the already well-known [Tc(CO)3]+ moiety, this review describes recent efforts in the coordination chemistry of technetium aiming at the exploitation of the metal-fragment strategy. Examples include TcIII ‘4 + 1’ and TcIII ‘SSS’ systems, along with a description of the TcV(N)(PNP) system and the transfer of this technology to other cores (MNPh and MO).