Technetium is a second-row group-VII transition metal, with an electron configuration of [Kr]5s24d5. The chemistry exhibited by Tc is vast as a result of the fact that it can occupy several different oxidation states (-1 to +7, although only +1 through +7 can exist in solution) and the large number of coordination geometries it can adopt . These two factors mean that Tc can bind to a large number of donor ligands. However, this flexibility can cause problems in the design of Tc radiopharmaceuticals, as well as the in vivo biodistribution of Tc coordination compounds. Technetium is a group-VII congener of rhenium, and as a result their chemistries are similar. The only major difference is that the redox potential of analogous complexes can differ significantly, with Tc complexes being more easily reduced. Due to their chemical similarities, rhenium has often been used as an alternative to technetium in preliminary radiopharmaceutical investigations.
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