Rhenium(V) complexes: From pure hard donors to mixed soft/hard functionalized phosphine ligands

A series of rhenium(V) complexes holding a distinctive core (mostly oxo, but also imido and nitrido), synthesized by our research groups in the last six years with the aim to increase knowledge on basic coordination chemistry of this metal and related technetium congener, is reviewed. The presence of typical cores confers a strong acidic character to the related moieties, which readily accept electron density from various nucleophilic ligands. Pure hard donors such as amine N and/or alcoholate O, as well as mixtures of hard and soft donors, including tertiary phosphines, are able to stabilize oxo-Re(V) complexes in distorted octahedral environments. Hetero-functionalized phosphines, which combine in the same framework the P(III) atom and suitable hard anchoring groups, possess enhanced coordination ability, and efficiently stabilize six-coordinated oxo-, nitrido- and imido-Re(V) complexes as well. The synthesis and complete characterization of these complexes offer an useful tool for designing potential Re and Tc radiopharmaceuticals.