The highly reactive mixed anhydrides [TcO3(OCOPh)] and [TcO3(OBF3)](-) were synthesized by treatment of [TcO4](-) with strong Lewis acids benzoyl chloride and BF3 center dot OEt2. These mixed anhydrides, prepared in situ, were used as precursors for the synthesis of complexes containing the [TcO3](+) core. Subsequent reactions with bi-or tridentate ligands resulted in new complexes comprised of the [TcO3](+) core. As examples with bidentate ligands, the classical complexes [TcO3Cl(bipy)] (1) (bipy = 2,2'-bipyridine) and [TcO3Cl(phen)] (2) (phen = 1,10-phenanthroline) have been prepared by this strategy and structurally characterized. The new compounds [TcO3(bpza)] (3) (bpza = di-1 H-pyrazol-1-ylacetate), [TcO3(bpza*)] (4) (bpza* = bis(3,5-dimethyl-1 H-pyrazol-1-yl)acetate), [TcO3(tPZM*)](+) (6) (tpzm* = 1,1,1 -methanetriyltris(3,5-dimethyl-1 H-pyrazole), and [ReO3(tPZm*)][ReO4] (7) are examples of complexes with tripod ligands. The complexes have been structurally characterized, and their Tc-99 NMR spectra have been recorded. As a common feature, the X-ray structures show a distinct widening of the O-Tc-O angles, almost to a tetrahedral angle. With the perspective of radiopharmaceutical applications, water stability and reactivities toward alkenes are described.