syn-ReO(DL-ECH(3)) (1) (DL-ECH(3) = trianionic form of ethylenedi-DL-cysteine) is the first M=O(N2S2) complex (M=Re, Tc-99) found by X-ray methods to have an NOS2 donor set in the basal plane. 1 has a distorted geometry with a relatively short Re-N bond trans to the oxo ligand. M=O(N2S2) complexes are an important class of radiopharmaceuticals and are normally five-coordinate with an N2S2 donor set in the basal plane. However, a few examples of six coordinate M=O(N2S2) complexes are known in which a pendant carboxyl group anti to the oxo ligand is weakly bound to the metal trans to the oxo ligand. In all previous examples with pendant syn carboxyl groups, the carboxyl group was not coordinated but was dangling. In future, the potential coordination of a syn carboxyl group must be considered in the design of radiopharmaceuticals.