The reduction of (Et4N)[ReIIIBr4(CO)2] (1) by 0.5 equiv of tetrakis-dimethylaminoethylene in acetonitrile yields directly the air-stable, 17-electron ReII synthon (Et4N)2[ReIIBr4(CO)2] (2) in nearly quantitative yield. The versatility of 2 as a synthon for ReII chemistry was demonstrated by substitution reactions of [ReIIBr4(CO)2]2− with different mono-, bi-, and tridentate ligands. The resulting ReII complexes form highly crystalline compounds, and the solid state structures of the neutral trans−cis-[ReIIBr2(CO)2(X)n] species (where X = imidazole, pyridine, or phenanthroline) could be determined. All complexes are stable under aerobic conditions, both as solids and in solution, and showed fully reversible one-electron ReII → ReI reductions between ca. −70 and −120 mV. Carbonyl stretching frequencies (νCO) of this new family of ReII complexes are observed in the 1990 cm−1 (A1) and 1830 (Eg) cm−1 regions. With complex 2, a wide variety of fundamental but so far unknown ReII complexes become accessible via facile substitution reactions.