The reduction of (Et4N)[(ReBr4)-Br-III(CO)(2)] (1) by 0.5 equiv of tetrakis-dimethylaminoethylene in acetonitrile yields directly the air-stable, 17-electron Re-II synthon (Et4N)(2)[(ReBr4)-Br-II(CO)(2)] (2) in nearly quantitative yield, The versatility of 2 as a synthon for Re-II chemistry was demonstrated by substitution reactions of [(ReBr4)-Br-II(CO)(2)](2-) with different mono-, bi-, and tridentate ligands. The resulting Re-II complexes form highly crystalline compounds, and the solid state structures of the neutral trans-cis-[(ReBr2)-Br-II(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 Re-II -> Re-I reductions between ca. -70 and -120 mV. Carbonyl stretching frequencies (nu(CO)) of this new family of Re-II complexes are observed in the 1990 cm(-1) (A(1)) and 1830 (E-g) cm(-1) regions. With complex 2, a wide variety of fundamental but so far unknown Re-II complexes become accessible via facile substitution reactions.