The chemistry of [(tacn)-N-CO-Re-III(CO)(2)Br]X (X = Cl or Br), obtained in good yield from the reaction of fac-[(tacn) Re-I(CO)(3)]Br (1, tacn = 1,4,7-triazacyclononane) with X-2 in water, is described. The [(tacn)-N-CO-Re-III(CO)(2)Br]X complex (2 with X = Br-; 2a with X = BrCl2-), which we have previously communicated, is characterized by an unusual three-membered ring acyl amide bond. Complex 2 is stable as a solid but is reactive in aqueous solution. Under basic conditions (1 M NaOH), reductive decarbonylation was observed, and the bis-carbonyl complex [(tacn)Re-I(CO)(2)Br] (3) was obtained in quantitative yield. The Br- ligand in 3 could be replaced by CN-, giving the neutral complex [(tacn) Re-I(CO)(2)(CN)] (4). In acidic media (1 M HBr), complex 2 partially converted to the monocarbonyl mu-oxo bridged dinuclear complex {[(tacn)Re-III(CO)Br](2)O}(2+) (5 as [PF6](-) salt). Under mild oxidative conditions the trioxo [(tacn) (ReO3)-O-VII](ReO4-) (6) was formed almost quantitatively, and small amounts of the uncommon Re-VI complex [(tacn) (ReO2Br)-O-VI](ReO4-) (7) were identified. Mechanistic investigations at the density functional level of theory (DFT) showed that the elementary steps in the formation of 2 from 1 and 3 from 2 involved reactions of the complexes with hydroxide. The calculated pathway is strongly exothermic (ca. -137 kcal/mol), confirming the energetically and kinetically highly favored formation of 3. The X-ray structures of 2a and 3-5 are reported and discussed.