The present study investigates structural and functional aspects of the redox chemistry of rhenium(III) chloride [Re3Cl9] (1) in aqueous and organic solvents, with emphasis on the dioxygen-activating capabilities of reduced rhenium clusters bearing the Re-3(8+) core. Dissolution of 1 in HCl (6 M) generates [Re-3(mu -Cl)(3)Cl-9](3-) (2a), which can be isolated as the tetraphenylphosphonium salt (2b). Anaerobic one-electron reduction of 1 by Hg in HCl (6-12 M) produces [(C6H5)(4)P](2)[Re-3(mu -Cl)(3)Cl-7(H2O)(2)].H2O (3), the structure of which features a planar [Re-3(mu -Cl)(3)Cl-3] framework (Re-3(8+) core), involving two water ligands that occupy out-of-plane positions in a trans arrangement. Compound 3 dissociates in the presence of CO, yielding [(C6H5)(4)P](2)[(Re2Cl8)-Cl-III] (4) and an unidentified red carbonyl species. In situ oxidation (O-2) Of the reduced Re-3(8+)-containing cluster in HCl (6 M) produces quantitatively 2a, whereas oxidation of 3 in organic media results in the formation of [(C6H5)(4)P](4)[{Re-3(mu -Cl)(3)Cl-7(mu -OH)}(2)]. 2CH(2)Cl(2) (5). The structure of 5 reveals that two oxygen ligands (hydroxo units) bridge asymmetrically two Re-3(9+) triangular clusters. The origin of these hydroxo units derives from the aquo ligands, rather than O-2, as shown by O-18(2) labeling studies. The hydroxo bridges of 5 can be replaced by chlorides upon treatment with Me3SiCl to afford the analogous [(C6H5)(4)P](4)[{Re-3(mu -Cl)(3)Cl-7(mu -Cl)}(2)]. 10CH(2)Cl(2) (6). The reaction of 5 with Hg in HCl (6 M)/tetrahydrofuran regenerates compound 3. Complexes 1-3 exhibit nitrile hydratase type activity, inducing hydrolysis of CH3CN to acetamide. The reaction of 3 with CH3CN yields [(C6H5)(4)P](2)[Re-3(mu -Cl)(3)Cl-6.5(CH3-CN)(1.5)(CH3C(O)NH)(0.5)] (7), the structure of which is composed of [Re-3(mu -Cl)(3)Cl-7(CH3CN)(2)](2-) (7a) and [Re-3(mu -Cl)(3)Cl(CH3CN)(CH3C(O)NH)](2-) (7b) (Re-3(8+) cores) as a disordered mixture (1:1). Oxidation of 7 with O-2 in CH3CN affords [(C6H5)(4)P](2)[Re-3(mu -Cl)(3)Cl-7(CH3C(O)NH)]. CH3CN (8) and small amounts of [(C6H5)(4)P][ReO4] (9). Compound 8 is also independently isolated from the reaction of 2b with wet CH3CN, or by dissolving 5 in CH3CN. In MeOH, 5 dissociates to afford [(C6H5)(4)P](2)[Re-3(mu -Cl)(3)Cl-8(MeOH)]. MeOH (10).