REDOX CHEMISTRY OF THE [RE-3(MU-CL)(3)X-9](3-) HALIDES (X = CL OR BR)- ISOLATION AND STRUCTURAL CHARACTERIZATION OF THE [(RE-3(MU-CL)(3)BR-6(H2O)(MU-O))(2)](2-) CLUSTER ANION

The triangulo Re-3(9+) cluster [Re-3(mu-Cl)(3)X-9](3-) (X = Cl 1a or B r 1b) in concentrated aqueous hydrohalogenic acid solutions underwent a facile one-electron reduction by various reducing agents (VCl2 . 4H( 2)O, Sn-SnCl2 or Hg) to give the air-sensitive Re-3(8+) anion [Re-3(mu -Cl)(3)X9-n(H2O)(n)]((4-n)-) (X = Cl 2a or Br 2b), where n may be 1; t he diamagnetism and EPR silence of this compound indicate that it may exist as a dimer with a direct or indirect rhenium-rhenium bond linkin g two Re-3(8+) trimetal cores. Oxidation of 2 by molecular oxygen in 6 mol dm(-3) HX (X = Cl or Br) solutions yielded 1 quantitatively, wher eas in the absence of acid and in aprotic solvents oxo-derivatives are formed; from 2b the hexanuclear anion [{Re-3(mu-Cl)(3)Br-6(H2O)(mu-O) }(2)](2-) 3b was obtained whose structural characterization, as its [P Ph4](+) salt 3c, shows that two mu-O ligands bridge two oxidized Re-3( 10+) units. The structure of the Re-3(9+) cluster [Co(en)(3)][Re-3(mu- Cl)(3)Br-8(H2O)]Br has been also determined by X-ray diffraction. The redox couple x-Re-3(9+)-(Re-3(8+))(x), where x = 1 or 2, derived upon mixing 6 mol dm(-3) HX (X = Cl or Br) solutions of 1 with an excess of mercury, catalyses efficiently the reduction of molecular oxygen to w ater.