MECHANISTIC STUDIES ON THE CIS-[VO2(H2O)(4)](+) AND [MO(CN)(8)](3-) OXIDATIONS OF THE RE-2(IV) COMPLEX [(C2O4)(2-)RE(MU-O)(2)RE(C2O4)(2)](4-)

Rate constants (25 degrees C) have been determined for the oxidation of the Re-2(IV), complex di-mu-oxo-bis[dioxalatorhenate(IV)]. [(C2O4)(2)Re(mu-O)(2)Re(C2O4)(2)](4-), I = 1.00 M (ClO4- or Cl-). Whereas the complex is itself stable in air at pH 7 over many days, at lower pH there is a slow decay of the 446 nm peak (epsilon = 6840 M(-1) cm(-1) per Re-2(IV)) to give in air the colourless' perrhenate(VII) [ReO4](-) product, peak at 225 nm (epsilon = 3400 M(-1) cm(-1)). The kinetics indicate a protonation step, equilibrium constant K = 2.4 M(-1), followed by the decay process k = 4.3 x 10(-5) s(-1). With the one-equivalent oxidants cis-[VO2(H2O)(4)](+) (1.0 V) and [Mo(CN)(8)](3-) (0.80 V), stoichiometries of 6 mol of oxidant per Re-2(IV) are obtained indicating conversion through to Re-VII. Two kinetic stages have been monitored with the oxidant in large excess. For the reaction with V-V as oxidant the first stage is dependent on [V-V] and [H+] as well as [Re-2(IV)], giving the rate constant k(1) = 3.31 x 10(4) M(-2) S-1. Since in the corresponding reaction with Mo-V as oxidant the reaction is independent of [H+] (k(Mo) = 2.27 x 10(4) M(-1) s(-1)), it is concluded that the V-V reactant introduces the [H+]-dependent term. The second stage of the V-V reaction gives the rate law dependence k(2obs) = k(2)[V-V] + k(0), but with the less strong Mo oxidant k(2obs) = k(0) which is approximate to 0.20 s(-1). The spectrum of the (ReReV)-Re-IV intermediate, peak at 529 nm (epsilon = 3800 M(-1) cm(-1)), was obtained by stopped-flow rapid-scan spectrophotometry. Reactions of less than full stoichiometric amounts of V-V with Re-2(IV) followed by QAE-Sephadex chromatography at 0 degrees C. gave orange-pink (peak at approximate to 495 nm) and maroon (approximate to 550 nm) intermediates, which undergo spontaneous decay.