THE KINETIC AND MECHANISTIC STUDIES OF RE DUCTION REACTION OF THE PEROXO COMPLEX OF MOLYBDENUM WITH HSO3- IN ACIDIC SOLUTION

A stopped-flow kinetic studies of the reduction of peroxomolybdnum (VI ) Mo4O12(O2)24- with HSO3- in aqueous solutions containing 4x10(-3) ap proximately 0.50 mol.dm-3 HClO4 was performed. A reaction scheme in ac cord with the experimental results was proposed to be: Mo4O12(O2)24- H2O half arrow right over half arrow left [Mo4O12(O2)(OH)(OOH)]4-[Mo4 (O2)(OOH) for short] (k1, k-1) Mo4(O2)(OOH) + HSO3- half arrow right o ver half arrow left Mo4(O2)OOSO2 + H2O (k2,k-2) Mo4(O2)OOSO2 + H+ + H2 O --> Mo4(O2) + H2SO4 (k3) Mo4(O2) + H2O half arrow right over half ar row left Mo4-OOH (k4, k-4) Mo4-OOH + HSO3- half arrow right over half arrow left Mo4-O-O-SO2 + H2O (k5, k-5) Mo4-O-O-SO2 + H+ + H2O --> Mo4( VI) + H2SO4 (k6) SO2 . H2O half arrow right over half arrow left HSO3- + H+ (k(a)) The steady-state approximation was applied to the postula ted intermediates and the rate expression was proposed to be: [S(IV)]/ k(obsd) = (k-1/2k1k2K(a))[H+] + 1/2k1 [S(IV)] + (k-1k-2/2k1k2k3) .(1/[ H+] + 1/K(a) + k3/k-2) The following kinetic parameters were determine d at 20-degrees-C and I = 0.5 mol/dm3 k1 = 7.4 +/- 0.3 dm6.mol-1.s-1, k-2/k2- (5.8 +/- 0.5) X 10(-2), k-2/k3 = (1.4 +/- 0.8) X 10(-4) In abo ve mechanism the protonation step of the (O22-) ligand in the peroxo c omplex has a rate slower than the nucleophilic displacement step. The previous result of Thompson's studies on the reactions MoO(O2)2 and HS O3- was also discussed and compared with our results.