The equilibrium speciation at 278 K and the dynamics in the systems pH+ + qMoO42- + sL = (H+)p- (MoO42-)q (H2O2)r Ls(L = SO42- or Cl-) in 0.3 M Na2(SO4) and 0.6 M Na(Cl) medium has been determined from 17O NMR integral and shift data in the range 0.7 ≥ pH ≥ 4.1 and at excess of peroxide ([H2O2]tot/[Mo]tot = 3). In 0.3 M Na2(SO4) medium species with the following compositions were found: MoX2- (1,1,2,0), MoX2 (2,1,2,0), MoX2S2- (2,1,2,1), MoX2S- (3,1,2,1), Mo2X42- (2,2,4,0), Mo2X4- (3,2,4,0) and Mo2X62- (2,2,6,0), where (Mo) corresponds to MoO42-, (X) to O22- or HOO- and (S) to SO42-. In 0.6 M Na(Cl) medium the diperoxomolybdosulfate complex MoX2S- is replaced with a novel diperoxomolybdochloride complex MoX2Cl- (2,1,2,1), where (Cl) corresponds to Cl-. The numbers in parentheses refer to the values of p,q,r and s in the formula above. The stoichiometries of each complex are given by the abbreviated formula MoqXrSs(2q + 2s - p)- and MoqXrCls(2q + s - p)-. The following formation constants with 3σ were obtained with values for 0.6 M Na(Cl) in parenthesis: log β1,1,2,0 = 11.61 (11.61); log β2,1,2,0 = 13.70 ± 0.10, pKa = 2.09 (13.86 ± 0.10, pKa = 2.25); log β2,1,2,1 = 14.57 ± 0.07 (13.87 ± 0.15); log β3,1,2,1 = 15.27 ± 0.17, pKa = 0.70, log β2,2,4,0 = 24.06 ± 0.05 (24.08 ± 0.04); log β3,2,4,0 = 25.99 ± 0.18, pKa = 1.93 (26.23 ± 0.17, pKa = 2.15); log β2,2,6,0 = 24.02 ± 0.15 (23.9 ± 0.3). The pKa value for HSO4- was determined to be 1.06 ± 0.11. At 278 K the MoX2 and Mo2X4 complexes are in chemical exchange and the rate of exchange increases upon protonation. At 312 K there is also measurable exchange between MoX2S and the MoX2 and Mo2X4 complexes. The dynamics in 0.6 M Na(Cl) medium seem to be very similar to those in the sulfate medium.
|Number of pages||6|
|Journal||Journal of the Chemical Society, Dalton Transactions|
|Publication status||Published - Dec 1 2002|
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