Potentiometric studies on the formation and dissociation of the L-cysteine complexes of di-μ-sulfido and di-μ-oxo molybdenum(v) [Mo2O2(μ-S)2(cis)2]2- and [Mo2O2(μ-O)2(cis)2]2-

Potentiometric studies at 25 degrees C, I = 0.50 mol dm(-3) (NaClO4), on the dissociation of cysteine from [Mo2O2(mu-S)(2)(cys)(2)](2-), cys = -SCH2CH(NH2)CO2- (here L-cysteine), induced by the addition of HClO4 indicated formation of two protonated species prior to [Mo2O2(mu-S)(2)(H2O)(6)](2+). In the reverse titration of product solutions with sodium hydroxide, reformation of the cysteine complex was observed with involvement of the same two protonated species. From both sets of data, formation equilibrium constants (log beta(21p) values) for 2cys(2-) + [Mo2O2(mu-S)(2)(H2O)(6)](2+) + pH(+) reversible arrow [Mo2O2(mu-S)(2)(cys)(2)H-p](p-2) are 36.24(3) (beta(210)), 38.63(3) (beta(211)) and 40.63(3) (beta(212)). The first two steps in the dissociation are defined as protonation and dissociation of the cysteine carboxylates, acid dissociation constants 2.00 and 2.39 respectively. In less extensive studies on the di-mu-oxo complex [Mo2O2(mu-O)(2)(cys)(2)](2-) the two cysteines are more weakly bound to the molybdenum, and a lower [H+] is required for dissociation. Precipitation of Mo-2(v) is observed at an intermediate stage of the titration. Final products were identified as [Mo2O2(mu-O)(2)(H2O)(6)](2+), and for the reverse reaction [Mo2O2(mu-O)(2)(cys)(2)](2-).