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(cys)2]2– and [Mo2O2(µ-O)2(cys)2]2–

Abstract

Potentiometric studies at 25 °C, I = 0.50 mol dm^–3 (NaClO₄), on the dissociation of cysteine from [Mo₂O₂(µ-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(µ-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 β_21p values) for 2cys^2– + [Mo₂O₂(µ-S)₂(H₂O)₆]²⁺ + pH⁺ ⇌ [Mo₂O₂(µ-S)₂(cys)₂H_p]^p–2 are 36.24(3) (β₂₁₀), 38.63(3) (β₂₁₁) and 40.63(3) (β₂₁₂). 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-µ-oxo complex [Mo₂O₂(µ-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^V₂ is observed at an intermediate stage of the titration. Final products were identified as [Mo₂O₂(µ-O)₂(H₂O)₆]²⁺, and for the reverse reaction [Mo₂O₂(µ-O)₂(cys)₂]^2–.