A new synthetic method for MS42- (M Mo, W). Evidence for catalysis of aqueous MO42-/MS42- interconversion by thiols

The reaction of aqueous molybdate with sulfide leading to [MoO4-xSx]2- (x = 1-4, hereafter referred to as S1-S4) in basic solution is greatly accelerated by thiols, as are the reverse hydrolyses. In aqueous solution buffered at pH 9, a 50-fold molar excess of 2-mercaptoethanol over molybdenum increased the rate of formation of S1 from molybdate and sulfide by at least a factor of 104 and also substantially increased the rates of S2→S3→S4 conversion. Dithiols behaved similarly to 2-mercaptoethanol in accelerating the formation of S1-S4 from molybdate and sulfide. The same molar excess of 2-mercaptoethanol over molybdenum was found to increase the rates of hydrolyses of S2 and S3 by ≈ 102 and ≈ 101, respectively, at pH 9. Thus, in basic aqueous solution, thiols appear to function as catalysts of oxo/sulfido ligand substitution on [MoO4-xSx]2-. Similar accelerating effects of thiol were observed on reactions of aqueous tungstate with sulfide. Based on these results a new method for the preparation of (NH4)2[MS4] (M Mo, W) was developed; this method combines aqueous MO42- and lithium sulfide in the presence of 2-mercaptoethanol in NH3/NH4+ buffer at pH 9.6. These results may be relevant to the biological chemistry of molybdate and tungstate.