A NEW SYNTHETIC METHOD FOR MS4(2-) (M=MO, W) - EVIDENCE FOR CATALYSIS OF AQUEOUS MO4(2-)/MS4(2-) 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 10(4) 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 almost-equal-to 10(2) and almost-equal-to 10(1), 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 MO4(2-) 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.