Calculation of the UV-visible spectra and the stability of Mo and Re oxysulfides in aqueous solution

Erickson and Helz (2000) established that molybdate, MoO4−2, is quickly and completely transformed to thiomolybdate, MoS4−2, by reaction with sulfide. They monitored the equilibria and kinetics of this process by measuring the UV-visible spectra of solutions containing the different oxythiomolybdate species. There is interest in the analogous reactions for the ReO4− species but little experimental data. We have therefore calculated quantum-chemically the equilibrium constants in solution for the sulfidation reactions of both MoO4−2 and ReO4−, as well as recalculating their UV-visible charge-transfer spectra. Calculations using configuration interaction singles and time-dependent hybrid Hartree-Fock density functional methods give good absolute values and trends in UV-vis energies for both series. For MoO4−2 the calculated equilibrium constants for the various sulfidation steps match reasonably well against the experimental values (within two log K units). For the ReO4− sulfidation reaction the first two steps are considerably less favorable than for MoO4−2, suggesting that the “geochemical switch” of Erickson and Helz, a rapid transformation of oxyanion to thioanion highly dependent on sulfide concentration, will be less effective in the Re case. However, both our calculations and experiment indicate that ReO4− and ReS4− are both easier to reduce than their Mo analogs, so that reduction of these Re(VII) species will be the preferred mechanism for their removal from seawater. A previous suggestion that the neutral species MoO4H2 is actually the hydrated octahedral Mo(OH)6 species is found to be incorrect, but the MoO3(OH2)3 species, a highly distorted six-coordinate complex, is almost competitive in energy with MoO4H2 plus two H2O.