Raman Spectroscopic Study of Tungsten(VI) Oxosulfato Complexes in WO3―K2S2O7―K2SO4 Molten Mixtures: Stoichiometry, Vibrational Properties, and Molecular Structure

The dissolution reaction of WO 3 in pure molten K 2 S 2 O 7 and in molten K 2 S 2 O-―K 2 SO 4 mixtures is studied under static equilibrium conditions in the X wo o = =0―0.33 mol fraction range at temperatures up to 860 °C. High temperature Raman spectroscopy shows that the dissolution leads to formation of W VI oxosulfato complexes, and the spectral features are adequate for inferring the structural and vibrational properties of the complexes formed. The band characteristics observed in the W=O stretching region (band wavenumbers, intensities, and polarization characteristics) are consistent with a dioxo W(=O) 2 configuration as a core unit within the oxosulfato complexes formed. A quantitative exploitation of the relative Raman intensities in the binary WO 3 ―K 2 S 2 O 7 system allows the determination of the stoichiometric coefficient, n, of the complex formation reaction WO 3 + nS 2 O 7 2― → C 2n― . It is found that n = 1; therefore, the reaction WO 3 + S 2 O 7 2― → WO 2 (SO 4 ) 2 2― with six-fold W coordination is proposed as fully consistent with the observed Raman features. The effects of the incremental dissolution and presence of K 2 SO 4 in WO 3 ―K 2 S 2 O 7 melts point to a WO 3 K 2 S 2 O 7 · K 2 SO 4 stoichiometry and a corresponding complex formation reaction in the ternary molten WO 3 ―K 2 S 2 O 7 ―K 2 SO 4 system according to WO 3 + S 2 O 7 2― + SO 4 2― → WO 2 (SO 4 ) 3 4― . The coordination sphere of W in WO 2 (SO 4 ) 2 2― (binary system) is completed with two oxide ligands and two chelating sulfate groups. A dimeric [{WO 2 (SO 4 ) 2 } 2 (μ-SO 4 ) 2 ] 8― configuration is proposed for the W oxosulfato complex in the ternary system, generated from inversion symmetry of a WO 2 (SO 4 ) 3 4― moiety resulting in two bridging sulfates. The most characteristic Raman bands for the W VI oxosulfato complexes pertain to W(=O) 2 stretching modes (i) at 972 (polarized) and 937 (depolarized) cm ―1 for the v s and v s W(=O) 2 modes of WO 2 (SO 4 ) 2 2― , and (ii) at 933 (polarized) and 909 (depolarized) cm ―1 for the respective modes of [{WO 2 (SO 4 ) 2 } 2 (μ-SO 4 ) 2 ] 8― .