Molybdenum(VI) Oxosulfato Complexes in MoO3–K2S2O7–K2SO4 Molten Mixtures: Stoichiometry, Vibrational Properties, and Molecular Structures

The structural and vibrational properties of molybdenum(VI) oxosulfato complexes formed in MoO3–K2S2O7 and MoO3–K2S2O7–K2SO4 molten mixtures under an O2 atmosphere and static equilibrium conditions were studied by Raman spectroscopy at temperatures of 400–640 °C. The corresponding composition effects were explored in the XMoO30 = 0–0.5 range.

MoO3 undergoes a dissolution reaction in molten K2S2O7, and the Raman spectra point to the formation of molybdenum(VI) oxosulfato complexes. The MoO stretching region of the Raman spectrum provides sound evidence for the occurrence of a dioxo Mo(O)2 configuration as a core. The stoichiometry of the dissolution reaction MoO3 + nS2O72– → C2n– was inferred by exploiting the Raman band intensities, and it was found that n = 1.

Therefore, depending on the MoO3 content, monomeric MoO2(SO4)22– and/or associated [MoO2(SO4)2]m2m– complexes are formed in the binary MoO3–K2S2O7 molten system, and pertinent structural models are proposed in full consistency with the Raman data. A 6-fold coordination around Mo is inferred. Adjacent MoO22+ cores are linked by bidentate bridging sulfates.

With increasing temperature at concentrated melts (i.e., high XMoO30), the observed spectral changes can be explained by partial dissociation of [MoO2(SO4)2]m2m– by detachment of S2O72– and formation of a MoOMo bridge. Addition of K2SO4 in MoO3–K2S2O7 results in a “follow-up” reaction and formation of MoO2(SO4)34– and/or associated [MoO2(SO4)3]m4m– complexes in the ternary MoO3–K2S2O7–K2SO4 molten system.

The 6-fold Mo coordination comprises two oxide ligands and four O atoms linking to coordinated sulfate groups in various environments of reduced symmetry. The most characteristic Raman bands for the molybdenum(VI) oxosulfato complexes pertain to the Mo(O)2 stretching modes: (1) at 957 (polarized) and 918 (depolarized) cm–1 for the νs and νas Mo(O)2 modes of MoO2(SO4)22– and [MoO2(SO4)2]m2m– and (2) at 935 (polarized) and 895 (depolarized) cm–1 for the respective modes of MoO2(SO4)34– and [MoO2(SO4)3]m4m–.

The results were tested and found to be in accordance with ab initio quantum chemical calculations carried out on [MoO2(SO4)3]4– and [{MoO2}2(SO4)4(μ-SO4)2]8– ions, in assumed isolated gaseous free states, at the DFT/B3LYP (HF) level and with the 3-21G basis set. The calculations included determination of vibrational infrared and Raman spectra, by use of force constants in the Gaussian 03W program.