Structural investigation of tetraperoxo complexes of Mo(VI) and W(VI): X-ray and theoretical studies

The family of very unstable tetraperoxo compounds has been prepared from aqueous solutions containing H₂O₂ and salts of Mo(VI) or W(VI). The crystal structures of Na₂[Mo(O₂)₄]·4H₂O and Na₂[W(O₂)₄]·4H₂O have been determined from single-crystal data, while the crystal structures of Rb₂[Mo(O₂)₄], Cs₂[Mo(O₂)₄], Rb₂[W(O₂)₄] and Cs₂[W(O₂)₄] have been determined from powder X-ray diffraction data. The compounds were also characterised by IR spectroscopy and the number of peroxo groups was determined by titration methods. By means of the density functional theory (DFT) method, the geometry and stability of tetraperoxo complexes have been studied. Even though in all tetraperoxo complexes the central atom Mo(VI), W(VI) or V(V) is surrounded by four peroxo groups and the geometry of the [Me(O₂)₄]ⁿ⁻ anion is essentially the same, the investigated compounds differ in stability and colour and crystallise in different crystallographic systems.