Synthesis, characterization and crystal structures of lithium tetraperoxomolybdate(VI) tetrahydrate Li2[Mo(O2)4] · 4H2O and lithium tetraperoxotungstate(VI) tetrahydrate Li2[W(O2)4] · 4H2O

The crystal structures of two very unstable tetraperoxo compounds have been determined from single-crystal X-ray diffraction data. Red needles of Li2[Mo(O2)4] · 4H2O crystallize in the space group P21/n with a = 6.57300(10), b = 10.0580(2), c = 13.2950(3) Å, β = 101.8350(9)°, Z = 4. Light yellow needles of Li2[W(O2)4] · 4H2O crystallize in the space group P21/n with a = 6.5575(3) Å, b = 10.0440(2) Å, c = 13.37900(10) Å, β = 101.344(3)°. In both tetraperoxo anions molybdenum and tungsten are surrounded by four O22- groups. The MOperoxo (M = Mo or W) distances span range 1.931(2)–1.990(2) Å and 1.934(3)–1.988(5) Å in tetraperoxomolybdate(VI) and tetraperoxotungstate(VI), respectively.