Five cationic transition metal (ethylenediamine) complexes (M=Cr(III)!, Co(III), Ni(II), and Cu(II)):paramolybdate anion (Mo7O246-) have been synthesis and characterized via their elemental analysis, magnetic susceptibility (mu (eff),) thermal analysis (TG and DTA), FTIR spectra, and X-ray diffraction (XRD. The FTIR study suggests that the compounds prepared be of the ion-pair type ([M(en)(n)](m). Mo7O24). Thermal study showed that molybdenum in the Cr(III), and Co(III) compounds is reduced from oxidation state (VI) to (V) at high temperature. The stoichiometries of the resulting mixed oxides at elevated temperatures (500-750 degreesC) are: Cr2O3. 7MoO(2.5), Co2O3. 7MoO(2.5), 6CoOCl . 7MoO(2.5), 3NiO . 7MoO(3) and 3CuO . 7MoO(3). Above 750 degreesC the molybdenum oxide in the ion-pair compounds start the sublimation process. X-ray diffraction of [Cr(en)(3)]. Mo7O24, [Co(en)(3)]. Mo7O24, and [CU(en)(2)(H2O)(2)]3 . Mo7O24 shows that these complexes are crystalline solids with a similar structure, while the [Ni(en)(H2O)(4)](3). Mo7O24 and [Co(en)(H2O)(2)Cl-2](6). Mo7O24] ion-pair compounds display a different structure. A novel technique based on photocatalysis to eliminate Cr(VI) ions, a toxic pollutant in the environment, was applied. The photoreduction of Cr(VT) to Cr(III) ion in aqueous suspensions using new-mixed oxides as photocatalysts (Cr2O3. MoO2.5, Co2O3. MoO2.5, NiO . MoO3, and CuO . MoO3) under air-equilibration and irradiation by a medium pressure mercury lamp (UV-VIS) was investigated. (C) 2000 Elsevier Science B.V. All rights reserved.