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New molybdenum complexes were prepared by the reaction of [MoVIO2(acac)2] or (NH4)2[MoVOCl5] with different N-substituted pyridoxal thiosemicarbazone ligands (H2L1 = pyridoxal 4-phenylthiosemicarbazone; H2L2 = pyridoxal 4-methylthiosemicarbazone, H2L3 = pyridoxal thiosemicarbazone). The investigation of monomeric [MoO2L1(CH3OH)] or polymeric [MoO2L1–3] molybdenum(VI) complexes revealed that molybdenum is coordinated with a tridentate doubly-deprotonated ligand. In the oxomolybdenum(V) complexes [MoOCl2(HL1–3)] the pyridoxal thiosemicarbazonato ligands are tridentate mono-deprotonated. Crystal and molecular structures of molybdenum(VI) [MoO2L1(CH3OH)]·CH3OH, and molybdenum(V) complexes [MoOCl2(HL1)]·C2H5OH, as well as of the pyridoxal thiosemicarbazone ligand methanol solvate H2L3·MeOH, were determined by the single crystal X-ray diffraction method.

The reaction of molybdenum(VI) oxide with oxalic acid and RCl where R=NH4 (1) and (C6H5)4P (2), results in the formation of molybdenum(VI) oxalate complexes of the general formula R2[Mo2O5(C2O4)2(H2O)2]. As determined by X-ray structure analysis both salts are built up of the dimeric anions containing Mo2O5 core. Molybdenum atoms exhibit distorted octahedral coordination. Whereas in the ammonium salt the MoOMo bridge is linear, the one in the corresponding tetraphenylphosphonium salt is angular with an angle of 162.42(15)° at the oxygen atom. This angular structure of 2 is also caused by additional strong intramolecular hydrogen bonds (2.719(4) and 2.661(4) A) between coordinated water molecules and oxalato-oxygens. Both complexes were characterized by chemical analysis, thermogravimetry, UV and IR spectroscopy as well as by conductivity measurements.