The oxygen atom exchange reaction between the terminal oxygen atom (V=O) of tetradentate Schiff base-oxovanadium(IV) complexes and water slowly proceeds in DMSO solutions. The rate constants k(obs) for the isotopic oxygen atom exchange reaction in DMSO solutions containing excess H 2 18O at 50 °C under an argon atmosphere were obtained for several complexes (6 x 10 -7 to 6 x 10 -5 s -1). The reaction proceeds faster for those complexes with electron-withdrawing groups at the Schiff base ligand moieties, and slower for the complexes with bulky hydrophobic groups. The oxygen atom exchange reaction and isomerization reaction under an argon atmosphere and in air were investigated for oxovanadium(IV) complexes that have two geometrical isomers. Water molecules attack the vanadium atom mainly from the same side of the oxo ligand for the oxovanadium(IV) complexes, whereas mainly from the opposite side of the oxo ligand for the oxovanadium(V) complexes produced by oxidation with air.
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