The substituted hydrazones H2L1-4 (L1-4 = dibasic tridentate ONO2(-) donor ligands) obtained by the condensation of 2-hydroxy-1-naphthaldehyde and 2-aminobenzoylhydrazine (H(2)hnal-abhz) (H2L1) , 2-hydroxy-1-naphthaldehyde and 2-hydroxybenzoylhydrazine (H(2)hnal-hbhz) (H2L2), 2-hydroxy-1-acetonaphthone and benzoylhydrazine (H(2)han-bhz) (H2L3), or 2-hydroxy-1-acetonaphthone and 2-aminobenzoylhydrazine (H(2)han-abhz) (H2L4) are prepared and characterized. Reaction of ammonium vanadate with the appropriate H2L1-4 results in the formation of oxidoethoxidovanadium(V) [(VO)-O-V(OEt)(L1-4)] (1-4) complexes. All compounds are characterized in the solid state and in solution by spectroscopic techniques (IR, UV-vis, H-1, C-13, and V-51 NMR, and electrospray ionization mass spectrometry). Single-crystal X-ray diffraction analysis of 1, 3, and 4 confirms the coordination of the corresponding ligands in the dianionic (ONO2-) enolate tautomeric form. In solution, the structurally characterized [(VO)-O-V(OEt)(L)] compounds transform into the monooxido-bridged divanadium(V,V) [((VOL)-O-V)(2)-mu-O] complexes, with the processes being studied by IR and H-1, C-13, and V-51 NMR. The density functional theory (DFT) calculated Gibbs free energy of reaction 2[(VO)-O-V(OEt)(L-4)] + H2O reversible arrow [((VOL4)-O-V)(2)-mu-O] + 2EtOH is only 2-3 kcal mol(-1), indicating that the dinuclear complexes may form in a significant amount. The electrochemical behavior of the complexes is investigated by cyclic voltammetry, with the V-V-V-IV E-1/2(red) values being in the range 0.27-0.44 V (vs SCE). Upon controlled potential electrolysis, the corresponding (L)(O)V-IV-O-V-V(O)(L) mixed-valence species are obtained upon partial reduction of the [((VOL)-O-V)(2)-mu-O] complexes formed in solution, and some spectroscopic characteristics of these dinuclear mixed-valence complexes are investigated using DFT calculations and by electron paramagnetic resonance (EPR), with the formation of V-IV-O-V-V species being confirmed by the observation of a 15-line pattern in the EPR spectra at room temperature.