Reaction of the non-oxo V-IV species [(VCl2)-Cl-IV(L-OO)(2)] [L-OO = acetylacetonate (acac(-)) or benzoylacetonate (bzac(-))] with a chelate nitrogen-donor ligand L-NN in acetonitrile leads to the reduction of V-IV to V-III and the formation of the mononuclear V-III compounds of the general formula [(VCl2)-Cl-III(L-OO)(L-NN)] (L-OO and L-NN are acac(-) and bipy for 1; acac- and 5,5'-me(2)bipy for 2; acac- and 4,4'-tb(2)bipy for 3; acac(-) and phen for 4; bzac(-) and bipy for 5; bzac(-) and phen for 6). The reduction of the V-IV complexes was monitored by GC-MS and H-1 NMR spectroscopy. Both one- and two-dimensional (2D COSY and 2D EXSY) H-1 NMR techniques were used to assign the observed 1H NMR resonances of 1-6 in CD2Cl2 or CDCl3 solution. It appeared that in solution these V-III complexes form two isomers which are in equilibrium: cis-[(VCl2)-Cl-III(L-OO)(L-NN)] reversible arrow trans-[(VCl2)-Cl-III(L-OO)(L-NN)]. 2D EXSY cross-peaks were clearly observed between bipy- and acac-hydrogen atoms of the two geometrical isomers of 1-3 as well as between bipy and acac(-) protons of the cis isomer, indicating a dynamic process that corresponds to cis-trans isomerization and a cis-cis racemization. The thermodynamic and kinetic parameters of the equilibrium between these two isomers were calculated for compounds 1 and 2 by using variable temperature (VT) NMR data. Both cis-trans isomerization and cis-cis racemization processes probably proceed with an intramolecular twist mechanism involving a trigonal prismatic transition state. Density functional calculations (DFT) also indicated such a rearrangement mechanism.