Towards empirical EPR parameters for characterizing V(IV) complexes

Improved empirical techniques are described for determining the solution structures (coordination numbers and ligand donor groups) of oxoV(IV) complexes from isotropic EPR parameters. These methods will enable more detailed structural information to be obtained than was available previously from such empirical correlations. Assignments of the structures of the V(IV)/oxalate complexes were found to differ from those proposed in the literature and have been important in developing the empirical parameters reported here. In acidic solutions (pH less than or equal to 1), V(V) oxalate complexes decay slowly to yield three V(IV) complexes, [V(O)(ox)(2)](2-), [V(O)(ox)(OH2)(3)](0) and cis-[V(O)(ox)(2)(OH2)](2-) (ox = oxalato(2-)), which have been characterized by EPR spectroscopy. These same three complexes are obtained from the direct reaction of V(IV) with oxalic acid. The structures of these complexes were assigned on the basis of the pH and [oxH(2)] dependences of the EPR signals and differ significantly to those reported in the literature. The equilibria among the five- and six-coordinate complexes, and the protonation/deprotonation equilibrium of the six-coordinate species are consistent with the well-studied isoelectronic Cr(V) system.