Complexation of oxovanadium(IV) by humic and tannic acids.

The complexes formed by oxovanadium(IV) with humic and tannic acids have been studied by ESR. The comparison with simple model systems has shown that the major sites which are involved in metal binding over the pH range 2-6 have spectral parameters similar to those of complexes with carboxylate groups or salicylate and o-catecholate chelating sets. A population of weaker sites has been also observed. However, tannic acid shows pure o-catecholato coordination and, due to the strong chelating ability of this donor set, displaces the oxo-ligand from the VO(IV) ion yielding stable complexes of the tris(catecholato)vanadate(IV) type.