Vanadium-51 and oxygen-17 nuclear magnetic resonance study of vanadate(V) equilibria and kinetics

Approximately 15 vanadate(V) and polyvanadate(V) species have been identified by 51V n.m.r. in the range pH 7–14, mostly for the first time. Oxygen-17 n.m.r. spectroscopy confirms in several cases that the vanadium coordination is tetrahedral, and that the 17O shifts are correlated closely with formal π-bond order. Most linear and cyclic catenations of VO4 tetrahedra are found up to V6, but there is no branched or triprotonated species or cyclic trimer. Several equilibrium constants and pKa values are reliably measurable. The ion [HVO4]2– rather than OH– is shown to be the dominant nucleophile in the two main ring-breaking reactions, and a simple kinetic pathway is proposed for the formation of decavanadate ion.