Characterization of decavanadate and decaniobate solutions by Raman spectroscopy

The decaniobate ion, (Nb 10 = [Nb 10 O 28 ] 6− ) being isoelectronic and isostructural with the decavanadate ion (V 10 = [V 10 O 28 ] 6− ), but chemically and electrochemically more inert, has been useful in advancing the understanding of V 10 toxicology and pharmacological activities. In the present study, the solution chemistry of Nb 10 and V 10 between pH 4 and 12 is studied by Raman spectroscopy. The Raman spectra of V 10 show that this vanadate species dominates up to pH 6.45 whereas it remains detectable until pH 8.59, which is an important range for biochemistry. Similarly, Nb 10 is present between pH 5.49 and 9.90 and this species remains detectable in solution up to pH 10.80. V 10 dissociates at most pH values into smaller tetrahedral vanadate oligomers such as V 1 and V 2 , whereas Nb 10 dissociates into Nb 6 under mildly (10 > pH > 7.6) or highly alkaline conditions. Solutions of V 10 and Nb 10 are both kinetically stable under basic pH conditions for at least two weeks and at moderate temperature. The Raman method provides a means of establishing speciation in the difficult niobate system and these findings have important consequences for toxicology activities and pharmacological applications of vanadate and niobate polyoxometalates. V 10 dissociates at most pH values into smaller tetrahedral vanadate oligomers such as V 4 and V 2 , whereas Nb 10 dissociates into Nb 6 under mildly or highly alkaline conditions.