Present General Status of Understanding of Heteropoly Electrolytes and a Tracing of Some Major Highlights in the History of Their Elucidation

The sol-gel synthesis of metal oxides can be performed via the hydrolysis and condensation of metal cations in aqueous solutions. This inorganic route provides a cheap and reliable way for industrial uses. However, many hydrolyzed precursors can be present simultaneously in the precursor solution and very few data are available on the formation of condensed species. The so-called Partial Charge Model provides a useful guide to describe and predict hydrolysis and condensation reactions in aqueous solutions. A charge-pH diagram can be established. It shows how condensation can be initiated via acid-base or redox reactions. The two basic condensation processes, olation and oxolation, are then discussed in terms of a charge-electronegativity diagram. They can lead either to small solute condensed species (polyanions or polycations) or to the formation of an infinite network (colloids, gels, or precipitates). Complexation by anionic species is finally discussed in the frame of an electronegativity-pH diagram. It also plays an important role during the formation of solid phases.

Aqueous chemistry of metal cations: Hydrolysis, condensation and complexation

Publisher Summary This chapter discusses protonation, oligomerization, and condensation reactions of vanadate(V), molybdate(VI), and tungstate(VI). The elements vanadium, molybdenum, and tungsten in their high oxidation states form the stable oxyanions [VO4]3-, [MoO4]2-, and [WO4]2-. In aqueous solution, these ions are easily protonated and show a strong tendency to form polyoxoanions by oxygen bridging, and the release of water molecules. Depending on pH and concentration, a great variety of ions can form in each case, giving rise to complex systems of simultaneous equilibria, which have proved difficult to characterize with certainty. The molybdate(VI) system, in particular, needs further clarification regarding the existence of some polyions. More kinetic and thermodynamic data would also help to improve the understanding of these systems and lead to a general inclusive explanation of the mechanism of polyoxoanion formation. In this respect, new information about some structural preferences of the different metals in mixed polyoxoanions is a useful addition to known facts regarding polyoxometalate structures.

Protonation, Oligomerization, and Condensation Reactions of Vanadate(V), Molybdate(vi), and Tungstate(vi)

Polyoxometalates (POMs) are a large group of anionic polynuclear metal-oxo clusters with discrete and chemically modifiable structures. In most aqueous POM solutions, numerous, and often highly negatively charged, species of different nuclearities are formed. It is rather difficult to determine the dominant POM species or their combination, which is responsible for the specific POM activity, during a particular application. Thus, the identification of all individual speciation profiles is essential for the successful implementation of POMs in solution applications. This review article summarizes species that are present in isopoly- and heteropolyvanadates, -niobates, -molybdates and -tungstates aqueous solutions and covers their stability and transformations. The ion-distribution diagrams over a wide pH range are presented in a comprehensive manner. These diagrams are intended for the targeted use of POMs, and in a clear form shows species that are in equilibrium at the given pH value. Thus, the data accumulated in this review can serve as both a starting point and a complete reference material for determining the composition of POM solutions. Some examples are highlighted where the POM speciation studies led to a detailed understanding of their role in applications. In doing so, we aim to motivate the POM community for more speciation studies and to make the subject more comprehensible, both for synthetic POM chemists and for scientists with different backgrounds interested in applying POMs in biological, medical, electrochemical, supramolecular and nanochemistry fields, or as homogeneous catalysts and other water-soluble materials.

Polyoxometalates in solution: speciation under spotlight

Heteropolyanions [WM3(H2O)2(XW9O34)2]12–(X = M = Zn or CoII) form on ageing concentrated solutions of sodium salts of the well known polyanions [Zn(H2O)(ZnW11O39)]8– and [CoII(H2O)(CoIIW11O39)]8–. Their high-yield syntheses are reported. An X-ray crystallographic study of Na12[WZn3(H2O)2(ZnW9O34)2]·46H2O [Space group p21/c, a= 13.027(4), b= 17.788(5), c= 24.124(4)A, β= 118.94(2)°, Z= 2, R′= 0.051 for 3783 reflections] and its high-resolution 183W NMR spectrum (ten lines, one with half the intensity of the others) reveal that the polyanion is isotypical of the earlier described [M4(H2O)2(PW9O34)2]10– anions (M = Zn or CoII) and elucidate the puzzling analytical composition Zn5W19. However, the lower point-group symmetry C2 (instead of C2h) points to the existence of enantiomers. This anion and the homologous cobalt(II) anion are always obtained as racemic mixtures. Both parent anions transform into substitution derivatives by replacement of atoms in M positions (Two or three for Zn, two only for Co) by transition elements: derivatives of MnII or MnIII, FeII or FeIII, CoII, NiII, CuII, Zn, PdII, PtII and VIV are reported. The structures of D,L-Na12[WCu3(H2O)2(ZnW9O34)2]·48H2O and D,L-K12[WZnV2O2(ZnW9O34)2]·30H2O are also described and compared. Chemical features, comparative electronic UV–VIS spectra, and 183W NMR spectra of these series are discussed.

Chiral polytungstometalates [WM3(H2O)2(XW9O34)2]12–(X = M = Zn or CoII) and their M-substituted derivatives. Syntheses, chemical, structural and spectroscopic study of some D,L sodium and potassium salts

K.G. Burtseva

Papers

17O and 183W NMR studies of the paratungstate anions in aqueous solutions