Cations isopoly

Isopolyanions. Isopoly anions are named by indicating with numerical prefixes the number of atoms of the characteristic element. It is not necessary to give the number of oxygen atoms when the charge of the anion or the number of cations is indicated. 

Why should the early transition metals form so many polyoxoanions The answer lies in the size of the M5/6+ cations and their -acceptor properties.1,5 The effective ionic radii of V5+ (0.68 A), Mo6+ (0.77 A) and W6+ (0.74 A) are consistent with the observation that these cations adopt four-, five- and six-fold coordination by oxide ion. With very few exceptions V, Mo and W atoms in heteropolyanions are six-coordinate. On the other hand Cr6+ (0.58 A) hap a maximum coordination number of four in oxides and oxoanions. Few isopoly- and heteropoly-chromates are known and they are all based on groups of corner-shared Cr04 tetrahedra [Cr207]2-, [Cr3O10]2-, [Cr4Oi3]2-, [03SOCrO3]2-, [02I0Cr03]-,... 

In recent times many advances in isopoly anion chemistry have been made by shifting reaction chemistry from aqueous to aprolic solution. This can often be done by employing a solubilizing cation such as tetrabutyfammonium ion. For example, when [(n-Bu)4N]0H and [(/i-Bu)4N][H7V ,OjK] are mixed in acetonitrile a new isopolyvanadate forms 53... 

An isopoly cation, [A1i304(0H)24(0H2) 2]7+, has the Keggin structure and can be intercalated into the layer structure of montmorillonite clay,... 

Myriads of structures of isopoly and heteropolyacid anion cages are found as minerals with many different metal cations completing the chemical structures. The basic tetrahedral or near tetrahedral local geometry of, for example, component SiO " units can be formed between neighbouring shells of decorations of cubic vertices. Three examples, which emphasize the orbit-by-orbit perspective are shown in Figures 2.28 to 2.30. 

A. Actinyl V s and Vi s give monomeric oxo cations in acid, in contrast to the d element proclivity towards polymerization (isopoly acid formation). Protactinium is similar to d elements in this regard. 

In contrast to the isopoly acids, many of the heteropoly acids are stable without depolymerization in quite strongly acid solutions they are often themselves strong acids. In general, heteropolymolybdates and tungstates of small cations, including those of some heavy metals, are water-soluble, but with larger cations insolubility is frequently found. Thus Cs+, Pb2+ and Ba2+ salts are usually insoluble, and NH, K + and Rb+ salts are sometimes insoluble salts of [(/ri-C5H5)2Fe]+,R4N+, R4P+ and alkaloids, are invariably insoluble. Table 26-C-2 lists the principal types of heteropolymolybdates,... 

Pillaring is another intercalation reaction that enables synthesis of metastable oxide material [38], Pillaring refers to intercalation of robust, thermally stable, molecular species that prop the layers apart and convert the two-dimensional interlayer space into micropores of molecular dimensions, similar to the pores in zeolites. Smectite clays [38], layered a-Zr(HPO )j, a-MoOj [39], perovskites [40] and double hydroxides (LDHs) [41] have all been pillared by cationic/anionic species such as alkylammonium ions, polyoxocations (e.g. Alj304(0H)24(H20)j2 " ) and isopoly and heteropolyanions (e.g. and PVjWgO / ). 

The nature of molybdic acid in water and in acidified solutions has been studied by relatively few workers, JONES (1) proposed the existence of isopoly-cations of molybdenum to explain the solubility of molybdic acid in acid solutions. More recently, AVESTON, ANACKER, and JOHNSON (2) report ultracentrifuge studies of molybdic acid in hydrochloric acid medium. They find a degree of polymerization of about 3 in 0,8 M HCl, but note that equilibrium is not attained. On the other hand they find no aggregation and apparent equilibrium in 6 M HCl, No quantitative results were reported of experiments in HCIO medium. 

The decavanadic ion is the largest isopoly anion formed in water by penta valent vanadium in the presence of alkali or protons. A change of solvent and/or cation may allow formation of other species, some of which exhibit very surprising structures. ... 

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