Lacunary polyanions

The number of complexes of this type is legion. The general formula is [XMhZ(L)039]" (or [X2Mi7Z(L)06i]"-) where Z is a secondary heteroatom and L is a terminal ligand on Z. In some cases L is another lacunary polyanion, an organic or organometallic moiety, or is absent. Scheme 5 illustrates some of the possibilities. 

When Z is a simple aquacation, two types of complex are formed depending upon the ionic radius of Z. For alkali, alkaline earth and most transition metal cations the product contains Z"+ in quasi-octahedral coordination. Equilibrium constants for reaction (6) have been determined for Li+, Na+, K+, Mg2+, Ca2+, Sr2+, Ba2+, Mn2+, Fe2+, Co2+, Ni, Cu2+ and Zn2+.93 For the transition metals, log K lies between 3 and 9, and is sensitive both to Z and to the lacunary polyanion involved. Larger cations, Sr24, Ba2+, and tri- and tetra-valent lanthanides and actinides are also able to bind two lacunary ligands in a manner similar to that illustrated in Figure 18. Although the stepwise formation of 1 1 and 2 1 complexes of the... 

Within the last few years there has been considerable activity directed towards the synthesis of organic and organometallic derivatives of heteropolyanions. Reactions of lacunary polyanions with RMC13, where R is an organic or organometallic radical, provide a simple route to such derivatives.97 Some examples of these reactions are presented in Scheme 5. For the most part the reactions proceed smoothly and the products are remarkably stable towards hydrolysis. Apart from their structural characterization via crystallography and NMR spectroscopy the chemistry of these complexes has not been completely described. 

The introduction of organic and organometallic moieties into lacunary polyanion structures has been described in Sections 38.4.2.1 and 38.4.2.2. Other organic derivatives of polyanions are discussed in the following sections. 

The dodecatungstosilicates are stable in acid solution. When the pH increases, hydrolytic cleavages of W—O bonds occur, leading to well-defined lacunary polyanions, with 11, 10, or 9 tungsten atoms. 

The lacunary polyanions can act as ligands with numerous metal cations, leading to mono-, di-, or trinuclear complexes according to the number of vacant sites. These complexes are the earliest model materials of mixed oxides, and they are important for catalytic purposes, particularly for the oxidation of organic substrates. 

Fig. 1. Genera) route for synthesis of dodecatungstosilicates and related lacunary polyanions.

This lacunary polyanion is used to synthetize adduct complexes [y-SiWioM204o]" , especially with V, Mo, W, and di- or trivalent cations of the Arst transition series. 

Recent years have seen the reports of increasing numbers of very large polyoxometalate anions. These may be divided into two classes (i) composite anions constructed of assemblies of simpler, usually lacunary, polyanions and (ii) mixed-valence polymolybdate anions which comprise common building blocks, e.g., Mo8, Mo6, and Mo2 clusters that have no separate, stable existence. 

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