Polyanions lacunary ligands

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... 

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. 

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. 

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