Cyclic Polyanions of Silicon, Germanium, Tin and Lead

The best known examples of polyanions of group 14 elements are the so-called Zintl anions, e.g. (tetrahedral), (trigonal bipyramidal) and 

More recently, a number of polyanions of the group 14 elements that have cyclic structures have been synthesised and structurally characterised. These 

The cyclic group 14 polyanions are usually found in ternary materials that contain two types of cations which, in addition to providing charge balance, perform different roles. The structures typically consist of cyclic polyanions that are stacked in an eclipsed fashion to form columns. One type of cation coordinates to the anions and separates them within the column, whereas the second type segregates the columns. These ternary materials are typically prepared by heating stoichiometric amounts of the three elements in an unreactive metal tube, e.g. niobium, at very high temperatures ( 800°C) followed by slow annealing at lower temperatures. 

As a specific example of these general principles, we will consider the structure of the ternary materials Na8BaM6 (M = Sn, Pb) prepared by direct reaction of the elements at 750 These isostructural compounds 

According to Wade s rules, one non-bonding electron pair is allocated to each metal atom in polyionic metal clusters of the p-block elements. Thus the planar five-membered M5 anions are 16-electron species comprised of two valence electrons contributed by each metal atom and the 6- charge. Since there are eight electron pairs for bonding five cluster atoms, Wade s rules predict an arachno structure ( + 3 electron pairs for n cluster atoms), i.e. a pentagonal bipyramid with two vacant sites. 

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