Lacunary derivatives

Several deficit or lacunary derivatives of the Keggin structure exist as individual species or as fragments of other heteropolyanion structures. Removal of one M06 octahedron (stoichiometric loss of MO"+) from a- or /3-Keggin anions leads to isomers of [XM11O39]"-. Removal of three adjacent octahedra leads to A- and B-type XM9034 anions. The structures of these are shown in Figure 11. 

The anion has almost D%h symmetry with two A-a-PWgC units (see Figure 11) that sandwich two W05(H20) octahedra and a WOs square pyramid, i.e. the equatorial plane contains two radial H20—W=0 groups and one W=0 group. The structure of [F W O ]14- is currently unknown, but it appears to be a lacunary derivative of P2W21 for it reacts with several divalent metal ions to give P2W19M2 species (see Section 38.4.2.2)., ... 

Several lacunary derivatives of the Dawson structure can be synthesized by controlled alkalinization of aqueous solutions, as shown in Scheme 4. 

FIGURE 28 [l<8 C P8W48O184]40 as an assembly of four P2W12 lacunary derivatives of the... 

Ostuni, A. and Pope, M.T. (2000) A large heteropolytungstotetracerate(III) based on a new divacant lacunary derivative of the Wells-Dawson tungstophosphate anion. Comptes Rendus de VAcademie des Sciences, Paris, Serie IIc Chemie, 3, 199-204. 

Figure 3. Upper [W Oig] in ball-and-stick and polyhedral representation Lower Lacunary derivative [WsOig] ...

The Keggin anion and three lacunary derivatives are illustrated in Figure 4. The trivacant (Wg) species are metastable in solution, but aqueous solutions of the mono vacant (Wn) anions are indefinitely stable in the appropriate pH range. 

The two known isomers of the (a)-Keggin anion with Csv (P) and C2v (y) symmetry (Figure 5a) also have accessible lacunary derivatives, but the only one of relevance to present discussion is the divacant y-[SiWio036] (Figure 5b). 

Figure 4. The Keggin anion a-[PWi204o] and three lacunary derivatives (a) [PWn039] ...

Figure 23 Polyhedral representation of the Lacunary derivatives of the a-Keggin ion left— a-XWn, middle— A-a-XW9, right— B-a-XW9. The metal ions form the centers of the polyhedra and the oxygen atoms form the apexes of the polyhedra. The central heteroatom is shown as a gray sphere.

W5 groups which are lacunary derivatives of [WgOig]. The four units are arranged in an almost tetrahedral arrangement with respect to the central EU3 unit see Figure 32. 

One of the lacunary derivatives of the a-Dawson anion forms crystals of composition, K28Li5H7PgW4gOig4 92H20 ( P8W48, (46)) with the cyclic structure shown in Figure 38. A tetramer of P2W12 groups leads to the D f, symmetry of the structure of the PgW48 anion. The crystal structure reveals that the central cavity of the anion contains a number of potassium cations. 

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