Lacunary anion

Among the compounds listed in Table 5 that would benefit from additional investigation are those involving lacunary anion isomers known to be metastable (entries 13-15, 22, 24, but see entries 10 and 11 in Table 4) and complexes of transition metal-centered lacunary anions (17-21). Mixed metal species (3, 10) even if homogeneous, inevitably exist as complex mixtures of isomers, as revealed by observation of multiple NMR signals (Shan et al., 1995). 

Ce4(0H2)9(0H)2(P2Wi6059)2] " [28] is a cluster with C2v virtual symmetry, in which four cerium ions are sandwiched between two novel lacunary anions [P2W16O59]... 

The lacunary anions [PWnOsg] ", [SiWn039] " and (a2-) [P2Wi706i] °" were first shown to form complexes with a variety of lanthanide cations (Ln POM =1 1 and 1 2) by Peacock and Weakley in 1971.[41] At that time only the 1 2 complexes could be isolated, and the ai isomer of [P2Wi706i] ° had not been discovered. The first actinide (tetravalent Th, U) analogs of the Peacock-... 

Reaction of the uranyl cation with the divacant lacunary anion y-[SiWio036] (Figure 5b) yields two species in approximately equal proportions according to Si- and W-NMR spectra. Crystals of one of these complexes have been structurally characterized as the tetrameric assembly [(M(H20))4(U02)4( i-OH)2(SiWio036)4] (M = Na, K ) shown in Figure 11. [53]... 

Sandwich-type Structures Based on Trivacant Lacunary Anions 651... 

The main lacunary versions of Keggin- and Dawson-type structures that can be isolated as pure salts, and that have kinetic stability in solution, are summarized in Table 12. Some molybdate analogues have been isolated160 or otherwise identified in equilibrium studies.19,161,162 All these lacunary anions have structures in which no MOe octahedron has more than two (cis) terminal oxygens, and are convenient synthetic precursors of substituted plenary anions and functionalized species. 

Table 12 Some isolable lacunary anions derived from Keggin- and Wells-Dawson-type structures.

Table 13 Plenary structures derived from A-type trivacant lacunary anions.

A large number and variety of polytungstate complexes incorporate pairs of trivacant lacunary anions that sandwich three or four metal atoms. Three types of lacunary anions involved in such sandwich structures are shown in Figure 12. 

The divalent metal derivatives of the A-type sandwich complexes (Table 14) are readily converted, e.g., by boiling aqueous solutions, to new sandwich species with the general formula [(XW9034)2M4(H20)2] that incorporate B-type lacunary anions. The first example of this type of anion, with X = P and M Co", had been reported in 1973212, but the range of heteroatoms... 

A closely related series of sandwich complexes (also shown in Table 15) is formed with the anions derived from the Wells-Dawson structure, [X2Wi5056]12 (X = P, As), since these possess a lacunary surface identical to that of B-[PW9034]9-. Since the precursor lacunary anions are easily isolated, the synthesis of the sandwich complexes is straightforward. Although several Baker Figgis-type isomeric possibilities exist for anions of this composition,233 until recently all confirmed structures have been found to be identical. The exception is [(P2W15056)2FeIII2Na2(H20)2]16-, which has a different connectivity of the lacunary components.235... 

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