The structural study of heteropolyanions began in early thirtieth, but still many unsolved problems remain. The present review begins with a short historical glance upon early synthetic works by Marignac in nineteen century and theoretical structural elucidation by Pauling in 1929.
The Section 2 contains nomenclature of heteropolyanions and explanation, by the aid of polyhedra model, of representative polyanions including well known Keggin structure PW
12O
403-, P
2W
18O
626-, partially hydrolyzed polyanions, SiW
11O
408-, P
2W
17O
6110, 9-heteropolyanion, H
3PMo
9O
31 (OH)
33-, 5-molybdo-2-phosphate anion P
2Mo
5O
236-, 12-molybdo-4-arsenate anion As
4Mo
12O
508-. Formation conditions of arsenate and molybdate containing polyanions are discussed (see Table 5) . Another type is “substituted”polyanions, 12-and 18-heteropolyanions in which one or two Mo or W atoms are replaced by divalent or trivalent transition metal cations.
Above mentioned polyanions contain the heteroatoms in tetrahedral oxygen coordination, MO
4.
But there are many heteropolyanions in which the heteroatoms are octahedrally coordinated by oxygen atoms. Examples shown are Co
2Mo
10O
3810- NiV
13 O
387- and MnMo
9O
326-.
The next section is devoted to explain recently solved structural problems of the geometrical isomers of 12-and 18-heteropolyacids. The Keggin structure SiW
12O
404- can have 4 isomers, α (Keggin structure itself), β, γ, δ, and e shown in Fig. 16. The crystal structure determinations of β-K
4SiW
12O
409H
2O and α-Ba
2SiW
12O
4016H
2O have shown that these structures correspond to β and α in Fig. 16. These five isomer models are successively made by 60° rotation of W
3O
13 unit in the Keggin structure.
In the last section, as two representative cases of crystal structure of heteropolyanions, Na
6Mo
5P
2O
23⋅13H
2O and α-Ba
2SiW
12O
40⋅16H
2O are illustrated.
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