School of Engineering and Science - Jacobs University

Fig. 1.3: Ball/stick and polyhedral representation of the alpha isomer of the Keggin structure with
different types of oxygen. Color codes: The blue polyhedra represent the tungsten and the red balls
represent oxygen
Keggin unit. The Keggin unit is usually anionic, and is balanced by cations in solid state.
The cations may be protons (typically coordinated with water as [H 5 O 2 ] + or [H 9 O 4 ] + ), in
which case the complex is acidic, and it is identified as a Heteropolyacid. Other typical
charge-balancing cations generally used are alkali metals and ammonium derivatives. As
mentioned above, the tetrahedral heteroatom at the center of the Keggin unit can be any
of a wide array of elemental cations. The most commonly studied structures are those
containing, As V , P V , Si IV , Ge IV as the heteroatom but other transition metals and non
metals have been observed playing this role. Moreover, trigonal-pyramidal (e.g. AsO 3− 3 ,
Figure 1.4) or ditetrahedral (e.g. O 3 POPO 4− 3 ,Figure 1.5) hetero groups as additional
building blocks allows further structural versatility. Furthermore, few types of addenda
atoms have been observed in Keggin structures; typically, the addenda atoms are molybdenum
or tungsten. Vanadium based structures are also reported, but its questionable
whether they form 1:12 heteroatom:addenda complexes; it appears more likely that they
form Keggin-like 1:13 or 1:14 complexes. Clusters with p-block elements (P, Si, Al, Ga and
Ge), transition metal elements (Fe(II/III), Co(I/II), Ni(II/IV), Zn(II)), and even two H +
have been synthesized. This position can be either tetrahedrally coordinated (as in Keggin
and Wells-Dawson anions) or octahedrally coordinated (as in the Anderson structure).
Figure 1.6 shows a [M 6 O 19 ] n− isopolyanion (A) and two heteropolyanions and Figures 1.7
and 1.8 illustrates two different types of heteropolyanions. The [M 6 O 19 ] n− isopolyanion
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