Structure, Synthesis, Stability, and Characterization

Heteropolyanions and isopolyanions are polymeric oxoanions (polyoxometalates) (2, 3, 5, 6). The structure of a heteropolyanion or polyoxoanion molecule itself is called a primary structure (5, 6, 77). There are various kinds of polyoxoanion structure (Section II.A. 1). In solution, heteropoly anions are present in the unit of the primary structure, being coordinated with solvent molecules and/ or protonated. Most heteropolyanions tend to hydrolyze readily at high pH (Section 1I.C). Protonation and hydrolysis of the primary structure may be major structural concerns in solution catalysis. Heteropoly compounds in the solid state are ionic crystals (sometimes amorphous) consisting of large polyanions, cations, water of crystallization, and other molecules. This three-dimensional arrangement is called the secondary structure. For understanding catalysis by solid heteropoly compounds, it is important to distinguish between the primary structure and the secondary structure (5, 6, 17). Recently, it has been realized that, in addition 

TOSHIO OKUHARA, NORITAKA M1ZUNO, AND MAKOTO MISONO 

The known addenda and heteroatoms incorporated in heteropolyanions are summarized in Table II (20). The structures in this table of polyanions with Se(IV), Te(lV), Sb(IIl), Bi(III), Ti(IV), and Zr(IV) still need to be con-finned, since tetradendral coordination of these ions with oxide ions is seldom observed (2). 

Known Addenda- D) and Hetero-(0) Atoms Incorporated in Heteropolyacids 

Compounds containing fluoride ions in metatungstate have been synthesized (27) (A) (Wl2F2038H2)H4, (B) (Wi2F3037H)H4, (C) (W12038F2H)H5, and (D) (W12039FH2)H5, where the central atoms are protons. 

---