complex compounds; acids or salts containing a complex anion (polyanion) formed by acid oxides in which the oxygen is partly replaced by the same kind of acid oxide.
Examples of isopoly compounds include pyrosulfuric acid (H2S2O7) and potassium dichromate (K2Cr2 O7), the coordination formulas of which are, respectively, H2[SO3(SO4)] and K2[CrO3(CrO4)]. Isopoly compounds are known mostly for anions containing sulfur, tantalum, niobium, chromium, molybdenum, tungsten, and uranium. X-ray diffraction has shown that the polyanions of isopoly compounds can be represented by short chains of tetrahedra or octahedra, with common edges or ver-texes. Isopoly compounds are prepared either by melting normal salts with acid oxides (for example, Na2WO4 + WO3 = Na2W2O7) or by acidifying aqueous solutions of neutral salts (2K2CrO4 + H2SO4 = K2Cr2O7 + K2SO4 + H2O). Isopoly compounds are used in analytical chemistry.
REFERENCESGrinberg, A.A. Vvedenie v khimiiu kompleksnykh soedinenii, 3rd ed. Moscow-Leningrad, 1966.
Cotton, F., and G. Wilkinson. Sovremennaia neorganicheskaia khimiia, part 3. Moscow, 1969. (Translated from English.)