Question

Give one example for each, write names of isopoly and heteropoly anions.

Answer 1

Answer:

Isopoly and heteropoly anions

The amphoteric metals of groups VB (vanadium, niobium, and tantalum) and VIB (chromium, molybdenum, and tungsten) in the +5 and +6 oxidation states, respectively, form weak acids that readily condense (polymerize) to form anions containing several molecules of the acid anhydride. If these condensed acids contain only one type of acid anhydride, they are called isopoly acids, and their salts are called isopoly salts. The acid anhydrides also can condense with other acids (e.g., phosphoric or silicic acids) to form heteropoly acids, which can form heteropoly salts. The condensation reactions, which occur reversibly in dilute aqueous solution, involve formation of oxo bridges by elimination of water from two molecules of the weak acid. The best-known and simplest example is the condensation of yellow chromate ion (CrO42−) to form the orange isopoly dichromate ion (Cr2O72−), an equilibrium reaction the extent of which depends on the pH. In acidic solution the isopoly anion Cr2O72−, predominates while in basic solution the simple ion CrO42− predominates.Heteropoly acids and their salts may be formed by coordination of the central atom with four to six oxo anions, which may be mononuclear (containing one metal ion each), as in H7[P(MoO4)6], or trinuclear (containing three metal ions each), as in H3[P(W3O10)4]. Incomplete replacement of oxygen atoms in PO43− ions by MoO3 groups can result in dimers (two-molecule polymers), as, for example, {OP[O(MoO3)3]3}26−. About 70 elements can act as central (hetero) atoms in heteropoly anions. Because each element may form more than one heteropoly anion and some of these anions can contain several different heteroatoms, thousands of heteropoly acids exist. Heteroatoms may be primary (these atoms are essential to the polyanion structure and thus not susceptible to chemical exchange) or secondary (these atoms can be removed by chemical reaction from the polyanion structure without destroying it). Heteropoly anions can be regarded as coordination compounds with polyanion ligands; e.g., [(H3N)5Cr(OH2)]3+ can be considered the parent of [(SiW11O39)Cr(OH2)]5−.