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⁻.

A variety of synthetic procedures are available for the preparation of isopoly acids and salts, which are usually less stable than heteropoly compounds. Heteropolymolybdates and heteropolytungstates are always prepared in solution, usually after acidifying and heating the theoretical amounts of reactants. In general, free heteropoly acids and salts, of which the heteropolymolybdates and heteropolytungstates are the best known, have very high molecular weights (some above 4,000) as compared with other inorganic electrolytes, are very soluble in water and organic solvents, are almost always highly hydrated with several hydrates existing, and are highly coloured. Some are strong oxidizing agents that can be reduced to stable, intensely deep blue species (heteropoly blues), which in turn can act as reducing agents, restoring the original colour on oxidation. The stoichiometry, oxidation-reduction potentials, and other characteristics of these reactions have been investigated by various methods. The free acids, which are polyprotic (contain several replaceable hydrogen ions), are fairly strong and nearly always stronger than the corresponding acids from which they are derived.

All heteropolymolybdate and heteropolytungstate anions are decomposed in strongly basic solution to form simple molybdate or tungstate ions and either an oxy anion or a hydrous metal oxide of the central metal atom, e.g.:

[P₂Mo₁₈O₆₂]⁶⁻ + 34OH⁻ -----> 18MoO₄²⁻ + 2HPO₄²⁻ + 16H₂O [NiW₆O₂₄H₆]⁴⁻ + 8OH^– -----> 6WO₄²⁻ + Ni(OH) ₂ + 6H₂O

Throughout specific ranges of pH and other conditions, most solutions of heteropolymolybdates and heteropolytungstates appear to contain predominantly one distinct species of anion, many of which are remarkably stable and nonlabile.

The first heteropoly compound, (NH₄)₃[PMo₁₂O₄₀], was obtained by the Swedish chemist Jöns Jacob Berzelius in 1826 as a yellow, crystalline precipitate, the formation of which is still used for the classical qualitative detection and quantitative estimation of phosphorus (after conversion to phosphate). By the beginning of the 20th century, hundreds of isopoly and heteropoly compounds were reported, many of which were based on incorrect analyses or failure to detect mixed crystals. Formulas were reported in terms of the old Berzelius dualistic theory as a combination of oxides, such as 3Na₂O∙Cr₂O₃∙12MoO₃∙20H₂O for Na₃CrMo₆O₂₄H₆∙7H₂O, and often merely expressed analytical results rather than structure. In addition to their use in analytical chemistry, heteropoly compounds have found use as catalysts, molecular sieves, corrosion inhibitors, photographic fixing agents, and precipitants for basic dyes.

Few structural studies of such compounds were carried out, but this lack did not prevent the elaboration of various unsuccessful theories to account for their structures. In 1907 Werner applied his coordination theory to the structure of 12-tungstosilicic acid, H₄ [SiW₁₂O₄₀], and its salts by assuming that the central group is an SiO₄⁴⁻ ion surrounded octahedrally by six RW₂O₆⁺ groups (R = a unipositive ion), four linked by primary (ionic) and two linked by secondary (coordinate covalent) valences. Difficulties were encountered by this system as well as by the later (1910–21), more elaborate Miolati-Rosenheim theory. Modern conclusive knowledge of the structures of heteropoly compounds did not begin until 1934, with J.F. Keggin’s determination of the structure of H₃ [PO₄W₁₂O₃₆]∙5H₂O by the most direct means, X-ray diffraction.

The structures of isopoly and heteropoly compounds consist of polyhedrons sharing corners and edges with one another. In heteropolymolybdates or heteropolytungstates, each molybdenum or tungsten atom is located at the centre of an octahedron, each vertex of which is occupied by an oxygen atom. These octahedrons can share corners or edges or both with other MoO₆ or WO₆ octahedrons. In [Mo₈O₂₆]⁴⁻ eight MoO₆ octahedrons share edges. In [PMo₁₂O₄₀]³⁻ the central phosphorus atom is located at the centre of a PO₄ tetrahedron, which is surrounded by 12 MoO₆ octahedrons, which share corners so that the correct number of oxygen atoms is utilized.

Acid-base

Coordination to a positive metal ion usually enhances the acidity (i.e., the tendency to lose protons) of hydrogen-containing ligands, such as water and ammonia. Thus, many metal ions in aqueous solution commonly exhibit acidic behaviour. Such behaviour is exemplified by hydrolysis reactions of the type shown in the following equilibrium:

[M(H₂O)_x]ⁿ⁺⇌ [M(OH)(H₂O)_{x− 1}]^{(n− 1)+}+ H⁺,

in which M represents the metal ion, n its charge, and x the number of coordinated water molecules.

The acidities of such aqua ions depend on the charge, size, and electronic configuration of the metal ion. This dependence is reflected in the values of acid dissociation constants, which range from about 10⁻¹⁴ (a value only slightly larger than for pure water, for which the dissociation constant = 10^−15.7) for the hydrated lithium ion, to about 10⁻² (a value equivalent to that of a fairly strong acid) for the hydrated uranium(4+) ion. Acid-base equilibria are rapidly established in solution, generally within a fraction of a second (see chemical reaction).

In some cases, hydrolysis of a metal ion may be accompanied by polymerization to form dinuclear or polynuclear hydroxo- or oxygen-bridged complexes.

Even very weakly acidic ligands, such as ammonia, can acquire appreciable acidity through coordination to a metal ion. Thus, the hexaammineplatinum(4+) ion dissociates according to the following equilibrium:

[Pt(NH₃)₆]⁴⁺⇌ [Pt(NH₂)(NH₃)₅]³⁺+ H⁺.

In addition to intrinsic strength, acids and bases have other properties that determine the extent of reactions. According to the hard and soft acids and bases (HSAB) theory, the metal cation and anion are considered to be acids and bases, respectively. Hard acids and bases are small and nonpolarizable, whereas soft acids and bases are larger and more polarizable. Interactions between two hard or soft acids or bases are stronger than ones between one hard and one soft acid or base. The theory can be used to explain solubilities, formation of metallic ores, and some reactions of metal cations with ligands.