Vokus Unfortunately, our editorial approach may not be able to accommodate all contributions. 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 eachas in H 7 [P MoO 4 6 ], or trinuclear containing three metal ions eachas in H 3 [P W 3 O 10 4 ]. Interactions between two hard or soft acids or bases are stronger than ones between one hard and one soft acid or base. In some cases, hydrolysis of a metal ion may be accompanied by polymerization to form dinuclear or polynuclear hydroxo- or oxygen-bridged complexes. A class of substitution reactions that affords the widest possible comparison of different metal ions is the replacement of water in the coordination spheres of metal- aqua complexes in aqueous solution.
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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. 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.
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]. 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. 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, 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. 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.
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.
Difficulties were encountered by this system as well as by the later —21 , more elaborate Miolati-Rosenheim theory. Modern conclusive knowledge of the structures of heteropoly compounds did not begin until , with J. 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. Important types of reactions of coordination compounds Acid-base Coordination to a positive metal ion usually enhances the acidity i. Thus, many metal ions in aqueous solution commonly exhibit acidic behaviour.
The acidities of such aqua ions depend on the charge, size, and electronic configuration of the metal 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. 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. Load Next Page.
JoJobei Your contribution may be further edited by our staff, and its publication is subject to our final approval. Thus, many metal ions in aqueous solution commonly exhibit acidic behaviour. These terms refer to kinetics reaction rates and should not be confused with the thermodynamic terms unstable and stablewhich refer to equilibrium. A heteropoly acid is a class of acid made up of a particular combination of hydrogen and oxygen with certain metals and non-metals. An idopoly of a dissociative reaction pathway for an octahedral complex of cobalt is as follows: In addition to their use in analytical chemistryheteropoly compounds have found use as catalystsmolecular sieves, corrosion inhibitors, photographic fixing agents, and precipitants for basic dyes. Internet URLs are the best. This type of reaction path is typical of octahedral complexes, many aqua complexes, and metal carbonyls such as tetracarbonylnickel.
ISOPOLY AND HETEROPOLY ACIDS AND SALTS PDF
Taudal Page 7 of 8. For example, as mentioned above, most cyanide complexes are extremely stable they possess very small dissociation constants ; yet, if their rate of exchange with carbonlabeled cyanide, as represented in the following equation. Acid-base equilibria are rapidly established in solution, generally within a fraction of a second see chemical reaction. Thank you for your feedback.