Metal Ammine Chlorides

All reactions are exothermic, but reaction enthalpies are different [96]. Using the Van t Hoff equation, the releasing temperature for NH3 at a constant reservoir pressure of 1.5 bar can be calculated. 

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Ionisation Isomerism.—This type of isomerism is very common in the metal-ammines. If two or more different acidic radicles are present in a molecule of metal-ammine, the acidic radicles may be firmly fixed in the co-ordination complex or may be outside of this. If they are outside the complex they are easily ionised and easily freed by other acids if, on the other hand, they are within the complex they are not ionised and are difficult to free by other acids. This distribution of the acidic radicles in the complex, or outside of it, gives rise to ionisation isomerism. Tor example, the compound Co(NH3)5Br(S04) is known in two forms, one violet and the other red. The violet modification in aqueous solution contains —S04" ions, which may be precipitated by barium chloride. The red variety gives no — S04" ions in aqueous solution, and barium sulphate is not precipitated by barium chloride. These two substances, bromo-pentammino-eobaltic sulphate and sulphato-pentammino-eobaltic bromide, are ionisation isomers, and are represented as ... 

Zinc halides unite with ammonia, readily forming a number of metal-ammines containing one, two, four, five, or six molecules of ammonia for every molecule of zinc salt. The ammino-derivatives of the chlorides are best known, although some of these described appear to be of doubtful individuality. 

The ammonolysed compounds or ammonio bases are formed when a solution of mercuric salt is treated with slight excess of ammonia. This does not give an addition compound such as metal-ammine, but a substituted derivative. For instance, from mercuric chloride infusible precipitate Hg(NH2)Cl is formed. The same substance may be produced from diammino-mercuric cliloride if an excess of ammonia be present after fusible precipitate is formed. 

Most authorities attribute the discovery of the first metal ammine to Tassaert, a Parisian chemist about whom virtually nothing is known — not even his first name. In his short article12 he is identified only as Citoyen Tassaert — Citizen Tassaert. Some chemists imply or even openly state that Tassaert was the first to prepare hexaamminecobalt(III) chloride, [Co(NH3)6]C13, the parent compound from which all cobalt ammines may be considered to be derived. Yet he merely observed the brownish mahogany color of the solution formed when excess aqueous ammonia is added to a solution of cobalt chloride or cobalt nitrate, and he failed to follow up his accidental discovery. 

As the number of known coordination compounds increased, theories to explain their constitution were devised.35 Thomas Graham (1805-1869) is credited with originating the first theory of metal ammines, the so-called ammonium theory, in which metal ammines are considered as substituted ammonium compounds. Graham attempted to explain the constitution of compounds such as diamminecopper(II) chloride by the formula (1). 

Friend88 specifically criticized Werner s theory and emphasized the differences between his own latent valencies and Werner s Nebenvalenzen. He applied his new theory of valency to ammonium salts, metal ammines and halide double salts , and he proposed that for hexacoordinate central atoms a hexatomic shell forms around the metal but that all the elements or groups are joined together by latent valencies and not necessarily to the metal itself. In 1916, Friend applied his cyclic theory of complexes in detail to the structures of the chlorides of hexaamminecobalt(III), chloro-pentaamminecobalt(III) and dichlorotetraamminecobalt(III) and emphasized four basic differences between his theory and Werner s.89... 

As summarized in Table 1, hollow Si02 nanocomposites could be formed only in the cases of ammine complex precursors, and non-hollow ones were formed in the cases of chloride precursors. The nucleation and crystal growth of metal ammine complexes in the reversed micelle may be responsible for the... 

In the remainder of this paper, extensions of the SEA method will be made to other noble and base metal ammines on silica, Pt chlorides on alumina, and both types of complexes over carbon. It will further be extended to achieve metals selectively adsorbed and highly dispersed onto a supported promoter, and to bimetallic catalysts. 

But Kekuld s stability criterion, or to be more accurate, instability criterion failed completely in the case of many coordination compounds, especially the metal-ammines, which were classified as molecular compounds by sheer dint of necessity even though they were extremely resistant to heat and chemical reagents. For example, look at Figure 1. Although hexaamminecobalt(III) chloride contains ammonia, it neither evolves this ammonia on mild heating nor does it react with acids to form ammonium salts. Also, despite its cobalt content, addition of a base to its aqueous solution fails to precipitate hydrated cobalt(III) hydroxide. It remained for Alfred Werner to explain successfully the constitution of such compounds, but the time was not yet ripe. Before considering Werner s coordination theory, we must examine one more theory of coordination compounds, perhaps the most successful of the pre-Wemer theories, namely, the Blomstrand-J0rgensen chain theory. 

Direct Titrations. The most convenient and simplest manner is the measured addition of a standard chelon solution to the sample solution (brought to the proper conditions of pH, buffer, etc.) until the metal ion is stoichiometrically chelated. Auxiliary complexing agents such as citrate, tartrate, or triethanolamine are added, if necessary, to prevent the precipitation of metal hydroxides or basic salts at the optimum pH for titration. Eor example, tartrate is added in the direct titration of lead. If a pH range of 9 to 10 is suitable, a buffer of ammonia and ammonium chloride is often added in relatively concentrated form, both to adjust the pH and to supply ammonia as an auxiliary complexing agent for those metal ions which form ammine complexes. A few metals, notably iron(III), bismuth, and thorium, are titrated in acid solution. 

A few examples will illustrate the use of those terms. Chloro-pentammino-cobaitic chloride is written as [Co(NII3)5.Cl]Cl2, nitrito-aquo-tetrammino-eobaltic chloride as [Co(N I3) (NH3),(N02).n20 (. l2, and chloro-aquo-tetrammino-eobaltic chloride as [Co(NI13)4C1(1120)JC1 2. It will be observed that the co-ordination number in each of the complexes is six, and this is characteristic not only of cobalt-ammines, but also of many other complexes. Some metals, however, yield complexes with co-ordination number four. For example, tetrammino-cuprio sulphate, [Cu(NH3)1]S04, and tctrammino-platinous chloride, [Pt(NH3)4]Cl2, belong to this class. On the other hand, the co-ordinatioi i number of hexannnino-platinic chloride, [Pt(NIl3)0]Cl. is six, but the central metallic atom is tetravalent, hence there are four chlorine atoms in the outer dissociable zone. 

Diammino-silver nitrate forms glistening rhombic or prismatic crystals which blacken on exposure to light. It is fairly stable, and may be heated to 100° C. without loss of ammonia. Further heating causes it to decompose, and finally to melt with evolution of nitrogen and ammonia, leaving a residue of metallic silver and ammonium nitrate. It is soluble in water, but partial dissociation takes place so that the solution is alkaline in reaction, and it therefore yields a precipitate of silver chloride and soluble chlorides. Certain of the metals—for example, zinc, cadmium, and copper—quickly reduce the ammine in solution to metallic silver. 

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