Metal ammine compounds

Decomposition of the metal ammines have probably been most extensively investigated. Some qualitative features of the thermal decomposition of metal ammine compounds are conveniently illustrated [1116— 1118] by the somewhat contrasting behaviour of the compounds [Cr(NH3)6]X3 and [Co(NH3)6]X3 where X is Cl- or Br . During decomposition of the chromium compound, the oxidation number of the metal remains unchanged, viz. 

Reaction involves more than a single step. The relative thermal stabilities of the hexammine salts, as determined by the temperatures of onset of the reaction, are Cl Br for the cobalt compounds but Br Cl I for the chromium compounds. When X = NOi, rapid oxidation developed into an exothermic explosive reaction of both the Cr and the Co salts. 

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This yellow, diamagnetic complex provides one of the few examples of metal hydrido compounds which contain only saturated ammine ligands. Osmiuin(v).—Further Os -arylimido-complexes have been synthesized via the reaction of phosphinimines with [OsOCl3(PPh3)2] ... 

Mixed valency occurs in minerals (e.g. 63 ), metal-chain compounds, dimers and oligomers and metal complexes, and even in organic and biological systems (Brown, 1980 Day, 1981). Among the dimeric and oligomeric metal complexes exhibiting mixed valency, the pyrazine-bridged Ru (II, III) ammine complex. 

In seeking for an explanation of the fact that certain molecules which were saturated, according to old valency theory, still possessed the power to combine with other molecules, Alfred Werner,15 in 1S91, introduced his co-ordination theory which, altered and added to as it has been developed, he used to explain the existence of complex salts, and especially the large class of compounds known as the metal-ammines. It does not take into account the internal structure of the atoms linked together, but is concerned with the combining capacity... 

In these formulae it is assumed that the (NH3) group has a capacity for forming homologous chains much like the methylene (CII2) group in the carbon compounds. The metal-ammines were represented, therefore, as built up of chains of ammonia molecules. By adopting such formulae he recognised that two different forms of chain formation are possible just as in the case of the carbon compounds, namely, the normal form and the iso or branched form. Thus ... 

Reitzenstein (Zeitsch. anorg. Ghem., 1898,18,152) has given a very complete account of the older theories and formulae of the metal-ammines, and has traced the development of those compounds up to the position at that time. 

That this reaction does not take place in the metal-ammines was proved in the following way.2 A tertiary amine was chosen, namely, pyridine, in which no labile hydrogen is possible, and therefore no forked chain could be formed if the compound were united with metal salt. It was found that when pyridine and the metallic salt are allowed to interact, metal-pyridine compounds are obtained analogous in every way to the metal-ammines. It seemed probable, therefore, that the metal-ammines do not contain the (NH4) group. 

Nomenclature.—A definite system of nomenclature has been adopted for metal-ammines, and the old names derived from the colour of the compounds have, in the main, been abandoned, as they lead to confusion. The system adopted is that suggested by Werner. Ammonia molecules present in the complex are indicated by the terms monam-mino, diammino, triammino, tetrammino, pentammino, hexammino, the word ammino, written with a double m, having been adopted to show the relationship with ammonia and to distinguish from the ammonium salts and the substituted ammonia compounds, namely, the amines. 

Formation of Metal-ammines.—The compounds may be formed in several ways, such as treating the dry metal salts with ammonia gas until absorption is complete by treating the salts with liquid ammonia or from an aqueous solution of the salt and aqueous ammonia. The method of formation depends on the stability of the amminc. In some eases the aminine has not been isolated in the solid state, and its presence in solution is only indicated by the behaviour of the liquid. 

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 ... 

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. 

As the various theories of the constitution of the metal-ammines have been discussed,1 only the constitutional formulae for these compounds, adopted at the present time, will be used. 

These are not, strictly speaking, members of the metal-ammines, inasmuch as all ammonia is replaced by acid groups, but they are the ultimate complex compounds arrived at in the systematic replacement of ammonia, and must therefore be briefly discussed at this stage. 

The best known of these compounds is potassium cobalti-nitrite, [Co(N02)6]K3.1 This salt was originally regarded as a double salt of cobaltic nitrite with potassium nitrite, and represented by the formula Co(NQ2)3.3KNOa. Such a formula, however, does not represent the reactions of the substance, as the nitrite radicle is held firmly, and nitrous aeid is not liberated when the compound is treated with cold dilute acids, as it would be if it were a double salt as the formula indicates. Molecular conductivity measurements also indicate that it is a complex salt comparable with the metal-ammines. Many compounds of cobalt of this type are known. They may be regarded as the salts of the complex acid hexanitrito-cobaltic acid, [Co (N02)6]H3. 

According to Lebeau,2 arsine does not enter into the composition of compounds analogous to the metal-ammines, and the latter in ammoniacal solution react with arsine to produce arsenides of the metals. 

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. 

According to some authorities, the first metal ammine to be isolated in the solid state was the reddish yellow hexaamminecobalt(III) oxalate [Co(NH3)6]2(C204)3, described in 1822 by Leopold Gmelin (1788-1853).16 Gmelin also discovered several new double salts potassium ferricyanide, or potassium hexacyanoferrate(III), K3[Fe(CN)6l>17 the cobalticyanides or hexacyanocobaltates(III), M3[Co(CN) ],18 and the platinocyanides or tetracyanoplatinates(II), M2[Pt(CN)4].19All these substances certainly deserve to rank among the earliest known coordination compounds. 

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). 

The next major theory of metal ammines was proposed by Carl Ernst Claus (1796—1864). In 1854, Claus rejected the ammonium theory and suggested a return to Berzelius view of complexes as conjugated compounds. He compared the platinum ammines not with ammonium salts nor with ammonium hydroxide but with metal oxides. He designated the coordinated ammonia molecule as passive, in contrast to the active, alkaline state in the ammonium salts, where it can easily be detected and replaced by other bases . 

G. B. Kauffman and I. Bernal, Overlooked opportunites in stereochemistry, part 2 the neglected connection between Werner s metal-ammines and Pope s organic onium compounds , J. Chem. Educ., 1989, 66, 293-300. 

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