Ammine nickel salts

The stoichiometry of decomposition of [Ni(NH3)4](NCS)2 was dependent on the method of salt preparation [1126]. Ammonia was lost in three successive steps (—NH3, —NH3, —2 NH3) from the solution-prepared salt, but the first intermediate could not be isolated from the similar reaction of material prepared by heterogenous reaction. The difference in behaviour was ascribed to differences in perfection of the crystallites resulting from the alternative preparative methods. 

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Simple nickel salts form ammine and other coordination complexes (see Coordination compounds). The octahedral configuration, in which nickel has a coordination number (CN) of 6, is the most common stmctural form. The square-planar and tetrahedral configurations (11), iu which nickel has a coordination number of 4, are less common. Generally, the latter group tends to be reddish brown. The 5-coordinate square pyramid configuration is also quite common. These materials tend to be darker in color and mostiy green (12). 

As just stated, rubeanic acid reacts to give colored complex compounds, not only with nickel, but also with cobalt and copper salts. Nevertheless, within certain concentration limits, nickel can be detected in the presence of these metals. Use is made of the different diffusion velocities of the metal ammine salts in thin filter paper, the diffusion velocity of nickel being the greatest. When a drop of an ammoniacal solution of copper, cobalt and nickel salts is placed on paper, or a drop of a neutral solution on paper is held over ammonia, the nickel accumulates in the outer zone of the spot. If a drop of the alcoholic reagent solution is then placed at the side and the drops coalesce, a blue ring of nickel rubeanate forms around the brown to green or brown circle due to the cobalt and copper compoimds. 

Because of ammine formation, when ammonia solution is added slowly to a metal ion in solution, the hydroxide may first be precipitated and then redissolve when excess ammonia solution is added this is due to the formation of a complex ammine ion, for example with copper(II) and nickel(II) salts in aqueous solution. 

Ephraim and Bolle 3 find that the stability of the ammines of general formula [M(NII3) ]R2 depends not only on the central atom but also on the anion. These influences oppose one another, and the stability of the whole molecule is therefore the resultant of the two influences consequently, very little parallelism may appear between analogous compounds. For instance, the temperatures of decomposition of the hexanunino-salts of nickel decrease in the order perchlorate, iodide, bromide, chlorate, nitrate, chloride, sulphate whilst in the ease of the liexammino-salts of zinc, the order for decreasing stability is iodide, bromide, chloride, perchlorate, sulphate, nitrate, chlorate. 

In general it is found that salts of strong acids yield more stable ammines than do those of weak acids, and in most cases the stability of the amminc is parallel with the stability of the corresponding ammonium salt. For instance, both hexammino-nickel iodide, [N NH JIj, and ammonium iodide decompose at higher temperatures than hexammino-nickel bromide, [Ni(NH3)0]Br2, and ammonium bromide. The more stable ammines are also the least soluble in water. 

Although iron, cobalt, and nickel occur in the same triad in Group VIII., the three elements differ considerably in their ability to form addition compounds with ammonia. Iron forms few ammino-salts, most of which are unstable, and its tendency to complex-salt formation of the ammine type appears in the complex cyanides and not in the ammines themselves. 

Cobalt and Nickel Ammines. First add a little, and then an excess of a concentrated ammonia solution to solutions of cobalt(II) and nickel(II) salts. What do you observe Boil the ammonium solution of the cobalt salt while shaking the test tube (for what purpose ). Explain the changes in its colour. Write the equations of the reactions. 

Nickel(ii) Complexes.—Ammine, Amine, and Related Ligands. [Ni(NH3)g](l3)2 has been prepared by dissolving a nickel(ii) salt in aqueous ammonia and adding iodine and a five-fold excess of The reaction of hydrazine hydrate with... 

By the lake method 0.1 y aluminum may be directly detected in the presence of 0.014 mg of cobalt or nickel. Larger amounts of these metals lower the sensitivity, partly by lake formation, and partly because of the formation of colored ammine salts. Under such conditions, the following procedure should be used. 

In the same way, 0.032 y nickel can be detected in the presence of 4800 times the amount of iron. The iron is fixed in the center of the fleck as Fe hydrous oxide, while the nickel ammine salt diffuses outward and reacts with rubeanic acid in a separate zone. 

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