Cobalt ammines, decompositions

This experiment begins a series of analytical investigations that will be used to confirm the product composition of your cobalt ammine coordination compound. We begin by determining the percentage composition by mass of cobalt in the coordination compound synthesized in Experiment 2.1. The first step involves quantitative decomposition and reduction of the cobalt(III) complex into the cobalt(II) ion. The decomposition of pentaammine-chlorocobalt(III)chloride [Co(NH3)5Cl]Cl2 is shown as an example in equation (2.6). 

The color and solubilities of these chromium(III) ammines are very similar to those of the corresponding cobalt(III) complexes.5 The chlorides, bromides, nitrates, and perchlorates in the acidopentamminechromium(III) series are not very soluble in water, whereas the analogous aquopentamminechromium(III) salts are soluble. In aqueous solutions, these chromium(III) ammines are much more readily decomposed than the corresponding cobalt(III) ammines. Decomposition may be perceptible within a few minutes. The absorption spectra of aqueous solutions of some acidopentamminechromium(III) salts have been studied.6,7... 

Hexammino-cobaltous Chloride, [Co(NH3)6]C12, is the most stable of the three ammines of cobaltous chloride and may be prepared in aqueous solution. If ammonia gas be passed into a concentrated aqueous solution of cobaltous chloride the greenish precipitate at first formed dissolves in excess of ammonia in absence of air, giving a red solution. From the liquid, on standing, pale red octahedral crystals of pentammino-cobaltous chloride separate. The crystals are stable in absence of air but lose ammonia if kept over sulphuric acid. On heating to 120° C. the substance loses four molecules of ammonia and is transformed into diammino-cobaltous chloride. It is soluble in aqueous ammonia without decomposition and insoluble in alcohol. With platinous chloride it forms a double salt, [Co(NH3)3]PtCl4.3 Cobaltous... 

The ammines of cobalt(II) are much less stable than those of cobalt(III) thermal decomposition of [Co(NH3)6]Cl2 is characterized by reversible loss of ammonia, whereas that of [Co(NH3)6]Cl3 is not. In his classic dichotomy of complexes, Biltz regarded [Co (NH 3)3] Cl 2 as the prototype of the normal complex and [Co(NH3)6]Cl3 as that of the Werner or penetration complex. Hexaamminecobalt-(II) chloride has been prepared by the action of gaseous ammonia on anhydrous cobalt (II) chloride or by displacing water from cobalt(II) chloride 6-hydrate with gaseous ammonia. It may also be synthesized in nonaqueous solvents by passing dry ammonia through solutions of cobalt(II) chloride in ethanol, acetone, or methyl acetate. Syntheses in the presence of water include heating cobalt(II) chloride 6-hydrate in a sealed tube with aqueous ammonia and alcohol and the treatment of aqueous cobalt(II) chloride with aqueous ammonia followed by precipitation of the product with ethanol. The latter method is used in this synthesis. Inasmuch as the compound is readily oxidized by air, especially when wet, the synthesis should be performed in an inert atmosphere. 

The black chloride is a lustrous crystalline solid. It is stable when perfectly dry. It cannot be dissolved in w ater (even at 0°) without decomposition to yield basic cobalt(II) chlorides. Treatment with concentrated hydrochloric acid yields [CoCU] " in solution, and neither the red isomer nor the chloropentaammine chloride can be obtained. If air is allowed to come in contact with basic solutions of the compound, oxidation to cobalt (III) ammines occurs. Treatment with concentrated ammonia solution (in a nitrogen atmosphere) sdelds [Co(NH3)6]. ... 

Diindenyl cobalt, Co (09117)2, may be produced by the interaction of indenyl potassium and the ammine [Co (NH ) 4] (SON) 2 in liquid ammonia (87). It forms black lustrous crystals which can be sublimed without decomposition they slowly sinter at 160° and melt rather gradually at about 180°. The compound is very soluble in benzene, ether, and alcohol but less soluble in petroleum ether, forming brown solutions. It is considerably less sensitive to oxidation than the dicyclopentadienyl, Co (05115)2, but powerful oxidizing agents convert it into the yellow [00(09117)2]+ ion (87), which may also be obtained by a Grignard reaction (151). 

As examples of series of related reactions, compensation effects have been described [53] for the thermal decompositions of [CoXj (aromatic amine)2] type complexes (7 reactions) and also for a series of cobalt (III) and chromium (III) complexes (22 compounds studied in which two compensation trends were identified). Later work [54] examined the dehydrations and deamminations of dioximine complexes (two compensation trends identified), and [Co(NCS)2(ammine)2]-type complexes (three compensation trends identified). The systems involving larger entropy changes required less energy for activation [53]. Separate compensation plots for the dehydrations and the decompositions of eleven alkali and alkaline-earth metal dithionates were described by Zsako et al. [55]. 

Zheng et al. [20] showed that KI decreases the temperature and diminishes the activation energy for the decompositions of five cobalt(III) ammine chloride coordination compounds containing the cations [Co(NH3)sX], where X=N3, NOj, Cr, C03 or NH3. The electron transfer step to Co " is regarded as exerting an important control on the decomposition. 

Joyner [76], reviewing a series of comparable kinetic studies of four solid cobalt(III) ammine azides, discussed the factors that determine the two alternative decomposition processes that yield either product CoN or cobalt(II) complexes. He concluded that the mechanisms are "independent of the nature of the cation, the nature of the salt and properties dependent on crystal structure." From a very careful appraisal of the evidence, he concluded that the course of the reaction is determined by "the very early interplay between the CoN and cobalt(II) reactions." This conclusion emphasizes the subtlety of the factors controlling decomposition behaviour. 

Attempts at identifying the influence of structme on stability have generally been inconclusive. For example, some alkali metal permanganates with comparable stmctures show similarities of decomposition behaviour [29], while, in contrast, the decompositions of several cobalt(lll) ammine azides show little evidence of structural influences [76], Significant differences in behaviour were found for the various crystal forms of the LiK tartrate hydrates [87] and, also, for the dehydrations of the isomorphic alums [20,43], However, some reactants, for example those prepared by the dehydration of hydrated metal carboxylates [5], may be amorphous to X-rays, thus preventing recognition of any control of stability by crystal structure. 

A heated temperature-programmable IR cell was used by Tanaka et al. (61) to study the thermal decomposition of a number of cobalt(III) ammine complexes. The disk matrix material was either KCl or KBr, and it was reported that they frequently became opaque to infrared radiation at elevated temperatures. A heated IR cell for use up to 200 C was also described by LeRoux and Montano (62). 

The photochemical decomposition of cobait(m)-ammine-oxalate complexes involves primarily redox processes, as does that of the cis -[Co(bipy)2Cy+ cation.The quantum yield for the photoredox aquation of [Cofphen) is very low - lower even than those for cobalt(m)-amine complexes. The quantum yield for photoreduction of [Co(fra/M-[14]-dieneX l2l is similar to that for photoreduction of [Co(tetren)Cl] + both of these quantum yields are much less than that for the photoreduction of [Co(NH3)5Cl] +. Thermal back-reactions are important in determining the overall kinetic pattern for the photoreaction of the [Co(tra s-[14]-diene)Cl2l+ cation. ... 

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