Ammines of cobalt

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... 

These hydroxo-salts are all sulphur-yellow crystalline substances. The acid residues are hydrolysable and hence outside the co-ordination complex, and the aqueous solutions, unlike the hydroxo-salts of chromium-and cobalt-ammines, are neutral to litmus, a fact which Werner suggests is due to the smaller tendency of the hydroxo-radicle attached to ruthenium to combine with hydrogen ions. This tendency is much less than in the case of the ammines of cobalt and chromium, but that it still exists is indicated by the increased solubility of these hydroxo-compounds in water acidified with mineral acids, and from such solutions aquo-nitroso-tetrammino-ruthenium salts are obtained thus ... 

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. 

Ma.nufa.cture. Nickel carbonyl can be prepared by the direct combination of carbon monoxide and metallic nickel (77). The presence of sulfur, the surface area, and the surface activity of the nickel affect the formation of nickel carbonyl (78). The thermodynamics of formation and reaction are documented (79). Two commercial processes are used for large-scale production (80). An atmospheric method, whereby carbon monoxide is passed over nickel sulfide and freshly reduced nickel metal, is used in the United Kingdom to produce pure nickel carbonyl (81). The second method, used in Canada, involves high pressure CO in the formation of iron and nickel carbonyls the two are separated by distillation (81). Very high pressure CO is required for the formation of cobalt carbonyl and a method has been described where the mixed carbonyls are scmbbed with ammonia or an amine and the cobalt is extracted as the ammine carbonyl (82). A discontinued commercial process in the United States involved the reaction of carbon monoxide with nickel sulfate solution. 

Ammonia forms a great variety of addition or coordination compounds (qv), also called ammoniates, ia analogy with hydrates. Thus CaCl2 bNH and CuSO TNH are comparable to CaCl2 6H20 and CuSO 4H20, respectively, and, when regarded as coordination compounds, are called ammines and written as complexes, eg, [Cu(NH2)4]S04. The solubiHty ia water of such compounds is often quite different from the solubiHty of the parent salts. For example, silver chloride, AgQ., is almost iasoluble ia water, whereas [Ag(NH2)2]Cl is readily soluble. Thus silver chloride dissolves ia aqueous ammonia. Similar reactions take place with other water iasoluble silver and copper salts. Many ammines can be obtained ia a crystalline form, particularly those of cobalt, chromium, and platinum. 

Cobalt exists in the +2 or +3 valence states for the majority of its compounds and complexes. A multitude of complexes of the cobalt(III) ion [22541-63-5] exist, but few stable simple salts are known (2). Werner s discovery and detailed studies of the cobalt(III) ammine complexes contributed gready to modem coordination chemistry and understanding of ligand exchange (3). Octahedral stereochemistries are the most common for the cobalt(II) ion [22541-53-3] as well as for cobalt(III). Cobalt(II) forms numerous simple compounds and complexes, most of which are octahedral or tetrahedral in nature cobalt(II) forms more tetrahedral complexes than other transition-metal ions. Because of the small stabiUty difference between octahedral and tetrahedral complexes of cobalt(II), both can be found in equiUbrium for a number of complexes. Typically, octahedral cobalt(II) salts and complexes are pink to brownish red most of the tetrahedral Co(II) species are blue (see Coordination compounds). 

Similarity with cobalt is also apparent in the affinity of Rh and iH for ammonia and amines. The kinetic inertness of the ammines of Rh has led to the use of several of them in studies of the trans effect (p. 1163) in octahedral complexes, while the ammines of Ir are so stable as to withstand boiling in aqueous alkali. Stable complexes such as [M(C204)3], [M(acac)3] and [M(CN)5] are formed by all three metals. Force constants obtained from the infrared spectra of the hexacyano complexes indicate that the M--C bond strength increases in the order Co < Rh < [r. Like cobalt, rhodium too forms bridged superoxides such as the blue, paramagnetic, fCl(py)4Rh-02-Rh(py)4Cll produced by aerial oxidation of aqueous ethanolic solutions of RhCL and pyridine.In fact it seems likely that many of the species produced by oxidation of aqueous solutions of Rh and presumed to contain the metal in higher oxidation states, are actually superoxides of Rh . ... 

Explosive Sensitivity of Cobalt Ammine Complexes , NOTS 1884, Navord 6639 (Oct 1957), AD 201554 (19) R.L. Carlin J.O. Edwards,... 

The long-running dispute over the mechanism of base hydrolysis of cobalt(III)-ammine and -amine complexes, SVj2 versus SVjlCB (better termed Dcb), was several years ago resolved in favor of the latter (73). Recent activity on reactions of this type has concentrated on attempting to locate the precise site of deprotonation of the complex, an exercise successfully accomplished for the complexes syn,anti-[Co (cyclen)(NH3)2]3+ and syn,[Pg.80]

The ammine complexes of Co3+ are prepared by adding excess ammonia to a solution of cobalt salt followed by air oxidation and boding. The brown solution turns pink on boiling. The cyanide complexes are made by adding excess potassium cyanide to a solution of cobalt salt. Acidification of the solution with a small amount of acetic or hydrochloric acid followed by boiling yields K3Co(CN)6. The aquo-halo mixed complexes are formed by stepwise substitution of H2O molecule with halide ion in the coordination sphere. In general, a mixed complex may be prepared by substitution with a specific anion. 

Consequently, reduction of cobalt(III) ammines in basic solution is not favorable. A variety of reducing agents has been used to effect reaction (11). The fortunate coincidences that cobalt(III) complexes are substitution inert while cobalt(II) systems are labile and that cobalt(II) is resistant to oxidation or further reduction in acid solution offer many advantages in the study of redox processes. Not surprisingly, work with cobalt(III) complexes forms the basis for much of the present understanding of oxidation-reduction reactions. 

Photolysis of cobalt(III) ammines in the solid state and in various organic media have been reported. Irradiation of crystals of potassium cobaltioxalate gives Co(II) and C02. Study of this reaction by ESR techniques has been reported.69 Photoreduction of Co(NH3)5(H20)l3 gives CoI42- as the cobalt-containing product similar results were obtained with the corresponding bromide and chloride. Solid-state photoreduction of Co(en)3Cl3 has also been reported.70... 

Much less is known about photoinitiated ligand exchange reactions of coordination complexes of cobalt(III) and other de ions than is the case for chromium(III). With the exception of the cobalt(III) ammines, however, available data suggest that photochemical ligand exchange reactions of cobalt(III) and d6 systems involve the thermally equilibrated 17 g and/or 37, states of the complex as intermediates. The reasoning is completely analogous to that described earlier with respect to chromium(III) photochemistry. 

A rather interesting statement of a similar point of view linking base hydrolyses of cobalt(III) ammines to reduction of the cobalt(III) center has recently appeared.139... 

Joyner et al, "Explosive Sensitivity of Cobalt Ammine Azides , Nature 196, No 4893(1962) 8) M.M. Braidech, "National... 

Note Destruction of cobaltic ammines, such as of hexammine by means of thiosulfate in acid soln was described by L.M. Orlova, ZavodLab 8, 502 (1939) CA 36, 6935 (1942), which is listed in Vol 1, p A286-R... 

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. 

The chromi-ammines are produced by the action of ammonia and ammonium salts on chromic salts, or by the action of ammonia in presence of ammonium salts on chromous salts and subsequent oxidation. The second method is analogous to that for the formation of cobalt-ammines. The chromi-ammines form a group of coloured substances, and comprise unstable and stable derivatives, some of which are very complex. 

A. Mononuclear Cobalt-ammines containing One Atom of Cobalt in the Molecule. 

These cobaltous addition compounds, therefore, resemble the preceding group of additive substances rather than the eobaltic salts, and the series illustrate the statement previously made that the most highly saturated compounds of an element have greater capacity for the formation of complex addition compounds. In this connection, for example, one might compare the relative instability of hexammino-cobaltous chloride, [Co(NH3)6]Cl2, with the high stability of the ammine of the more saturated eobaltic chloride, hexammino-cobaltic chloride, [Co(NH3)6]C13.2... 

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