Cobalt III

The chemistry of Co(III) in dilute aqueous acidic solution is complicated by (/) oxidation of the solvent, it) complex formation with counter-ions, Hi) hydrolysis and iv) apparently extensive dimerisation. These phenomena are discussed further in the section on oxidation of water. 

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CoS04,7H20. Few cobalt(III) oxy acid salts are known. 002(504)3,ISHjO is formed by electrolytic oxidation and forms alums Co(N03)3 contains co-ordinated nitrate (C0F3 plus NjOs). 

Octahedral substitution reactions (e.g. those involving cobalt(III) complexes) may proceed by both Sf l or 8 2 reactions. In the S l case a slow dissociative mechanism (bond breaking) may take place. Reaction with the substituting... 

When the water ligands around a cation are replaced by other ligands which are more strongly attached, the redox potential can change dramatically, for example for the cobalt(II)-cobalt(III) system we have... 

Also, in anhydrous conditions, silver reacts with fluorine and forms silver difluoride AgFj and cobalt gives cobalt(III) fluoride, C0F3, these metals showing higher oxidation states than is usual in their simple salts. 

The thiocyanate ion SCN forms an intensely red-coloured complex (most simply represented as [Fe(SCN)(H20)5] ) which is a test for iron(III). However, unlike cobalt(III), iron(lll) does not form stable hexammines in aqueous solution, although salts containing the ion [FefNHj) ] can be obtained by dissolving anhydrous iron(III) salts in liquid ammonia. 

Cobalt compounds have been in use for centuries, notably as pigments ( cobalt blue ) in glass and porcelain (a double silicate of cobalt and potassium) the metal itself has been produced on an industrial scale only during the twentieth century. Cobalt is relatively uncommon but widely distributed it occurs biologically in vitamin B12 (a complex of cobalt(III) in which the cobalt is bonded octahedrally to nitrogen atoms and the carbon atom of a CN group). In its ores, it is usually in combination with sulphur or arsenic, and other metals, notably copper and silver, are often present. Extraction is carried out by a process essentially similar to that used for iron, but is complicate because of the need to remove arsenic and other metals. 

Like iron and the next transition element, nickel, cobalt is not generally found in any oxidation state above + 3, and this and + 2 are the usual states. The simple compounds of cobalt(III) are strongly oxidising ... 

Cobaltilll) oxide is obtained as a brown precipitate Co Oj.aq when cobalt(II) hydroxide is oxidised in alkaline conditions (or when a cobalt(III) is decomposed by aqueous alkali). On heating it gives the black mixed oxide C03O4. 

Hydrated cobalt III) sulphate, Co2(S04)3. JSHjO is obtained when cobalt(II) sulphate is oxidised electrolytically in moderately concentrated sulphuric acid solution it is stable when dry but liberates oxygen from water. Some alums, for example KCo(S04)2.12H,0 can be obtained by crystallisation from sulphuric acid solutions. In these and the sulphate, the cation [CofHjO) ] may exist it is both acidic and strongly oxidising. 

Cobaltilll) nitrate Co(N03)3 has been prepared by the reaction of dinitrogen pentoxide with cobalt(III) fluoride. 

Cobalt(III) contains six 3d electrons in the presence of six appropriate ligands, arranged octahedrally, a large splitting of the d... 

For this reaction, charcoal is a catalyst if this is omitted and hydrogen peroxide is used as the oxidant, a red aquopentammino-cobalt(lll) chloride, [Co(NH3)jH20]Cl3, is formed and treatment of this with concentrated hydrochloric acid gives the red chloro-p0itatnmino-coba. t(lll) chloride, [Co(NH3)5Cl]Cl2. In these latter two compounds, one ammonia ligand is replaced by one water molecule or one chloride ion it is a peculiarity of cobalt that these replacements are so easy and the pure products so readily isolated. In the examples quoted, the complex cobalt(III) state is easily obtained by oxidation of cobalt(II) in presence of ammonia, since... 

Decomposition of most cobalt(III) complexes by boiling with alkali gives a brown precipitate of the hydrated oxide C02O3. aq (p.402). This will quantitatively oxidise iodide to iodine. 

Hydrogen Peroxide Analysis. Luminol has been used for hydrogen peroxide analysis at concentrations as low as 10 M using the cobalt(III) triethanolamine complex (280) or ferricyanide (281) as promoter. With the latter, chemiluminescence is linear with peroxide concentration from... 

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). 

Indeed the cobalt(III) ion is sufficientiy unstable in water to result in release of oxygen and formation of cobalt(II) ion. Under alkaline conditions the oxidation is much more facile and in the presence of complexing agents, eg, ammonia or cyanide, the oxidation may occur with ease or even spontaneously. 

Cobalt(II) nitrate hexahydrate [10026-22-9], Co(N02)2 6H20, is a dark reddish to reddish brown, monoclinic crystalline material containing about 20% cobalt. It has a high solubiUty in water and solutions containing 14 or 15% cobalt are commonly used in commerce. Cobalt nitrate can be prepared by dissolution of the simple oxide or carbonate in nitric acid, but more often it is produced by direct oxidation of the metal with nitric acid. Dissolution of cobalt(III) and mixed valence oxides in nitric acid occurs in the presence of formic acid (5). The ttihydrate forms at 55°C from a melt of the hexahydrate. The nitrate is used in electronics as an additive in nickel—ca dmium batteries (qv), in ceramics (qv), and in the production of vitamin B 2 [68-19-9] (see Vitamins, VITAMIN B22)-... 

Cobalt(II) can be separated from cobalt(III) as the acetylacetonate (acac) compounds by extraction of the ben2ene soluble cobalt(Ill) salt (14). Magnesium hydroxide has been used to selectively adsorb cobalt(Il) from an ammonia solution containing cobalt(Il) and cobalt(Ill) (15). 

Bromo-2-pyridyla2o)-5-diethylamiQophenol (5-Br-PADAP) is a very sensitive reagent for certain metals and methods for cobalt have been developed (23). Nitroso-naphthol is an effective precipitant for cobalt(III) and is used in its gravimetric determination (24,25). Atomic absorption spectroscopy (26,27), x-ray fluorescence, polarography, and atomic emission spectroscopy are specific and sensitive methods for trace level cobalt analysis (see... 

Cobalt compounds can be classified as relatively nontoxic (33). There have been few health problems associated with workplace exposure to cobalt. The primary workplace problems from cobalt exposure are fibrosis, also known as hard metal disease (34,35), asthma, and dermatitis (36). Finely powdered cobalt can cause siUcosis. There is Htfle evidence to suggest that cobalt is a carcinogen in animals and no epidemiological evidence of carcinogenesis in humans. The LD q (rat) for cobalt powder is 1500 mg/kg. The oral LD q (rat) for cobalt(II) acetate, chloride, nitrate, oxide, and sulfate are 194, 133, 198, 1700, 5000, and 279 mg/kg, respectively the intraperitoneal LD q (rat) for cobalt(III) oxide is 5000 mg/kg (37). 

Paint pigments do not change colors on appHcation. Other common colors are violet from cobalt(II) phosphate [18475-47-3] pink from cobalt and magnesium oxides, aureolin yellow from potassiuim cobalt(III) nitrite [13782-01-9], KCo(N02)4, and cerulean blue from cobalt staimate [6546-12-5]. Large quantities of cobalt are used at levels of a few ppm to decolori2e or whiten glass and ceramics. Iron oxide or titanium dioxide often impart a yellow tint to various domestic ware. The cobalt blue tends to neutrali2e the effect of the yellow. 

In contrast, chromium (ITT) and cobalt(III) form 2 1 dye metal complexes that have nonplanar stmctures. Geometrical isomerism exists. The (9,(9 -dihydroxyazo dyes (22) form the Drew-Pfitzner or y rtype (23) (A = C = O) whereas o-hydroxy—o -carboxyazo dyes (24) form the Pfeiffer-Schetty or fac type (25), where A = CO 2 and C = O. 

K3[CoFfi], whose anion is notable not only as the only hexahalogeno complex of cobalt(III) but also for being high-spin and hence paramagnetic with a magnetic moment at room temperature of nearly 5.8 BM. 

Complexes of cobalt(III) with O-donor ligands are generally less stable than those with Af-donors although the dark-green [Co(acac)3] and M iCo-( 204)3] complexes, formed from the chelating ligands acetylacetonate and oxalate, are stable. Other carboxylato complexes such as those of... 

There is also clear evidence of a change from predominantly class-a to class-b metal charactristics (p. 909) in passing down this group. Whereas cobalt(III) forms few complexes with the heavier donor atoms of Groups 15 and 16, rhodium(III), and more especially iridium (III), coordinate readily with P-, As- and S-donor ligands. Compounds with Se- and even Te- are also known. Thus infrared. X-ray and nmr studies show that, in complexes such as [Co(NH3)4(NCS)2]" ", the NCS acts as an A -donor ligand, whereas in [M(SCN)6] (M = Rh, Ir) it is an 5-donor. Likewise in the hexahalogeno complex anions, [MX ] ", cobalt forms only that with fluoride, whereas rhodium forms them with all the halides except iodide, and iridium forms them with all except fluoride. 

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