Cobalt complexes ammonia complex, acidity

Erdmann,1 in 1866, prepared the first member of this series, namely, ammonium tetranitrito-diammino-cobaltate, [Co(NH3)2(N02)4] NH4. The salt is sometimes referred to as Erdmann s salt on that account. Later, Gibbs prepared other salts of the same type, and showed that in these salts the cobalt atom, united with ammonia and acidic radicles, forms a negative radicle.2 Werner then showed that these salts form the connecting link between the neutral un-ionised complex triacido-triammino-eobalt compounds and the double salt, such as potassium eobalti-nitrite. Thus, by replacement of ammonia molecules by acid radicles a transition takes place from trinitrito-triammino - cobalt to potassium tetranitrito - diammino - cobaltate, [Co(NH3)2(N02)4]K, then to potassium pentamtrito-ammino-eobalfate. [Co(NH3)(N02)5]K, and finally to hexanitrito-cobaltate, [Co(N02)e]Iv3. Tetra-acido-diammino-cobaltates are therefore the salts of the acid tetra-acido-diammino-cobaltic acid, [Co(NH3)2R4]H. 

The amount of ammonia volatilized was determined from the amount of standard sulfuric acid consumed in the traps. Ethylenediamine was determined by the salicylaldehyde method (14). The sample for cobalt(II) determination was made slightly acidic with hydrochloric acid immediately upon removal of the sample from the reaction flask, in order to prevent further oxidation of the cobalt (18). The carbon was removed by filtration, and the cobalt (II) concentration was determined spectrophotometrically as the cobalt-ammonium thiocyanate complex, (NH4)2Co(NCS)4 (28). 

A multicomponent positive-imaging process using ammonia release has been described by Ricoh.211 The components of the system are (1) a cobalt(III) hexaammine complex, (2) a quinone photoreductant, (3) a chelating agent such as dimethylglyoxime, (4) a leuco dye (triarylmethane type), (5) a photooxidant (biimidazole) and (6) an organic acid (toluenesulfonic acid). 

Cobalt can also be separated from nickel in alkaline conditions by first adding ammonia and hydrogen peroxide to form cobalt(III) pentamine complex, and then crystallizing the bulk of the nickel as the sparingly soluble nickel ammonium sulfate (Figure 6.8). The separation of nickel from cobalt can be achieved by precipitation via oxidation to nickel(III) hydroxide with Caro s acid at near-neutral pH (Figure 6.9).257... 

The tris(Ar-hydroxethyIethylenediamine)cobalt(III) chloride that was used in these reactions was reported to be an orange crystalline solid (111). In a subsequent investigation (69), attempts to prepare this compound by the air oxidation of a mixture of cobalt (II) and the amine failed. The compound was, however, prepared by the displacement of ammonia from [Co(NH3)e]Cl3 by iV-hydroxyethylethylenediamine and a dark red compound was obtained. Attempts to react the hydroxy groups in this red complex with a variety of reagents (nitric acid, thionyl chloride, benzoyl chloride, and acetyl chloride) were as unsuccessful as the previously reported attempts to react the hydroxy groups in the orange cobalt complex. 

As with most complexation and drug solubility situations, pH b a critical variable. Cocaine base b not soluble in water, and if the drug b in thb form rather than a soluble salt, no reaction occurs. Acid b needed to ensure that the cocaine b in the water-soluble ionic form to allow for the formation of a complex. The color b the result of an ion-pair compound formed from the cationic cocaine and the anionic cobalt complex. As with all amine bases, such as ammonia, the base becomes protonated in acidic solution. The pKg of the base determines the ratio of the protonated, ionized form to the neutral form. It is possible to add too much HQ, because cobalt forms a water-soluble pink complex with chloride [CoCy . The pH can also influence the type of complex and ion pair formed. Under acidic conditions, the ion pair favored b [Co(cocaine)2l(SCN)2 (which b pinkbh and soluble in water), while in the neutral-to-basic ranges, the ion pair b assigned the structure [cocaine-H ]2 [Co(SCN)4] (which b a blue solid and soluble in chloroform). The important points of the cobalt thiocyanate reaction with cocaine are summarized in Figures 7.24r-7.26. 

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... 

The precipitate is soluble in free mineral acids (even as little as is liberated by reaction in neutral solution), in solutions containing more than 50 per cent of ethanol by volume, in hot water (0.6 mg per 100 mL), and in concentrated ammoniacal solutions of cobalt salts, but is insoluble in dilute ammonia solution, in solutions of ammonium salts, and in dilute acetic (ethanoic) acid-sodium acetate solutions. Large amounts of aqueous ammonia and of cobalt, zinc, or copper retard the precipitation extra reagent must be added, for these elements consume dimethylglyoxime to form various soluble compounds. Better results are obtained in the presence of cobalt, manganese, or zinc by adding sodium or ammonium acetate to precipitate the complex iron(III), aluminium, and chromium(III) must, however, be absent. 

Armannsson [659] has described a procedure involving dithizone extraction and flame atomic absorption spectrometry for the determination of cadmium, zinc, lead, copper, nickel, cobalt, and silver in seawater. In this procedure 500 ml of seawater taken in a plastic container is exposed to a 1000 W mercury arc lamp for 5-15 h to break down metal organic complexes. The solution is adjusted to pH 8, and 10 ml of 0.2% dithizone in chloroform added. The 10 ml of chloroform is run off and after adjustment to pH 9.5 the aqueous phase is extracted with a further 10 ml of dithizone. The combined extracts are washed with 50 ml of dilute ammonia. To the organic phases is added 50 ml of 0.2 M-hydrochloric acid. The phases are separated and the aqueous portion washed with 5 ml of chloroform. The aqueous portion is evaporated to dryness and the residue dissolved in 5 ml of 2 M hydrochloric acid (solution A). Perchloric acid (3 ml) is added to the organic portion, evaporated to dryness, and a further 2 ml of 60% perchloric acid added to ensure that all organic matter has been... 

Numerous d cobalt(III) complexes are known and have been studied extensively. Most of these complexes are octahedral in shape. Tetrahedral, planar and square antiprismatic complexes of cobalt(lII) are also known, but there are very few. The most common ligands are ammonia, ethylenediamine and water. Halide ions, nitro (NO2) groups, hydroxide (OH ), cyanide (CN ), and isothiocyanate (NCS ) ions also form Co(lII) complexes readily. Numerous complexes have been synthesized with several other ions and neutral molecular hgands, including carbonate, oxalate, trifluoroacetate and neutral ligands, such as pyridine, acetylacetone, ethylenediaminetetraacetic acid (EDTA), dimethylformamide, tetrahydrofuran, and trialkyl or arylphosphines. Also, several polynuclear bridging complexes of amido (NHO, imido (NH ), hydroxo (OH ), and peroxo (02 ) functional groups are known. Some typical Co(lll) complexes are tabulated below ... 

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. 

In some instances the metal complex may become the anode instead of the cathode. The acidic radicles have, in this case, increased at the expense of ammonia until there is a greater number of acidic radicles in the complex than corresponds to the valency of the metallic atom thus [Co(NH8)s.(NO,)J. If valency is determined by the above method it is found, since cobalt is trivalent, and (N02)4 has a total valency of four, that the valency of the complex, namely, three minus four, has a unit negative value. The complex is thus anodic and unites with one atom of a monovalent metal or its equivalent. The complex radicle cited, therefore, united with potassium yields the substance [Co(NH.))2.(N02)4]K, or potassium tctranitrito-diammino-eobalt,. 

Such a configuration should on replacement of one ammonia molecule by acidic or other monovalent radicle yield only one compound, and this is proved to be the case. On the other hand, if two acidic, or other groups replace two ammonia groups in the complex, isomerism should be possible, yielding two isomers of the formula [M(NII3)4R2 R. In the case of dinitro-tctrammino-cobaltic nitrate, [Co(NIi2)4(N02)2 N03, two isomeric forms are known to exist, one brown in colour, the other yellow. The two substances may be represented by the following formula —... 

The ammonia addition compounds of cobaltous salts are unstable, and on that account there is no long series of these salts as in the ease of the ammino-cobaltie salts, where ammonia groups may be partially replaced by acidic groups, water, and other molecules, and where the complex is sufficiently stable to allow of different salts being prepared by double decomposition. 

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