Carbonato complexes, of cobalt

These equations are in line with Eq. (30), such that kx denotes the ring-opening rate constant of the protonated carbonato complex and / 2 is the decarboxylation rate constant of the ring-opened bicarbonato complex. Values of these rate constants and the acid dissociation constants of some protonated carbonato complexes of cobalt(TII) (see Table III) reflect the ligand dependence with respect to charge variations, steric constraint, and donor properties of the non-labile ligands. 

The hydrolysis of chelated carbonato complexes of cobalt(III) is much faster in acid than in neutral solution. Explain. 

In certain instances, the coordinated oxygen atom may undergo reaction. The complete and elegant work carried out on carbonato-ammine complexes of cobalt (III) illustrates this situation. As Equation 34 reveals, the acid hydrolysis of [ (NH3)5CoC03]+ involves substitution at the carbon-oxygen bond, rather than the cobalt-oxygen bond (4). 

COBALT(III), RHODIUM(III), AND IRIDIUM(III) CARBONATO COMPLEXES OF THE PENTAAMMINE TYPE... 

The procedure described here is based on the observation that amine monohydroxo complexes of cobalt(III), rhodium(IIl), and iridium(III) react directly with carbon dioxide to form the corresponding carbonato complexes,2 3 without effect on the configuration of the amine ligands.4 The amine monoaqua complex is allowed to react with lithium carbonate or carbon dioxide gas at room temperature at pH 8.0 for a few minutes, and the carbonato complex is isolated by adding alcohol. The procedure has been used to prepare salts of the following cations pentaammine(carbonato)-cobalt(III),2 ds-ammine(carbonato)bis(ethylenediamine)cobalt(III),5 trans-... 

The following procedure is based on the reaction of an aqueous solution of cobalt(II) chloride with the equivalent amount of (2-aminoethyl)carbamic acid, followed by oxidation with hydrogen peroxide and the subsequent formation of bis(ethylene-diamine)cobalt(III) ions. The bis(ethylenediamine)cobalt(lII) species are converted to the carbonato complex by reaction with lithium hydroxide and carbon dioxide. During the entire preparation a vigorous stream of carbon dioxide is bubbled through the reaction mixture. This procedure appears to be essential in order to minimize the formation of tris(ethylenediamine)cobalt(III) chloride as a by-product. However, the formation of a negligible amount of the tris salt cannot be avoided. The crude salts have a purity suitable for preparative purposes. The pure salts are obtained by recrystallization from aqueous solution. 

To 27.5 g. (0.1 mole) of crude (carbonato)bis(ethylenediamine)-cobalt(III) chloride is added 200 ml. of 1.00 N hydrochloric acid. The carbonato complex is dissolved with evolution of carbon dioxide gas and formation of a red solution consisting primarily of the corresponding cw-diaqua species. The solution is evaporated in the steam bath until an almost dry paste has been formed. The purple residue is filtered and washed with three 20-ml. portions of ice-cold water. Drying in air yields 19.5 g. of purple crystals of cu-dichlorobis(ethylenediamine)cobalt(III) chloride. The mother liquor and the washings are again evaporated almost to dryness to yield a second crop of crystals, 5.9 g. The total yield is 25.4 g. (84% based on (carbonato)bis(ethylenediamine)cobalt(III) chloride). The analysis and the visible absorption spectrum of the two fractions are identical. Anal. Calcd. for [Co(en)2Cl2 ] C1 H20 Co, 19.42 N, 18.46 C, 15.82 Cl, 35.05 H, 5.98. Found Co, 19.50 N, 18.57 C, 15.77 C1, 35.15 H, 6.01. 

Cobalt(III)-carbonato complexes provide a very useful starting point for the preparation of other cobalt(III) complexes so that high yield synthetic methods are of great practical importance. Table 67 lists preparations for the major structural types (241 >-(244) and variations for substituted or related ligand systems are based on these. Recent reviews covering the wider aspects of transition metal carbonates are available.872 874... 

Table 69 Rates of CC>2

Tetraammine(carbonato)cobalt(III)] perchlorate (5.74 g, 0.020 mole) is dissolved in 30 mL of 3 M perchloric acid at room temperature with stirring. The carbonato complex dissolves rapidly evolving carbon dioxide. After 5 minutes the solution is filtered and cooled with ice. To the cold (approximately 5°)... 

Johri, K. N., Kaushik, N. K., Bakshi, K. Thin-layer chromatographic separation of copper (II), nickel(II), and cobalt(II) as thio-carbonato-complexes and determination by ring colorimetry. Chromatographia 5, 326 (1972)... 

Some years later, Robert Auten, an undergraduate student at the University of Illinois, achieved an optical inversion in the reaction just mentioned (9). Patterning his work on that of Walden, Auten used silver carbonate instead of potassium carbonate and obtained a levo-rotatory carbonato complex. It was soon found that the choice of carbonate is not the important factor, and Dwyer, Sargeson and Reid, in Australia, showed that the pH of the solution is the deciding factor (10). After his initial success, Auten looked for inversions in the reactions of the dichloro complex with oxalate and nitrite, but did not find them. It is most fortunate that he tried the carbonate reaction first, for otherwise, we would have concluded that optical inversions do not take place in the reactions of cobalt complexes, and probably would not have tried carbonate. 

A large majority of diacidobis(l, 2-ethanediamine)cobalt(III) complexes can be synthesized from the (carbonato)bis(l, 2-ethanediamine)cobalt(III) ion. This is now readily available in high yield and purity.1 cis- and trans-Dichlorobis(l, 2-ethanediamine)cobalt(III) chlorides are obtained from the carbonato complex with hydrochloric acid using appropriate conditions. These isomeric dichloro salts are widely used starting materials for further syntheses. In this respect, the analogous dibromo complexes can be more convenient starting materials because bromide is substituted from the cobalt center more readily than is chloride ion. The milder conditions minimize the production of cobalt(II), a common side reaction. 

Into a solution of 3.3 g (0.009 mole) of cis-diamminecarbonatodi-(pyridine)cobalt(III) chloride monohydrate in 10 mL of water is poured 1.2 g (0.0038 mole) of sodium (—, 2-ethanediamine-bis(oxalato)cobaltate(III) monohydrate3 with stirring. The solution is cooled in an ice bath, and the sides of the vessel are scratched with a glass rod, whereupon the diastereoisomeric salt of the (— )589 carbonato complex deposits. (If the diastereoisomer does not deposit, a small amount of ethanol is added.) The whole is kept in an ice bath for a while. The diastereoisomer is collected on a filter, washed with cold water, and recrystallized several times from water at 35° to increase the optical purity. The recrystallized salt is collected on a filter, washed with cold water, ethanol, and diethyl ether, and dried under vacuum. The yield is 0.3 g. Anal. Calcd. for [Co(C03)(NH3)2(py)2][Co(ox)2(en)]-2.5H20 C, 31.35 H, 4.49 N, 12.90. Found C, 31.37 H, 4.67 N, 12.82. The diastereoisomer is separated as before. 

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