Pentaammine cobalt salts

The synthesis of pentaammine(carbonato)cobalt(III) salts is readily accomplished by an air-oxidation method starting with cobalt(II) nitrate and ammonium carbonate in aqueous ammonia.1 However, a suitable general method for making other types of monodentate amino(carbonato) metal salts is not available in the literature. The air-oxidation technique can be applied only to complexes where the ligands are all ammonia molecules or where the central metal ion is cobalt. 

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-... 

Figure 5. Absorption spectrum of pentaammine (S)-2-chloropropionato cobalt-(III) tetraphenylborate in ("lower set) water (as ClOf salt), hexamethylphosphor-triamide (hmpa), dimethyl sulfoxide, N-metkylformamide (nmf), N,N-dimethyl-formamide (dmf) ("upper set) acetonitrile, methanol, ethanol, acetone, pyridine (py) and tetrahydrofuran (thf). The reference contained tetraphenylborate (as Na salt) at the same concentration as the solution of the complex.

Figure 6. CD spectrum of pentaammine (S)-2-chloropropionato -cobalt(III) tetraphenylborate in various solvents. Abbreviations are as for Figure 5. In water, the perchlorate salt was used.

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