Octahedral cobalt(III) complexes of the chloropentammine type. XII. The preparation, properties, and reactions of the chloropentakismethylaminecobalt(III) cation

Abstract

Although chloropentaamminecobalt(n~) chloride can be obtained from cobalt(11) chloride, ammonia, ammonium chloride, and hydrogen peroxide by the method of Willard and Hall,l attempts to prepare the corresponding pentakismethylamine complex lead to the formation of a blue precipitate. We have now successfully prepared the complex [ C O C~( C H~N H~)~] C~~ by the action of buffered aqueous methylamine on solid trans-dichlorotetrakispyridinecobalt(~rr) chloride hexahydrate. The chloride so obtained can be converted into the nitrate by the action of ammonium nitrate.However, attempts to extend the method to higher homologous alkylamines were not successful.Both salts are purple-red in colour, and are more soluble in water than the corresponding chloropentaamminecobalt(~n) compounds. In basic and neutral aqueous solutions, the complex cation decomposes completely to yield the brown insoluble cobalt oxide. If, however, it is dissolved in dilute nitric acid, the coordinated chloride is slowly replaced by water. The product is presumably the aquopentakismethylaminecobalt(~~~) cation, but attempts to prepare salts of this aquo ion have so far proved unsuccessful.The visible absorption spectrum of a solution of chloropentakismethylaminecobalt(1rr) nitrate in dilute nitric acid (Amax 555 mp, emax 63.2 1. mole-1 cm-1; Amax 382 mp, emax 65 .O 1. mole-1 cm-1) was virtually identical with that for the corresponding chloride salt, indicating that the conversion of the chloride into nitrate by the action of ammonium nitrate leaves the complex cation intact. Compared with the spectrum of the corresponding pentaammine complex, the first absorption peak (d-d band) for the present cation occurs a t a longer wavelength (and with a higher extinction coefficient), reflecting the relative strengths of the field produced by the five ammonia and the five methylamine ligands.The hydrolysis of chloropentakismethylaminecobalt(n~) nitrate was studied in dilute nitric acid, and the first-order rate constants obtained (see Experimental)