Preparative Study and Characterization of the Cis-Diamminetetranitrocobaltate(III) Ion. A Missing Link in the Ammine-Nitro Cobalt(III) Series.

Abstract

The preparative conditions and stability of cis-[Co(NO2)4(NH3)2]-, which had been a missing link in the ammine-nitro cobalt(III) series until recently, were studied using 59Co NMR spectroscopy. The reaction of Na3[Co(NO2)6] with liquid ammonia mainly gave cis-[Co(NO2)4(NH3)2]- together with mer- and fac-[Co(NO2)3(NH3)3] and cis-[Co(NO2)2(NH3)4]+; the formation of trans-[Co(NO2)4(NH3)2]- was not observed. The reaction of aqueous [Co(NO2)6]3- with aqueous ammonia also gave the cis isomer as a major product. On the contrary, the cis isomer did not form during the conventional preparation from CoCl2 in an aqueous solution. The cis isomer was very stable in the solid state, but the isomer slowly isomerized in water (t1/2>5 days at room temperature, ca. 0.1mol dm-3) to the trans isomer. The reaction was accompanied by the formation of fac-[Co(NO2)3(NH3)3] which then isomerized to the mer isomer, and by the formation of several unknown species.