PREPARATIVE STUDY AND CHARACTERIZATION OF THE CIS-DIAMINETETRANITROCOBALTATE(III) ION - A MISSING LINK IN THE AMINE-NITRO COBALT(III) SERIES

The preparative conditions and stability of cis-[Co(NO2)(4)(NH3)(2)](- ), which had been a missing link in the ammine-nitro cobalt(III) serie s until recently, were studied using Co-59 NMR spectroscopy. The react ion of Na-3[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 aque ous ammonia also gave the cis isomer as a major product. On the contra ry, the cis isomer did not form during the conventional preparation fr om CoCl2 in an aqueous solution. The cis isomer was very stable in the solid state, but the isomer slowly isomerized in water (t(1/2) > 5 da ys at room temperature, ca. 0.1 mol dm(-3)) to the trans isomer. The r eaction was accompanied by the formation of fac-[Co(NO2)(3)(NH3)(3)] w hich then isomerized to the mer isomer, and by the formation of severa l unknown species.