The preparative conditions and stability of
cis-[Co(NO
2)
4(NH
3)
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 Na
3[Co(NO
2)
6] with liquid ammonia mainly gave
cis-[Co(NO
2)
4(NH
3)
2]
- together with
mer- and
fac-[Co(NO
2)
3(NH
3)
3] and
cis-[Co(NO
2)
2(NH
3)
4]
+; the formation of
trans-[Co(NO
2)
4(NH
3)
2]
- was not observed. The reaction of aqueous [Co(NO
2)
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 CoCl
2 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 days at room temperature,
ca. 0.1mol dm
-3) to the trans isomer. The reaction was accompanied by the formation of
fac-[Co(NO
2)
3(NH
3)
3] which then isomerized to the mer isomer, and by the formation of several unknown species.
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