[Co(η 2 -CO 3 )(NH 3 ) 4 ](NO 3 )·0.5H 2 O and [(NH 3 ) 3 Co(μ-OH) 2 (μ-CO 3 )Co(NH 3 ) 3 ][NO 3 ] 2 ·H 2 O were prepared by prolonged aerial oxidation of a solution of Co(NO 3 ) 2 ·6H 2 O and ammonium carbonate in aqueous ammonia. The formation of these side products highlights the richness of the chemistry of these systems and the possibility of by products if methods are not strictly adhered to. The X-ray crystal structures of [Co(η 2 -CO 3 )(NH 3 ) 4 ][NO 3 ]·0.5H 2 O and [(NH 3 ) 3 Co(μ-OH) 2 (μ-CO 3 )Co(NH 3 ) 3 ][NO 3 ] 2 ·H 2 O reveal a monomeric octahedral cobalt center with η 2 -bound CO 3 2− in the former, while the latter consists of a dimeric array where the two cobalt centers are bridged by two OH − and one μ 2 -CO 3 2− groups with three terminal NH 3 ligands for each Co center. In both complexes extensive hydrogen bonding interactions are evident.