When kept in dilute acetonitrile solutions, complexes [CoB6][Co(CO)4]2 [B = C5H5N, Me2NC(O)H] form gels CoB x (MeCN) y ](OH)2·2[CoCO3·2CoO·2H2O] (x = 2, 3, y = 4, 3), while the polyfunctional complex [Co{(EtO)3SiCH2CH2CH2NH2)}][Co(CO)4]2 gives a mixed carbonate-siloxane gel. The gel formation is accompanied by complete decomposition of [Co(CO)4]- anions under the action of air oxygen and moisture, to give CoCO3·2CoO·2H2O. Mechanistically, the gel and film formation involves absorption of molecular oxygen by the hexamine- or hexamidecobalt(II) cation and subsequent decomposition of theunstable oxygenated carbonylcobalt complex.