cis-[CoL2](3+) (1a(3+)), trans-[CoL2](3+) (2a(3+)), cis-[Co(MeL)(2)](3+) (1b(3+)), and trans-[Co(MeL)(2)](3+) (2b(3+)), L = 1,4-diazepan-6-amine (daza) and MeL = 6-methyl-1,4-diazepan-6-amine (Medaza), were allowed to react as templates in acetonitrile with paraformaldehyde and triethylamine. Several Co-III complexes, where two adjacent amino groups of two ligand moieties are interlinked by an oxidimethaneamine bridge, were obtained. Connection of a primary with a secondary amino group (prim sec bridging) was found to be predominant. The singly and doubly bridged daza- and Medaza-derivatives 7a(3+), 9a(3+) and 7b(3+), 9b(3+) were characterized by crystal-structure analysis. The bridging process resulted in a slight lengthening of the mean Co-N distance, a red shift of the A(1g)-T-1g transition, and an increase of the Co-III/Co-II reduction potential. Several minor components, which could be only partially separated by chromatographic methods, were also formed. The daza-derivatives 6a(3+) (prim prim bridged) and 10a(3+) (bidentate coordination of one daza frame) formed in small quantities. The Medaza derivatives 3b(3+) and 4b(3+) (trans configuration of the Medaza frames, with additional pending carbinolamino groups), and 8b(3+) (with a methylideneimino group) represent intermediates of the condensation process. Their structure was again corroborated by X-ray diffraction. All bridged species (6a(3+), 7a(3+), 8b(3+), 9a(3+), 9b(3+), and 10a(3+)) exhibited exclusively a cis orientation of the two diazepane frames, even if the trans configured 2a(3+) or 2b(3+) were used as starting materials. Molecular mechanics calculations indicate that in the bridged species with a trans configuration steric strain is substantially more pronounced. In alkaline aqueous media, 9a(3+) and 9b(3+) revealed a complete degradation of the bridges whereby the original 1a(3+) and 1b(3+) reformed. The pseudo-first-order rate constant k(obs) of the degradation reaction was found to depend linearly on OH- concentration. The degradation of the first bridge is about 100 times faster than the degradation of the second. The mechanism of formation and degradation of such oxidimethaneamine bridges is discussed.