Reaction of nickel(II) acetate with H3L (2-(5-bromo-2-hydroxyphenyl)-1,3-bis[4-(5-bromo-2-hydroxyphenyl)-3-azabu t-3-enyl]-1,3-imidazolidine) yields [Ni2L(OAc)(H2O)(2)](.)3MeCN(.)2H(2)O (1(.)3MeCN(.)2H(2)O), crystallographically characterized. 1 is unstable in solution for a long time and hydrolyzes to give [Ni2L(O-OC6H3BrCHO)(H2O)](.)2.25MeCN-H2O (2(.)2.25MeCN(.)H(2)O). In addition, 1 uptakes CO2 from air in a basic methanol/acetonitrile solution, yielding {[Ni2L(MeOH)](2)(CO3)}(.)1.5MeOH-MeCN-H2O (3(.)1.5WOH(.)MeCN-H2O). The X-ray characterization of 3 reveals that it is a tetranuclear nickel cluster, which can be considered as the result of a self-assembly process from two dinuclear [Ni2L](+) blocks, joined by a mu(4)-eta(2):eta(2)-O,O carbonate ligand. The coordination mode of the carbonate anion is highly unusual and, to the best of our knowledge, it has not been described thus far for first-row transition metal complexes or magnetically studied until now. Magnetic characterization of 1 and 3 shows net intramolecular ferromagnetic coupling between the metal atoms in both cases, with S = 2 and S = 4 ground states for 1 and 3, respectively.