Atmospheric CO2 fixation of [Ni-II(3-MeOsaltn)(H2O)(2)]center dot 2.5H(2)O [3-MeOsaltn = N,N'-bis(3-methoxy-2-oxybenzylidene) -1,3-propanediaminato], Ln(III)(NO3)(3)center dot 6H(2)O, and triethylamine occurred in methanol/acetone, giving a first series of carbonato-bridged Ni-2(II),Ln(2)(III) complexes [(mu(4)-CO3)(2){Ni-II(3-MeOsaltn)(MeOH)-Ln(III)(NO3)}(2)] (1Gd, 1Tb, and 1Dy). When the reaction was carried out in acetonitrile/water, it gave a second series of complexes [(mu(4)-CO3)(2){Ni-II(3-MeOsaltn)(H2O)-Ln(III)(NO3)}(2)]center dot 2CH(3)CN center dot 2H(2)O (2Gd, 2Tb, and 2Dy). For both series, each Ni(2)(II)Ln(2)(III) structure can be described as two di-mu-phenoxo-bridged Ni(II)Ln(III) binuclear units bridged by two carbonato CO32- units to form a carbonato-bridged (mu(4)-CO3)(2){Ni(2)(II)Ln(2)(III)) structure. The high-spin Ni-II ion has octahedral coordination geometry, and the Ln(III) ion is coordinated by O-9 donor atoms from Ni-II(3-MeOsaltn), bidentate NO3-, and one and two oxygen atoms of two CO32- ions. The NO3- ion for the first series roughly lie on Ln-O(methoxy) bonds and are tilted toward the outside, while for the second series, the two oxygen atoms roughly lie on one of the Ln-O(phenoxy) bonds due to the intramolecular hydrogen bond. The temperature-dependent magnetic susceptibilities indicated a ferromagnetic interaction between the Ni-II and Ln(III) ions (Ln(III) = Gd-III, Tb-III, Dy-III) for all of the complexes, with a distinctly different magnetic behavior between the two series in the lowest-temperature region due to the Ln(III)-Ln(III) magnetic interaction and/or different magnetic anisotropies of the Tb-III or Dy-III ion. Alternating-current susceptibility measurements under the 0 and 1000 Oe direct-current (dc) bias fields showed no magnetic relaxation for the (Ni2Gd2III)-Gd-II complexes but exhibited an out-of-phase signal for (Ni2Tb2III)-Tb-II and (Ni2Dy2III)-Dy-II, indicative of slow relaxation of magnetization. The energy barriers, Delta/k(B), for the spin flipping were estimated from the Arrhenius plot to be 12.2(7) and 6.1(3) K for 1Tb and 2Tb, respectively, and 18.1(6) and 14.5(4) K for 1Dy and 2Dy, respectively, under a dc bias field of 1000 Oe. Compound 1Dy showed relatively slow relaxation of magnetization reorientation even at zero dc applied field with Delta/k(B) = 6.6(4) K.