Carboxylato (R = Bu-t and Et) and carbonato bridges have been utilized for nickel(II)-based aggregates [Ni-4(mu-H2L)(2)(mu(3)-OH)(2)(mu 1,3-O(2)CBut)(2)](NO3)(2)center dot H2O center dot 2DMF (1 center dot H2O center dot 2DMF), Ni-4(mu-(HL)-H-hy)(2)(mu(3)-OMe)(2)(mu(1,1)-N-3)(2)(mu(1,3)-O2CEt)(2)]center dot 4H(2)O (2 center dot 4H(2)O), and Ni-6(mu(4)-L)(mu(3)-L)(2)(mu(6)-CO3)(H2O)(8)](ClO4)center dot 9H(2)O (3 center dot 9H(2)O). Building blocks [Ni-2(mu-H2L)](3+), [Ni-2(mu-(HL)-H-hy)](3+), and [Ni-2(mu-L)](+) originating from [Ni-2(mu-H2L)](3+) have been trapped in these complexes. The complexes have been characterized by X-ray crystallography, magnetic measurements, and density functional theory (DFT) analysis. In 1, the magnetic interactions are transmitted through the mu 3-phenoxido/mu 3-hydroxido/syn-syn-tBuCO2-, mu 3-phenoxido/mu 3- hydroxido, and double mu 3-phenoxido/double mu 3-hydroxido bridges with J = +11.4 cm(-1), J1 = -2.1 cm(-1), and J2 = -2.8 cm(-1), respectively. In 2, the interactions are ferromagnetic, with J1 = +27.5 cm(-1), J2 = +20.62 cm(-1), and J3 = +1.52 cm(-1) describing the magnetic couplings through the mu-phenoxidoo/mu 3-methoxido, mu-azido/mu 3-methoxido, and mu 3-methoxido/mu 3-methoxido exchange pathways, respectively. Complex 3 gives J1 = -3.30 cm(-1), J2 = +1.7 cm(-1), and J3 = -12.8 cm(-1) for exchange pathways mediated by mu-phenoxido/mu-carbonato, mu-alkoxido/mu-alkooxido/mu-syn-syn-carbonato, and the mu-phenoxido/mu-carbonato, respectively. Interestingly, 1 and 3 below 20 K and 35 K, respectively, show an abrupt increase of the chi T-M product to reach a magnetic-field-dependent maximum, which is associated with a slightly frequency-dependent out-of-phase alternating-current peak. DFT calculations have also been performed on 1-3 to explain the exchange interaction mechanisms and to support the magnitude and sign of the magnetic coupling constants between the Ni-II ions.