We present the magnetic properties of three recently synthesized binuclear molecular complexes [NiNd], [NiGd], and [ZnGd] investigated by dc magnetization and proton nuclear magnetic resonance (NMR) measurements. The high-temperature magnetic properties are related to the independent paramagnetic behavior of the two magnetic metal ions within the binuclear entities both in [NiNd] and [NiGd]. On lowering the temperature, the formation of a magnetic dimer, with a low-spin ground state due to antiferromagnetic interaction (J/kB -25 K) between Ni2+ and Nd3+, is found in the case of [NiNd], while in [NiGd], a ferromagnetic interaction (J/kB 3.31 K) between the magnetic ions leads to a high-spin (S = 9/2) ground state. The temperature dependence of the proton nuclear spin lattice relaxation rate T1 -1 in [NiNd] is driven by the fluctuation of the hyperfine field at the nuclear site due to relaxation of the magnetization. At high temperatures, the independent Ni2+ and Nd3+ spins fluctuate fast, while at low temperatures, we observe a slowing down of the fluctuation in the total magnetization of the dimer because of the insurgence of antiferromagnetic spin correlations. The relaxation mechanism in [NiNd] at low temperatures is interpreted by a single temperature-dependent correlation frequency ωc T3.5, which reflects the lifetime broadening of the exchange-coupled spins via spin-phonon interaction. The proton NMR signal in [NiGd] could just be detected at room temperature due to the shortening of relaxation times when T is decreased. The magnetic properties of [ZnGd] are the ones expected from a weakly interacting assembly of isolated moments except for anomalies in the susceptibility and NMR results below 15 K, which currently cannot be explained. © 2011 American Physical Society.