The 3,5-dichlorobenzoate anion, L–, serves as a bridging ligand and 2,2'-bipyridine, bipy, as a terminal bidentate ligand to yield, through hydrothermal syntheses, the tetranuclear clusters Dy2Co2L10(bipy)2, 1, and Ln2Ni2L10(bipy)2, where Ln is the trivalent La, 2, Gd, 3, Tb, 4, Dy, 5, or Ho, 6, ion. Single-crystal X-ray diffraction reveals that the six complexes are all isomorphous with the monoclinic P21/c space group and with lattice parameters that decrease with the lanthanide contraction. The two cobalt(II) or nickel(II) and two Ln(III) cations are linked by the 10 L– anions to generate Dy2Co2 or Ln2Ni2 3d–4f cationic heteronuclear clusters with a slightly bent Co···Dy···Dy···Co or Ni···Ln···Ln···Ni arrangement. Direct current magnetic susceptibility studies reveal that the complexes are essentially paramagnetic, with room-temperature ?MT values close to the expected values for two cobalt(II) or nickel(II) and two Ln(III) cations. The temperature dependence of ?MT for 1 and 5 is well reproduced by ab initio calculations with the inclusion of weak magnetic exchange between the cobalt(II) or nickel(II) and a dysprosium(III) and between two dysprosium(III) ions. The calculated magnetic exchange parameters are JDy–Co = 0.2 cm–1 and JDy–Dy = 0.02 cm–1 for 1 and JDy–Ni = -0.2 cm–1 and JDy–Dy = 0.03 cm–1 for 5. Alternating current magnetic susceptibility studies reveal that 1 and 5 exhibit slow magnetic relaxation with effective energy barriers, Ueff, for the reversal of the magnetization for 1 of 82(2) cm–1 in a 0 Oe dc bias field and 79.4(5) cm–1 in a 1000 Oe dc bias field and, for 5, 73(1) cm–1 in a 0 dc bias field; the calculated energies of 66.1(1) and 61.0(1) cm–1 for the first excited spin–orbit state of dysprosium(III) in 1 and 5 agree rather well with these effective energy barriers. The entire Arrhenius plots of the logarithm of t, the relaxation rate of the magnetization in 1 and 5, have been fit with contributions from quantum tunneling, direct Raman scattering, and Orbach thermal processes. The observation of a low-temperature magnetization reversal mechanism in 5 but not in 1 may be understood through the calculated exchange energy spectrum in their ground state.