A family of 3d–4f [MII2LnIII2(μ3-OH)2(O2CtBu)10]2– “butterflies” (where MII = Mg, Co, Ni, and Cu; LnIII = Y, Gd, Tb, Dy, Ho, and Er) and [MnIII2LnIII2(μ3-O)2(O2CtBu)10]2– molecules (where LnIII = Y, Gd, Tb, Dy, Ho, and Er) has been synthesized and characterized through single-crystal X-ray diffraction, SQUID magnetometry, and ab initio calculations. All dysprosium- and some erbium-containing tetramers showed frequency-dependent maxima in the out-of-phase component of the susceptibility associated with slow relaxation of magnetization, and hence, they are single-molecule magnets (SMMs). AC susceptibility measurements have shown that the SMM behavior is entirely intrinsic to the Dy and Er sites and the magnitude of the energy barrier is influenced by the interactions between the 4f and the 3d metal. A trend is observed between the strength of the 3d-4f exchange interaction between and the maximum observed in the χ″M(T).