Heterometallic polymeric coordination compounds [(Ln(hfac)(2)(CH3OH)}(2){Cu(dmg)(Hdmg)}(2)](n) (abbreviated as [Ln(2)Cu(2)](n)) involving light lanthanoid ions, Pr, Nd, Sm, and Eu, were synthesized, where H(2)dmg and Hhfac stand for dimethylglyoxime and 1,1,1,5,5,5-hexafluoropentane-2,4-dione, respectively. The X-ray crystallographic analysis shows that their structures are isomorphous to those of the known heavy lanthanoid analogs. The exchange couplings were evaluated by high-frequency electron paramagnetic resonance and pulsed-field magnetization studies, giving ferromagnetic exchange parameters: J(Pr-cu)/k(B) = 0.039(1) K, J(Nd-cu)/k(B) = 0.38(1) K, J(sm-cu)/k(B) = 0.95(2) K, and no interaction between Eu and Cu ions. A significant trend is found in the order of the periodic table. The ferro- and antiferromagnetic 4f-3d couplings were characterized for the light and heavy lanthanoid derivatives, respectively. The magnitude of J(Ln-Cu) decreases with a decrease of the 4f-spin portion. i.e., the number of unpaired electrons, whether the coupling is ferro- or antiferromagnetic. (C) 2011 Elsevier Ltd. All rights reserved.