Series of DFT calculations on [L1Ni(H2O) 2Gd(NO3)3] (1) and their transition metal substituted models {CrIIIGdIII}, {FeIIGd III}, {MnIIGdIII} and {CuIIGd III} have been performed to probe the electronic effects of the metal substitution on the sign and strength of magnetic coupling. Our results unfold an interesting observation that the strength of the exchange is related to the d-orbital occupancy with occupancy on the eg orbitals resulting in stronger ferromagnetic exchange while progressively filling the t2g orbitals leads to a reduction in the computed J values. Further, we extend our studies to the trinuclear {3d-Ln-3d} [L2M2Ln][ClO 4] (LH3 = (S)P[N(Me)N = CH-C6H 3-2-OH-3-OMe]3) (here M = Cu2+, Ni2+ and Mn2+ and Ln = GdIII and LaIII) complexes possessing phosphorus, salicylideneaminooxy and Schiff base supported ligands to corroborate our findings in dinuclear models. By computing the nearest neighbor and next-nearest neighbor interactions in all three complexes, our studies reveal that the observation of strong exchange with eg occupancy is preserved also in the trinuclear complexes. This will have implications in the area of molecular coolants where CuII and GdIII are the preferred choice as they inherit no/negligible anisotropy, but our finding suggests that CuII invariably also offers strong exchange interaction - a parameter unsuitable for ideal molecular coolants. Metals ions such as MnII and CrIII are perhaps better choice for molecular refrigeration as they possess large spin values, negligible anisotropy and mediate relatively a weak magnetic exchange interaction. © 2013 Elsevier Ltd. All rights reserved.