We investigate how ligand substitution affects the intramolecular spin exchange interactions, studying a prototypical family of single-molecule magnets comprising dodecanuclear cluster molecules [(Mn8Mn4O12)-Mn-III-O-IV(COOR)(16)]. We identify a simple scheme based on accumulated Pauling electronegativity numbers (AEN) of the carboxylate ligand groups (R). The redistribution of the electron density, controlled by the AEN of a ligand, changes the degree of hybridization between 3d electrons of manganese and 2p electrons of oxygen atoms, thus changing the exchange interactions. This scheme, despite its conceptual simplicity, provides a strong correlation with the exchange energies associated with carboxylate bridges and is confirmed by the electronic structure calculations taking into account the Coulomb correlations in magnetic molecules.