A method of increasing the axial zero-field splitting parameter for transition metal complexes of utility in the assembly of magnetic clusters is demonstrated through the use of heavy atoms as auxiliary ligands. The octahedral complexes [Cr(dmpe)(2)(CN)X](+) (dmpe = 1,2-bis(dimethylphosphino)ethane, X = Cl, Br, I) and Cr(dmpe)2(CN)X (X = Cl, I) are synthesized and structurally characterized. Variable-field magnetization measurements show the magnitude of D for these complexes to increase significantly as the halide ligand varies from chloride to iodide, ranging from 0.11 cm(-1) for [Cr(dmpe)(2)(CN)Cl](+) to 6.26 cm(-1) for Cr(dmpe)(2)(CN)I.