The fac/mer-ratios of the iron(II) complexes with the derivatives of an unnatural amino acid, 5'-N-acetylamino-2,2'-bipyridine-5-carboxylic acid, were investigated. It was found that the amide derivatives with secondary amines selectively yielded the mer-isomers independently of the ligand structures. On the other hand, for the amides with primary amines, the ratios of the fac- and mer-complexes increased and decreased, respectively, with the increasing sizes of the substituents, suggesting stabilization of the fac-isomers by electrostatic interaction (or hydrogen bondings) between the ligands.