The β-diketone scaffold is a commonly used synthetic intermediate, and is a functional group found in natural products such as curcuminoids. This core structure can also act as a chelating ligand for a variety of metals. In order to assess the potential of this scaffold for medicinal inorganic chemistry, seven different κ-O,O'-chelating ligands were used to construct Ru(II) complexes with polypyridyl co-ligands, and their biological activity was evaluated. The complexes demonstrated promising structure-dependent cytotoxicity. Three complexes maintained high activity in a tumor spheroid model, and all complexes demonstrated low toxicity in a zebrafish model. From this series, the best compound exhibited a ~ 30-fold window between cytotoxicity in a 3-D tumor spheroid model and potential toxicity. These results suggest that κ-O,O'-ligands can be incorporated into Ru(II)-polypyridyl complexes to create favorable candidates for future drug development.