Mononuclear ruthenium complexes with a pentadentate ligand, N,N-bis[(isoquinolin-1-yl)methyl][6-(pyridin-2-yl)pyridin-2-yl]methanamine (DIQ-Bpy), were synthesized and characterized by 1H NMR spectroscopy, elemental analysis, electrochemistry, and theoretical calculations. The oxidation of water by [Ru(DIQ-Bpy)(H2O)]2+ was observed in the presence of excess amounts of CeIV. Relative to [Ru(DPA-Bpy)(H2O)]2+ [DPA-Bpy = N,N-bis(2-pyridinylmethyl)-2,2′-bipyridine-6-methanamine], the substitution of pyridine groups in DPA-Bpy with electron-withdrawing isoquinolines results in higher redox potential and lower activity for the oxidation of water by [Ru(DIQ-Bpy)(H2O)]2+. A kinetic study of water oxidation by [Ru(DPA-Bpy)(H2O)]2+ suggests a mononuclear pathway for the oxidation of water. The noncovalent interaction between isoquinoline groups in [Ru(DIQ-Bpy)(H2O)]2+, which favors the formation of dinuclear species, might account for the lower activity for water oxidation by [Ru(DIQ-Bpy)(H2O)]2+.