The cationic complex in the title compound, RuCl(C10H8N2)(C19H20N3O3P)PF6·0.83CH3CN·0.17H2O, is a water-oxidation precatalyst functionalized for TiO2 attachment via terpyridine phosphonate. The The RuII atom in the complex has a distorted octahedral geometry due to the restricted bite angle 159.50 (18)° of the terpyridyl ligand. The dihedral angle between the least-squares planes of the terpyridyl and bipyridyl moieties is 86.04 (14)°. The mean Ru-N bond length for bipyridine is 2.064 (5) Å, with the bond opposite to Ru-Cl being 0.068 Å shorter. For the substituted terpyridine, the mean Ru-N distance involving the outer N atoms trans to each other is 2.057 (6) Å, whereas the bond length involving the central N atom is 1.944 (5) Å. The Ru-Cl distance is 2.4073 (15) Å. The P atom of the phosphonate group lies in the same plane as its adjacent pyridyl ring, with the ordinary character of the bond between P and Ctpy 1.801 (6) Å allowing for free rotation of the terpyridine substituent around the P-Ctpy axis. The acetonitrile solvent molecule was refined to be disordered with two water molecules; occupancies for the acetontrile and water molecules were 0.831 (9) and 0.169 (9), respectively. Also disordered was the PF6− counter-ion (over three positions) and one of the ethoxy substituents (with two positions). The crystal structure shows significant intra- and intermolecular H...X contacts, especially some involving the Cl− ligand.