A Ru-II-pentadentate polypyridyl complex [Ru-II(K-N-5-bpy2PYMe)Cl](+) (1(+), bpy2PYMe = 1-(2-pyridyl)-1,1-bis(6-2,2'-bipyridyl)ethane) and its aqua derivative [Ru-II(kappa-N-5-bpy2PYMe)(H2O)](2+) (2(2+)) were synthesized and characterized by experimental and computational methods. In MeOH, 1(+) exists as two isomers in different proportions, cis (70%) and trans (30%), which are interconverted under thermal and photochemical conditions by a sequence of processes: chlorido decoordination, decoordination/recoordination of a pyridyl group, and chlorido recoordination. Under oxidative conditions in dichloromethane, trans-1(2+) generates a [Ru-III(kappa-N4-bpy2PYMe)Cl-2](+) intermediate after the exchange of a pyridyl ligand by a Cl- counterion, which explains the trans/cis isomerization observed when the system is taken back to Ru(II). On the contrary, cis-I2+ is in direct equilibrium with trans-1(2+), with absence of the kappa-N-4-bis-chlorido Rum-intermediate. All these equilibria were modeled by density functional theory calculations. Interestingly, the aqua derivative is obtained as a pure trans-[Ru-II(kappa-N(5)bpy(bpyMe)PYMe)](+) isomer (trans-2(2+), while the addition of a methyl substituent to a single bpy of the pentadentate ligand leads to the formation of a single cis isomer for both chlorido and aqua derivatives [Ru-II(kappa-N-5-bpy(bpyMe)PYMe)Cl](+) (3(+)) and [Ru-II(kappa-N-5-bpy(bpyMe)PYMe)(H2O)](2+) (4(2+)) due to the steric constraints imposed by the modified ligand. This system was also structurally and electrochemically compared to the previously reported [Ru-II(PY5Me(2))X](n+) system (X = Cl, n = 1 (5(+)); X = H2O, n = 2 (6(2+)), which also contains a kappa-N-5-Ru-II coordination environment, and to the newly synthesized [Ru-II(PY4Im)X](n+) complexes (X = Cl, n = 1 (7(+)); X = H2O, n = 2 (8(2+)), which possess an electron-rich kappa-(NC)-C-4-Ru-II site due to the replacement of a pyridyl group by an imidazolic carbene.