A Ru ^II-pentadentate polypyridyl complex [Ru ^II(κ-N ⁵-bpy2PYMe)Cl] ⁺ (1 ⁺, bpy2PYMe = 1-(2-pyridyl)-1,1-bis(6-2,2′-bipyridyl)ethane) and its aqua derivative [Ru ^II(κ-N ⁵-bpy2PYMe)(H ₂O)] 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(κ-N ⁴-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-1 ²⁺ is in direct equilibrium with trans-1 ²⁺, with absence of the κ-N ⁴-bis-chlorido Ru ^III-intermediate. All these equilibria were modeled by density functional theory calculations. Interestingly, the aqua derivative is obtained as a pure trans-[Ru ^II(κ-N ⁵-bpy2PYMe)(H ₂O)] ²⁺ 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(κ-N ⁵-bpy(bpyMe)PYMe)Cl] ⁺ (3 ⁺) and [Ru ^II(κ-N ⁵-bpy(bpyMe)PYMe)(H ₂O)] 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 = H ₂O, n = 2 (6 ²⁺)), which also contains a κ-N ⁵-Ru ^II coordination environment, and to the newly synthesized [Ru ^II(PY4Im)X] ⁿ⁺ complexes (X = Cl, n = 1 (7 ⁺); X = H ₂O, n = 2 (8 ²⁺)), which possess an electron-rich κ-N ⁴C-Ru ^II site due to the replacement of a pyridyl group by an imidazolic carbene.