Cis-Bis(2,2'-bipyridine)dichlororuthenium(II) hydrate, commonly known as cis-[Ru(bpy)₂Cl₂]·xH₂O, is a coordination complex featuring a ruthenium(II) center coordinated by two 2,2'-bipyridine (bpy) ligands and two chloride ligands in a cis configuration. It is a red-orange to brown solid that is soluble in polar organic solvents and water (depending on hydration state). This complex is a key precursor in the synthesis of a wide range of polypyridyl ruthenium complexes. Its primary and most significant application is as a versatile starting material for the preparation of photochemically and electrochemically active ruthenium complexes. Most notably, it is the direct precursor to the benchmark dye sensitizer [Ru(bpy)₂(NCS)₂] (N3 dye) used in dye-sensitized solar cells (DSSCs). It is also used to synthesize other important complexes like [Ru(bpy)₃]²⁺, a quintessential photosensitizer and redox mediator in artificial photosynthesis, photocatalysis, and electrochemiluminescence (ECL) applications. Beyond its role in materials science, cis-[Ru(bpy)₂Cl₂] serves as a catalyst or catalyst precursor in various organic transformations, including oxidation and transfer hydrogenation reactions. Due to its photosensitivity, it should be stored protected from light. It is considered moderately toxic and an irritant, requiring standard laboratory safety precautions during handling.