Three anion–cation compounds 1–3 with formula [M(phen)₃][Cd₄(SPh)₁₀]·Sol (M = Ru²⁺, Fe²⁺, and Ni²⁺, Sol = MeCN and H₂O) have been synthesized and characterized by single-crystal analysis. Both the cations and anion are well-known ions, but the properties of the co-assembled compounds are interesting. Molecular structures and charge-transfer between the cations and anions in crystal and even in solution are discussed. These compounds are isomorphous and short inter-ion interactions are found in these crystals, such as ππ stacking and C–Hπ contacts. Both spectroscopic and theoretical calculated results indicate that there is anion–cation charge-transfer (ACCT) between the Ru-phen complex dye and the Cd-SPh cluster, which plays an important role in their photophysical properties. The intensity of the fluorescent emission of the [Ru(phen)₃]²⁺ is enhanced when the cation interacts with the [Cd₄(SPh)₁₀]²⁻ anion. The mechanism for the enhancement of photoluminescence has been proposed.